diff --git a/cranelift/Cargo.toml b/cranelift/Cargo.toml
index b674fbf7f9..b3c7672c0c 100644
--- a/cranelift/Cargo.toml
+++ b/cranelift/Cargo.toml
@@ -34,6 +34,8 @@ term = "0.5.1"
 capstone = { version = "0.4", optional = true }
 wabt = { version = "0.4", optional = true }
 target-lexicon = "0.0.3"
+pretty_env_logger = "0.2.4"
+file-per-thread-logger = "0.1.1"
 
 [features]
 default = ["disas", "wasm"]
diff --git a/cranelift/README.md b/cranelift/README.md
index b304a97891..b325058da1 100644
--- a/cranelift/README.md
+++ b/cranelift/README.md
@@ -8,7 +8,7 @@ into executable machine code.
 
 [![Documentation Status](https://readthedocs.org/projects/cranelift/badge/?version=latest)](https://cranelift.readthedocs.io/en/latest/?badge=latest)
 [![Build Status](https://travis-ci.org/CraneStation/cranelift.svg?branch=master)](https://travis-ci.org/CraneStation/cranelift)
-[![Gitter chat](https://badges.gitter.im/CraneStation/CraneStation.svg)](https://gitter.im/CraneStation/Lobby/~chat)
+[![Gitter chat](https://badges.gitter.im/CraneStation/CraneStation.svg)](https://gitter.im/CraneStation/Lobby)
 
 For more information, see [the
 documentation](https://cranelift.readthedocs.io/en/latest/?badge=latest).
@@ -127,7 +127,7 @@ Building the documentation
 --------------------------
 
 To build the Cranelift documentation, you need the [Sphinx documentation
-generator](https://www.sphinx-doc.org/):
+generator](http://www.sphinx-doc.org/) as well as Python 3::
 
     $ pip install sphinx sphinx-autobuild sphinx_rtd_theme
     $ cd cranelift/docs
diff --git a/cranelift/src/clif-util.rs b/cranelift/src/clif-util.rs
index 0f7edd4ce3..890bf95be7 100644
--- a/cranelift/src/clif-util.rs
+++ b/cranelift/src/clif-util.rs
@@ -1,5 +1,5 @@
-#![deny(trivial_numeric_casts)]
-#![warn(unused_import_braces, unstable_features, unused_extern_crates)]
+#![deny(trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
 #![cfg_attr(
     feature = "cargo-clippy",
     warn(
@@ -14,11 +14,13 @@ extern crate cranelift_codegen;
 extern crate cranelift_filetests;
 extern crate cranelift_reader;
 extern crate docopt;
+extern crate file_per_thread_logger;
 extern crate filecheck;
 #[macro_use]
 extern crate serde_derive;
 #[cfg(feature = "disas")]
 extern crate capstone;
+extern crate pretty_env_logger;
 extern crate term;
 
 cfg_if! {
@@ -31,6 +33,7 @@ cfg_if! {
 }
 extern crate target_lexicon;
 
+use cranelift_codegen::dbg::LOG_FILENAME_PREFIX;
 use cranelift_codegen::{timing, VERSION};
 use docopt::Docopt;
 use std::io::{self, Write};
@@ -46,16 +49,17 @@ const USAGE: &str = "
 Cranelift code generator utility
 
 Usage:
-    clif-util test [-vT] <file>...
-    clif-util cat <file>...
-    clif-util filecheck [-v] <file>
-    clif-util print-cfg <file>...
-    clif-util compile [-vpT] [--set <set>]... [--target <triple>] <file>...
-    clif-util wasm [-ctvpTs] [--set <set>]... [--target <triple>] <file>...
+    clif-util test [-vTd] <file>...
+    clif-util cat [-d] <file>...
+    clif-util filecheck [-vd] <file>
+    clif-util print-cfg [-d] <file>...
+    clif-util compile [-vpTd] [--set <set>]... [--target <triple>] <file>...
+    clif-util wasm [-ctvpTsd] [--set <set>]... [--target <triple>] <file>...
     clif-util --help | --version
 
 Options:
     -v, --verbose   be more verbose
+    -d, --debug     enable debug output on stderr/stdout
     -T, --time-passes
                     print pass timing report
     -t, --just-decode
@@ -90,6 +94,7 @@ struct Args {
     flag_target: String,
     flag_time_passes: bool,
     flag_print_size: bool,
+    flag_debug: bool,
 }
 
 /// A command either succeeds or fails with an error message.
@@ -106,6 +111,12 @@ fn clif_util() -> CommandResult {
         })
         .unwrap_or_else(|e| e.exit());
 
+    if args.flag_debug {
+        pretty_env_logger::init();
+    } else {
+        file_per_thread_logger::initialize(LOG_FILENAME_PREFIX);
+    }
+
     // Find the sub-command to execute.
     let result = if args.cmd_test {
         cranelift_filetests::run(args.flag_verbose, &args.arg_file).map(|_time| ())
diff --git a/lib/codegen/Cargo.toml b/lib/codegen/Cargo.toml
index 5bddad6418..4d8f48f012 100644
--- a/lib/codegen/Cargo.toml
+++ b/lib/codegen/Cargo.toml
@@ -17,6 +17,7 @@ failure = { version = "0.1.1", default-features = false, features = ["derive"] }
 failure_derive = { version = "0.1.1", default-features = false }
 hashmap_core = { version = "0.1.9", optional = true }
 target-lexicon = { version = "0.0.3", default-features = false }
+log = { version = "0.4.3", default-features = false, features = ["release_max_level_warn"] }
 # It is a goal of the cranelift-codegen crate to have minimal external dependencies.
 # Please don't add any unless they are essential to the task of creating binary
 # machine code. Integration tests that need external dependencies can be
diff --git a/lib/codegen/src/binemit/relaxation.rs b/lib/codegen/src/binemit/relaxation.rs
index f23d3218f8..ad13dc21c1 100644
--- a/lib/codegen/src/binemit/relaxation.rs
+++ b/lib/codegen/src/binemit/relaxation.rs
@@ -140,7 +140,7 @@ fn relax_branch(
     isa: &TargetIsa,
 ) -> CodeOffset {
     let inst = cur.current_inst().unwrap();
-    dbg!(
+    debug!(
         "Relaxing [{}] {} for {:#x}-{:#x} range",
         encinfo.display(cur.func.encodings[inst]),
         cur.func.dfg.display_inst(inst, isa),
@@ -156,7 +156,7 @@ fn relax_branch(
         .find(|&enc| {
             let range = encinfo.branch_range(enc).expect("Branch with no range");
             if !range.contains(offset, dest_offset) {
-                dbg!("  trying [{}]: out of range", encinfo.display(enc));
+                debug!("  trying [{}]: out of range", encinfo.display(enc));
                 false
             } else if encinfo.operand_constraints(enc)
                 != encinfo.operand_constraints(cur.func.encodings[inst])
@@ -166,10 +166,10 @@ fn relax_branch(
                 // which the existing operands don't satisfy. We can't check for
                 // validity directly because we don't have a RegDiversions active so
                 // we don't know which registers are actually in use.
-                dbg!("  trying [{}]: constraints differ", encinfo.display(enc));
+                debug!("  trying [{}]: constraints differ", encinfo.display(enc));
                 false
             } else {
-                dbg!("  trying [{}]: OK", encinfo.display(enc));
+                debug!("  trying [{}]: OK", encinfo.display(enc));
                 true
             }
         }) {
diff --git a/lib/codegen/src/binemit/shrink.rs b/lib/codegen/src/binemit/shrink.rs
index 63b0329278..5027aeeeb7 100644
--- a/lib/codegen/src/binemit/shrink.rs
+++ b/lib/codegen/src/binemit/shrink.rs
@@ -54,7 +54,7 @@ pub fn shrink_instructions(func: &mut Function, isa: &TargetIsa) {
                 if best_enc != enc {
                     func.encodings[inst] = best_enc;
 
-                    dbg!(
+                    debug!(
                         "Shrunk [{}] to [{}] in {}, reducing the size from {} to {}",
                         encinfo.display(enc),
                         encinfo.display(best_enc),
diff --git a/lib/codegen/src/dbg.rs b/lib/codegen/src/dbg.rs
index 0b32bf2bf5..f19c222caf 100644
--- a/lib/codegen/src/dbg.rs
+++ b/lib/codegen/src/dbg.rs
@@ -1,133 +1,8 @@
-//! Debug tracing macros.
-//!
-//! This module defines the `dbg!` macro which works like `println!` except it writes to the
-//! Cranelift tracing output file if enabled.
-//!
-//! Tracing can be enabled by setting the `CRANELIFT_DBG` environment variable to something
-/// other than `0`.
-///
-/// The output will appear in files named `cranelift.dbg.*`, where the suffix is named after the
-/// thread doing the logging.
-#[cfg(feature = "std")]
-use std::cell::RefCell;
-#[cfg(feature = "std")]
-use std::env;
-#[cfg(feature = "std")]
-use std::ffi::OsStr;
+//! Debug tracing helpers.
 use std::fmt;
-#[cfg(feature = "std")]
-use std::fs::File;
-#[cfg(feature = "std")]
-use std::io::{self, Write};
-#[cfg(feature = "std")]
-use std::sync::atomic;
-#[cfg(feature = "std")]
-use std::thread;
 
-#[cfg(feature = "std")]
-static STATE: atomic::AtomicIsize = atomic::ATOMIC_ISIZE_INIT;
-
-/// Is debug tracing enabled?
-///
-/// Debug tracing can be enabled by setting the `CRANELIFT_DBG` environment variable to something
-/// other than `0`.
-///
-/// This inline function turns into a constant `false` when debug assertions are disabled.
-#[cfg(feature = "std")]
-#[inline]
-pub fn enabled() -> bool {
-    if cfg!(debug_assertions) {
-        match STATE.load(atomic::Ordering::Relaxed) {
-            0 => initialize(),
-            s => s > 0,
-        }
-    } else {
-        false
-    }
-}
-
-/// Does nothing
-#[cfg(not(feature = "std"))]
-#[inline]
-pub fn enabled() -> bool {
-    false
-}
-
-/// Initialize `STATE` from the environment variable.
-#[cfg(feature = "std")]
-fn initialize() -> bool {
-    let enable = match env::var_os("CRANELIFT_DBG") {
-        Some(s) => s != OsStr::new("0"),
-        None => false,
-    };
-
-    if enable {
-        STATE.store(1, atomic::Ordering::Relaxed);
-    } else {
-        STATE.store(-1, atomic::Ordering::Relaxed);
-    }
-
-    enable
-}
-
-#[cfg(feature = "std")]
-thread_local! {
-    static WRITER : RefCell<io::BufWriter<File>> = RefCell::new(open_file());
-}
-
-/// Write a line with the given format arguments.
-///
-/// This is for use by the `dbg!` macro.
-#[cfg(feature = "std")]
-pub fn writeln_with_format_args(args: fmt::Arguments) -> io::Result<()> {
-    WRITER.with(|rc| {
-        let mut w = rc.borrow_mut();
-        writeln!(*w, "{}", args)?;
-        w.flush()
-    })
-}
-
-/// Open the tracing file for the current thread.
-#[cfg(feature = "std")]
-fn open_file() -> io::BufWriter<File> {
-    let curthread = thread::current();
-    let tmpstr;
-    let mut path = "cranelift.dbg.".to_owned();
-    path.extend(
-        match curthread.name() {
-            Some(name) => name.chars(),
-            // The thread is unnamed, so use the thread ID instead.
-            None => {
-                tmpstr = format!("{:?}", curthread.id());
-                tmpstr.chars()
-            }
-        }.filter(|ch| ch.is_alphanumeric() || *ch == '-' || *ch == '_'),
-    );
-    let file = File::create(path).expect("Can't open tracing file");
-    io::BufWriter::new(file)
-}
-
-/// Write a line to the debug trace file if tracing is enabled.
-///
-/// Arguments are the same as for `printf!`.
-#[cfg(feature = "std")]
-#[macro_export]
-macro_rules! dbg {
-    ($($arg:tt)+) => {
-        if $crate::dbg::enabled() {
-            // Drop the error result so we don't get compiler errors for ignoring it.
-            // What are you going to do, log the error?
-            $crate::dbg::writeln_with_format_args(format_args!($($arg)+)).ok();
-        }
-    }
-}
-
-/// `dbg!` isn't supported in `no_std` mode, so expand it into nothing.
-#[cfg(not(feature = "std"))]
-#[macro_export]
-macro_rules! dbg {
-    ($($arg:tt)+) => {};
-}
+/// Prefix added to the log file names, just before the thread name or id.
+pub static LOG_FILENAME_PREFIX: &str = "cranelift.dbg.";
 
 /// Helper for printing lists.
 pub struct DisplayList<'a, T>(pub &'a [T])
diff --git a/lib/codegen/src/ir/layout.rs b/lib/codegen/src/ir/layout.rs
index 9f03969061..a9dc08d786 100644
--- a/lib/codegen/src/ir/layout.rs
+++ b/lib/codegen/src/ir/layout.rs
@@ -316,7 +316,7 @@ impl Layout {
                 next_inst = self.insts[inst].next.expand();
             }
         }
-        dbg!("Renumbered {} program points", seq / MAJOR_STRIDE);
+        debug!("Renumbered {} program points", seq / MAJOR_STRIDE);
     }
 }
 
diff --git a/lib/codegen/src/isa/x86/abi.rs b/lib/codegen/src/isa/x86/abi.rs
index d0305d918b..21796e2a10 100644
--- a/lib/codegen/src/isa/x86/abi.rs
+++ b/lib/codegen/src/isa/x86/abi.rs
@@ -217,6 +217,7 @@ fn callee_saved_gprs(isa: &TargetIsa) -> &'static [RU] {
     }
 }
 
+/// Get the set of callee-saved registers that are used.
 fn callee_saved_gprs_used(isa: &TargetIsa, func: &ir::Function) -> RegisterSet {
     let mut all_callee_saved = RegisterSet::empty();
     for reg in callee_saved_gprs(isa) {
diff --git a/lib/codegen/src/legalizer/boundary.rs b/lib/codegen/src/legalizer/boundary.rs
index 640c3a82b0..6f3143028b 100644
--- a/lib/codegen/src/legalizer/boundary.rs
+++ b/lib/codegen/src/legalizer/boundary.rs
@@ -270,7 +270,7 @@ where
     // Reconstruct how `ty` was legalized into the `arg_type` argument.
     let conversion = legalize_abi_value(ty, &arg_type);
 
-    dbg!("convert_from_abi({}): {:?}", ty, conversion);
+    debug!("convert_from_abi({}): {:?}", ty, conversion);
 
     // The conversion describes value to ABI argument. We implement the reverse conversion here.
     match conversion {
@@ -279,7 +279,7 @@ where
             let abi_ty = ty.half_width().expect("Invalid type for conversion");
             let lo = convert_from_abi(pos, abi_ty, None, get_arg);
             let hi = convert_from_abi(pos, abi_ty, None, get_arg);
-            dbg!(
+            debug!(
                 "intsplit {}: {}, {}: {}",
                 lo,
                 pos.func.dfg.value_type(lo),
@@ -586,7 +586,7 @@ pub fn handle_return_abi(inst: Inst, func: &mut Function, cfg: &ControlFlowGraph
     // the legalized signature. These values should simply be propagated from the entry block
     // arguments.
     if special_args > 0 {
-        dbg!(
+        debug!(
             "Adding {} special-purpose arguments to {}",
             special_args,
             pos.func.dfg.display_inst(inst, None)
diff --git a/lib/codegen/src/legalizer/call.rs b/lib/codegen/src/legalizer/call.rs
index 365ba3988d..21753d19db 100644
--- a/lib/codegen/src/legalizer/call.rs
+++ b/lib/codegen/src/legalizer/call.rs
@@ -8,7 +8,8 @@ use flowgraph::ControlFlowGraph;
 use ir::{self, InstBuilder};
 use isa::TargetIsa;
 
-/// Expand a `call` instruction.
+/// Expand a `call` instruction. This lowers it to a `call_indirect`, which
+/// is only done if the ABI doesn't support direct calls.
 pub fn expand_call(
     inst: ir::Inst,
     func: &mut ir::Function,
diff --git a/lib/codegen/src/lib.rs b/lib/codegen/src/lib.rs
index 23959e67ac..10e8a7a461 100644
--- a/lib/codegen/src/lib.rs
+++ b/lib/codegen/src/lib.rs
@@ -2,7 +2,7 @@
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
-#![cfg_attr(feature = "std", warn(unstable_features))]
+#![cfg_attr(feature = "std", deny(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature="cargo-clippy", allow(
 // This requires Rust 1.27 or later.
@@ -49,6 +49,9 @@ extern crate failure_derive;
 #[cfg_attr(test, macro_use)]
 extern crate target_lexicon;
 
+#[macro_use]
+extern crate log;
+
 pub use context::Context;
 pub use legalizer::legalize_function;
 pub use verifier::verify_function;
@@ -59,15 +62,12 @@ pub const VERSION: &str = env!("CARGO_PKG_VERSION");
 
 #[macro_use]
 pub extern crate cranelift_entity as entity;
-
 pub extern crate cranelift_bforest as bforest;
 
-#[macro_use]
-pub mod dbg;
-
 pub mod binemit;
 pub mod cfg_printer;
 pub mod cursor;
+pub mod dbg;
 pub mod dominator_tree;
 pub mod flowgraph;
 pub mod ir;
diff --git a/lib/codegen/src/regalloc/coalescing.rs b/lib/codegen/src/regalloc/coalescing.rs
index b9415524a0..c7db253767 100644
--- a/lib/codegen/src/regalloc/coalescing.rs
+++ b/lib/codegen/src/regalloc/coalescing.rs
@@ -6,7 +6,6 @@
 //! parameter will belong to the same virtual register as the EBB parameter value itself.
 
 use cursor::{Cursor, EncCursor};
-#[cfg(feature = "std")]
 use dbg::DisplayList;
 use dominator_tree::{DominatorTree, DominatorTreePreorder};
 use flowgraph::{BasicBlock, ControlFlowGraph};
@@ -116,7 +115,7 @@ impl Coalescing {
         virtregs: &mut VirtRegs,
     ) {
         let _tt = timing::ra_cssa();
-        dbg!("Coalescing for:\n{}", func.display(isa));
+        debug!("Coalescing for:\n{}", func.display(isa));
         self.preorder.compute(domtree, &func.layout);
         let mut context = Context {
             isa,
@@ -185,7 +184,7 @@ impl<'a> Context<'a> {
                 continue;
             }
 
-            dbg!(
+            debug!(
                 " - checking {} params at back-edge {}: {}",
                 num_params,
                 pred_ebb,
@@ -229,7 +228,7 @@ impl<'a> Context<'a> {
                 if Some(def_ebb) == self.func.layout.entry_block()
                     && self.func.signature.params[def_num].location.is_stack()
                 {
-                    dbg!("-> isolating function stack parameter {}", arg);
+                    debug!("-> isolating function stack parameter {}", arg);
                     let new_arg = self.isolate_arg(pred_ebb, pred_inst, argnum, arg);
                     self.virtregs.union(param, new_arg);
                     continue;
@@ -296,7 +295,7 @@ impl<'a> Context<'a> {
         let inst = pos.built_inst();
         self.liveness.move_def_locally(param, inst);
 
-        dbg!(
+        debug!(
             "-> inserted {}, following {}({}: {})",
             pos.display_inst(inst),
             ebb,
@@ -365,7 +364,7 @@ impl<'a> Context<'a> {
 
         pos.func.dfg.inst_variable_args_mut(pred_inst)[argnum] = copy;
 
-        dbg!(
+        debug!(
             "-> inserted {}, before {}: {}",
             pos.display_inst(inst),
             pred_ebb,
@@ -381,7 +380,7 @@ impl<'a> Context<'a> {
     /// closure of the relation formed by EBB parameter-argument pairs found by `union_find_ebb()`.
     fn finish_union_find(&mut self) {
         self.virtregs.finish_union_find(None);
-        dbg!("After union-find phase:{}", self.virtregs);
+        debug!("After union-find phase:{}", self.virtregs);
     }
 }
 
@@ -412,7 +411,7 @@ impl<'a> Context<'a> {
     fn check_vreg(&mut self, vreg: VirtReg) -> bool {
         // Order the values according to the dominator pre-order of their definition.
         let values = self.virtregs.sort_values(vreg, self.func, self.preorder);
-        dbg!("Checking {} = {}", vreg, DisplayList(values));
+        debug!("Checking {} = {}", vreg, DisplayList(values));
 
         // Now push the values in order to the dominator forest.
         // This gives us the closest dominating value def for each of the values.
@@ -434,7 +433,7 @@ impl<'a> Context<'a> {
             let ctx = self.liveness.context(&self.func.layout);
             if self.liveness[parent.value].overlaps_def(node.def, node.ebb, ctx) {
                 // The two values are interfering, so they can't be in the same virtual register.
-                dbg!("-> interference: {} overlaps def of {}", parent, value);
+                debug!("-> interference: {} overlaps def of {}", parent, value);
                 return false;
             }
         }
@@ -458,7 +457,7 @@ impl<'a> Context<'a> {
             self.cfg,
             self.preorder,
         );
-        dbg!(
+        debug!(
             "Synthesizing {} from {} branches and params {}",
             vreg,
             self.vcopies.branches.len(),
@@ -546,7 +545,7 @@ impl<'a> Context<'a> {
         }
 
         let _vreg = self.virtregs.unify(self.values);
-        dbg!("-> merged into {} = {}", _vreg, DisplayList(self.values));
+        debug!("-> merged into {} = {}", _vreg, DisplayList(self.values));
         true
     }
 
@@ -568,7 +567,7 @@ impl<'a> Context<'a> {
         // registers and the filtered virtual copies.
         let v0 = self.virtregs.congruence_class(&param);
         let v1 = self.virtregs.congruence_class(&arg);
-        dbg!(
+        debug!(
             " - set 0: {}\n - set 1: {}",
             DisplayList(v0),
             DisplayList(v1)
@@ -621,7 +620,7 @@ impl<'a> Context<'a> {
                 if node.set_id != parent.set_id
                     && self.liveness[parent.value].reaches_use(inst, node.ebb, ctx)
                 {
-                    dbg!(
+                    debug!(
                         " - interference: {} overlaps vcopy at {}:{}",
                         parent,
                         node.ebb,
@@ -646,7 +645,7 @@ impl<'a> Context<'a> {
                 && self.liveness[parent.value].overlaps_def(node.def, node.ebb, ctx)
             {
                 // The two values are interfering.
-                dbg!(" - interference: {} overlaps def of {}", parent, node.value);
+                debug!(" - interference: {} overlaps def of {}", parent, node.value);
                 return false;
             }
         }
diff --git a/lib/codegen/src/regalloc/coloring.rs b/lib/codegen/src/regalloc/coloring.rs
index b824246d5b..56691eecf3 100644
--- a/lib/codegen/src/regalloc/coloring.rs
+++ b/lib/codegen/src/regalloc/coloring.rs
@@ -124,7 +124,7 @@ impl Coloring {
         tracker: &mut LiveValueTracker,
     ) {
         let _tt = timing::ra_coloring();
-        dbg!("Coloring for:\n{}", func.display(isa));
+        debug!("Coloring for:\n{}", func.display(isa));
         let mut ctx = Context {
             usable_regs: isa.allocatable_registers(func),
             cur: EncCursor::new(func, isa),
@@ -156,7 +156,7 @@ impl<'a> Context<'a> {
 
     /// Visit `ebb`, assuming that the immediate dominator has already been visited.
     fn visit_ebb(&mut self, ebb: Ebb, tracker: &mut LiveValueTracker) {
-        dbg!("Coloring {}:", ebb);
+        debug!("Coloring {}:", ebb);
         let mut regs = self.visit_ebb_header(ebb, tracker);
         tracker.drop_dead_params();
         self.divert.clear();
@@ -216,7 +216,7 @@ impl<'a> Context<'a> {
         let mut regs = AvailableRegs::new(&self.usable_regs);
 
         for lv in live.iter().filter(|lv| !lv.is_dead) {
-            dbg!(
+            debug!(
                 "Live-in: {}:{} in {}",
                 lv.value,
                 lv.affinity.display(&self.reginfo),
@@ -294,7 +294,7 @@ impl<'a> Context<'a> {
         tracker: &mut LiveValueTracker,
         regs: &mut AvailableRegs,
     ) -> bool {
-        dbg!(
+        debug!(
             "Coloring {}\n    from {}",
             self.cur.display_inst(inst),
             regs.input.display(&self.reginfo),
@@ -362,7 +362,7 @@ impl<'a> Context<'a> {
             if let Affinity::Reg(rci) = lv.affinity {
                 let rc = self.reginfo.rc(rci);
                 let reg = self.divert.reg(lv.value, &self.cur.func.locations);
-                dbg!(
+                debug!(
                     "    kill {} in {} ({} {})",
                     lv.value,
                     self.reginfo.display_regunit(reg),
@@ -380,7 +380,7 @@ impl<'a> Context<'a> {
         }
 
         // This aligns with the "    from" line at the top of the function.
-        dbg!("    glob {}", regs.global.display(&self.reginfo));
+        debug!("    glob {}", regs.global.display(&self.reginfo));
 
         // This flag is set when the solver failed to find a solution for the global defines that
         // doesn't interfere with `regs.global`. We need to rewrite all of `inst`s global defines
@@ -419,7 +419,7 @@ impl<'a> Context<'a> {
         // Finally, we've fully programmed the constraint solver.
         // We expect a quick solution in most cases.
         let output_regs = self.solver.quick_solve(&regs.global).unwrap_or_else(|_| {
-            dbg!("quick_solve failed for {}", self.solver);
+            debug!("quick_solve failed for {}", self.solver);
             self.iterate_solution(throughs, &regs.global, &mut replace_global_defines)
         });
 
@@ -454,7 +454,7 @@ impl<'a> Context<'a> {
         regs.input = output_regs;
         for lv in defs {
             let loc = self.cur.func.locations[lv.value];
-            dbg!(
+            debug!(
                 "    color {} -> {}{}",
                 lv.value,
                 loc.display(&self.reginfo),
@@ -700,7 +700,7 @@ impl<'a> Context<'a> {
                 ConstraintKind::FixedReg(reg) | ConstraintKind::FixedTied(reg) => {
                     self.add_fixed_output(lv.value, op.regclass, reg, throughs);
                     if !lv.is_local && !global_regs.is_avail(op.regclass, reg) {
-                        dbg!(
+                        debug!(
                             "Fixed output {} in {}:{} is not available in global regs",
                             lv.value,
                             op.regclass,
@@ -736,7 +736,7 @@ impl<'a> Context<'a> {
                     let rc = self.reginfo.rc(rci);
                     self.add_fixed_output(lv.value, rc, reg, throughs);
                     if !lv.is_local && !global_regs.is_avail(rc, reg) {
-                        dbg!(
+                        debug!(
                             "ABI output {} in {}:{} is not available in global regs",
                             lv.value,
                             rc,
@@ -812,7 +812,7 @@ impl<'a> Context<'a> {
                         // We need to make sure that fixed output register is compatible with the
                         // global register set.
                         if !lv.is_local && !global_regs.is_avail(op.regclass, reg) {
-                            dbg!(
+                            debug!(
                                 "Tied output {} in {}:{} is not available in global regs",
                                 lv.value,
                                 op.regclass,
@@ -848,7 +848,7 @@ impl<'a> Context<'a> {
                     debug_assert!(added, "Ran out of registers in {}", rc);
                 }
                 Err(SolverError::Global(_value)) => {
-                    dbg!(
+                    debug!(
                         "Not enough global registers for {}, trying as local",
                         _value
                     );
@@ -863,7 +863,7 @@ impl<'a> Context<'a> {
 
     /// Try to add an `rc` variable to the solver from the `throughs` set.
     fn try_add_var(&mut self, rc: RegClass, throughs: &[LiveValue]) -> bool {
-        dbg!("Trying to add a {} reg from {} values", rc, throughs.len());
+        debug!("Trying to add a {} reg from {} values", rc, throughs.len());
 
         for lv in throughs {
             if let Affinity::Reg(rci) = lv.affinity {
@@ -995,7 +995,7 @@ impl<'a> Context<'a> {
     /// the constraints on the instruction operands.
     ///
     fn replace_global_defines(&mut self, inst: Inst, tracker: &mut LiveValueTracker) {
-        dbg!("Replacing global defs on {}", self.cur.display_inst(inst));
+        debug!("Replacing global defs on {}", self.cur.display_inst(inst));
 
         // We'll insert copies *after `inst`. Our caller will move the cursor back.
         self.cur.next_inst();
@@ -1042,14 +1042,14 @@ impl<'a> Context<'a> {
             lv.endpoint = copy;
             lv.is_local = true;
 
-            dbg!(
+            debug!(
                 "  + {} with {} in {}",
                 self.cur.display_inst(copy),
                 local,
                 loc.display(&self.reginfo)
             );
         }
-        dbg!("Done: {}", self.cur.display_inst(inst));
+        debug!("Done: {}", self.cur.display_inst(inst));
     }
 
     /// Process kills on a ghost instruction.
diff --git a/lib/codegen/src/regalloc/reload.rs b/lib/codegen/src/regalloc/reload.rs
index a712616af8..f07f35cea0 100644
--- a/lib/codegen/src/regalloc/reload.rs
+++ b/lib/codegen/src/regalloc/reload.rs
@@ -72,7 +72,7 @@ impl Reload {
         tracker: &mut LiveValueTracker,
     ) {
         let _tt = timing::ra_reload();
-        dbg!("Reload for:\n{}", func.display(isa));
+        debug!("Reload for:\n{}", func.display(isa));
         let mut ctx = Context {
             cur: EncCursor::new(func, isa),
             encinfo: isa.encoding_info(),
@@ -119,7 +119,7 @@ impl<'a> Context<'a> {
     }
 
     fn visit_ebb(&mut self, ebb: Ebb, tracker: &mut LiveValueTracker) {
-        dbg!("Reloading {}:", ebb);
+        debug!("Reloading {}:", ebb);
         self.visit_ebb_header(ebb, tracker);
         tracker.drop_dead_params();
 
diff --git a/lib/codegen/src/regalloc/solver.rs b/lib/codegen/src/regalloc/solver.rs
index 1140aa7536..283a461378 100644
--- a/lib/codegen/src/regalloc/solver.rs
+++ b/lib/codegen/src/regalloc/solver.rs
@@ -532,7 +532,7 @@ impl Solver {
     /// In either case, `to` will not be available for variables on the input side of the
     /// instruction.
     pub fn reassign_in(&mut self, value: Value, rc: RegClass, from: RegUnit, to: RegUnit) {
-        dbg!(
+        debug!(
             "reassign_in({}:{}, {} -> {})",
             value,
             rc,
@@ -545,7 +545,7 @@ impl Solver {
             // added as a variable previously. A fixed constraint beats a variable, so convert it.
             if let Some(idx) = self.vars.iter().position(|v| v.value == value) {
                 let v = self.vars.remove(idx);
-                dbg!("-> converting variable {} to a fixed constraint", v);
+                debug!("-> converting variable {} to a fixed constraint", v);
                 // The spiller is responsible for ensuring that all constraints on the uses of a
                 // value are compatible.
                 debug_assert!(
@@ -578,7 +578,7 @@ impl Solver {
     /// This function can only be used before calling `inputs_done()`. Afterwards, more input-side
     /// variables can be added by calling `add_killed_var()` and `add_through_var()`
     pub fn add_var(&mut self, value: Value, constraint: RegClass, from: RegUnit) {
-        dbg!(
+        debug!(
             "add_var({}:{}, from={})",
             value,
             constraint,
@@ -593,7 +593,7 @@ impl Solver {
     ///
     /// This function should be called after `inputs_done()` only. Use `add_var()` before.
     pub fn add_killed_var(&mut self, value: Value, constraint: RegClass, from: RegUnit) {
-        dbg!(
+        debug!(
             "add_killed_var({}:{}, from={})",
             value,
             constraint,
@@ -608,7 +608,7 @@ impl Solver {
     ///
     /// This function should be called after `inputs_done()` only. Use `add_var()` before.
     pub fn add_through_var(&mut self, value: Value, constraint: RegClass, from: RegUnit) {
-        dbg!(
+        debug!(
             "add_through_var({}:{}, from={})",
             value,
             constraint,
@@ -635,7 +635,7 @@ impl Solver {
             if let Some(v) = self.vars.iter_mut().find(|v| v.value == value) {
                 // We have an existing variable entry for `value`. Combine the constraints.
                 if let Some(rc) = v.constraint.intersect(constraint) {
-                    dbg!("-> combining constraint with {} yields {}", v, rc);
+                    debug!("-> combining constraint with {} yields {}", v, rc);
                     v.constraint = rc;
                     return;
                 } else {
@@ -647,7 +647,7 @@ impl Solver {
 
             // No variable, then it must be a fixed reassignment.
             if let Some(a) = self.assignments.get(value) {
-                dbg!("-> already fixed assignment {}", a);
+                debug!("-> already fixed assignment {}", a);
                 debug_assert!(
                     constraint.contains(a.to),
                     "Incompatible constraints for {}",
@@ -656,12 +656,12 @@ impl Solver {
                 return;
             }
 
-            dbg!("{}", self);
+            debug!("{}", self);
             panic!("Wrong from register for {}", value);
         }
 
         let new_var = Variable::new_live(value, constraint, from, live_through);
-        dbg!("-> new var: {}", new_var);
+        debug!("-> new var: {}", new_var);
 
         self.regs_in.free(constraint, from);
         if self.inputs_done && live_through {
@@ -777,7 +777,7 @@ impl Solver {
         if is_global {
             let mut new_var = Variable::new_live(value, rc, reg, true);
             new_var.is_global = true;
-            dbg!("add_tied_input: new tied-global value: {}", new_var);
+            debug!("add_tied_input: new tied-global value: {}", new_var);
             self.vars.push(new_var);
             self.regs_in.free(rc, reg);
         } else {
@@ -899,7 +899,7 @@ impl Solver {
             )
         });
 
-        dbg!("real_solve for {}", self);
+        debug!("real_solve for {}", self);
         self.find_solution(global_regs)
     }
 
@@ -982,7 +982,7 @@ impl Solver {
             .extend(self.assignments.values().filter_map(Move::with_assignment));
 
         if !(self.moves.is_empty()) {
-            dbg!("collect_moves: {}", DisplayList(&self.moves));
+            debug!("collect_moves: {}", DisplayList(&self.moves));
         }
     }
 
@@ -1024,7 +1024,7 @@ impl Solver {
                 if let Some((rc, reg)) = m.from_reg() {
                     avail.free(rc, reg);
                 }
-                dbg!("move #{}: {}", i, m);
+                debug!("move #{}: {}", i, m);
                 i += 1;
                 continue;
             }
@@ -1058,7 +1058,7 @@ impl Solver {
             let m = self.moves[i].clone();
             let toprc = m.rc().toprc();
             if let Some(reg) = avail.iter(toprc).next() {
-                dbg!(
+                debug!(
                     "breaking cycle at {} with available {} register {}",
                     m,
                     toprc,
@@ -1089,7 +1089,7 @@ impl Solver {
             // a last resort.
             let slot = num_spill_slots;
             num_spill_slots += 1;
-            dbg!("breaking cycle at {} with slot {}", m, slot);
+            debug!("breaking cycle at {} with slot {}", m, slot);
             let old_to_reg = self.moves[i].change_to_spill(slot);
             self.fills.push(Move::Fill {
                 value: m.value(),
diff --git a/lib/codegen/src/regalloc/spilling.rs b/lib/codegen/src/regalloc/spilling.rs
index 457b07c663..d1244c0774 100644
--- a/lib/codegen/src/regalloc/spilling.rs
+++ b/lib/codegen/src/regalloc/spilling.rs
@@ -90,7 +90,7 @@ impl Spilling {
         tracker: &mut LiveValueTracker,
     ) {
         let _tt = timing::ra_spilling();
-        dbg!("Spilling for:\n{}", func.display(isa));
+        debug!("Spilling for:\n{}", func.display(isa));
         let reginfo = isa.register_info();
         let usable_regs = isa.allocatable_registers(func);
         let mut ctx = Context {
@@ -118,7 +118,7 @@ impl<'a> Context<'a> {
     }
 
     fn visit_ebb(&mut self, ebb: Ebb, tracker: &mut LiveValueTracker) {
-        dbg!("Spilling {}:", ebb);
+        debug!("Spilling {}:", ebb);
         self.cur.goto_top(ebb);
         self.visit_ebb_header(ebb, tracker);
         tracker.drop_dead_params();
@@ -198,14 +198,12 @@ impl<'a> Context<'a> {
             if let Affinity::Reg(rci) = lv.affinity {
                 let rc = self.reginfo.rc(rci);
                 'try_take: while let Err(mask) = self.pressure.take_transient(rc) {
-                    dbg!("Need {} reg for EBB param {}", rc, lv.value);
+                    debug!("Need {} reg for EBB param {}", rc, lv.value);
                     match self.spill_candidate(mask, liveins) {
                         Some(cand) => {
-                            dbg!(
+                            debug!(
                                 "Spilling live-in {} to make room for {} EBB param {}",
-                                cand,
-                                rc,
-                                lv.value
+                                cand, rc, lv.value
                             );
                             self.spill_reg(cand);
                         }
@@ -213,7 +211,7 @@ impl<'a> Context<'a> {
                             // We can't spill any of the live-in registers, so we have to spill an
                             // EBB argument. Since the current spill metric would consider all the
                             // EBB arguments equal, just spill the present register.
-                            dbg!("Spilling {} EBB argument {}", rc, lv.value);
+                            debug!("Spilling {} EBB argument {}", rc, lv.value);
 
                             // Since `spill_reg` will free a register, add the current one here.
                             self.pressure.take(rc);
@@ -237,7 +235,7 @@ impl<'a> Context<'a> {
         constraints: &RecipeConstraints,
         tracker: &mut LiveValueTracker,
     ) {
-        dbg!("Inst {}, {}", self.cur.display_inst(inst), self.pressure);
+        debug!("Inst {}, {}", self.cur.display_inst(inst), self.pressure);
         debug_assert_eq!(self.cur.current_inst(), Some(inst));
         debug_assert_eq!(self.cur.current_ebb(), Some(ebb));
 
@@ -279,7 +277,7 @@ impl<'a> Context<'a> {
             if op.kind != ConstraintKind::Stack {
                 // Add register def to pressure, spill if needed.
                 while let Err(mask) = self.pressure.take_transient(op.regclass) {
-                    dbg!("Need {} reg from {} throughs", op.regclass, throughs.len());
+                    debug!("Need {} reg from {} throughs", op.regclass, throughs.len());
                     match self.spill_candidate(mask, throughs) {
                         Some(cand) => self.spill_reg(cand),
                         None => panic!(
@@ -333,7 +331,7 @@ impl<'a> Context<'a> {
 
             // Only collect the interesting register uses.
             if reguse.fixed || reguse.tied || reguse.spilled {
-                dbg!("  reguse: {}", reguse);
+                debug!("  reguse: {}", reguse);
                 self.reg_uses.push(reguse);
             }
         }
@@ -406,7 +404,7 @@ impl<'a> Context<'a> {
             if need_copy || ru.spilled {
                 let rc = self.reginfo.rc(ru.rci);
                 while let Err(mask) = self.pressure.take_transient(rc) {
-                    dbg!("Copy of {} reg causes spill", rc);
+                    debug!("Copy of {} reg causes spill", rc);
                     // Spill a live register that is *not* used by the current instruction.
                     // Spilling a use wouldn't help.
                     //
@@ -489,7 +487,7 @@ impl<'a> Context<'a> {
             let rc = self.reginfo.rc(rci);
             self.pressure.free(rc);
             self.spills.push(value);
-            dbg!("Spilled {}:{} -> {}", value, rc, self.pressure);
+            debug!("Spilled {}:{} -> {}", value, rc, self.pressure);
         } else {
             panic!("Cannot spill {} that was already on the stack", value);
         }
diff --git a/lib/codegen/src/timing.rs b/lib/codegen/src/timing.rs
index 707efbf3f8..b340796578 100644
--- a/lib/codegen/src/timing.rs
+++ b/lib/codegen/src/timing.rs
@@ -179,7 +179,7 @@ mod details {
     /// This function is called by the publicly exposed pass functions.
     pub(super) fn start_pass(pass: Pass) -> TimingToken {
         let prev = CURRENT_PASS.with(|p| p.replace(pass));
-        dbg!("timing: Starting {}, (during {})", pass, prev);
+        debug!("timing: Starting {}, (during {})", pass, prev);
         TimingToken {
             start: Instant::now(),
             pass,
@@ -191,7 +191,7 @@ mod details {
     impl Drop for TimingToken {
         fn drop(&mut self) {
             let duration = self.start.elapsed();
-            dbg!("timing: Ending {}", self.pass);
+            debug!("timing: Ending {}", self.pass);
             let old_cur = CURRENT_PASS.with(|p| p.replace(self.prev));
             debug_assert_eq!(self.pass, old_cur, "Timing tokens dropped out of order");
             PASS_TIME.with(|rc| {
diff --git a/lib/codegen/src/unreachable_code.rs b/lib/codegen/src/unreachable_code.rs
index 36ec7e673c..74cc6f4ea4 100644
--- a/lib/codegen/src/unreachable_code.rs
+++ b/lib/codegen/src/unreachable_code.rs
@@ -24,14 +24,14 @@ pub fn eliminate_unreachable_code(
             continue;
         }
 
-        dbg!("Eliminating unreachable {}", ebb);
+        debug!("Eliminating unreachable {}", ebb);
         // Move the cursor out of the way and make sure the next lop iteration goes to the right
         // EBB.
         pos.prev_ebb();
 
         // Remove all instructions from `ebb`.
         while let Some(inst) = pos.func.layout.first_inst(ebb) {
-            dbg!(" - {}", pos.func.dfg.display_inst(inst, None));
+            debug!(" - {}", pos.func.dfg.display_inst(inst, None));
             pos.func.layout.remove_inst(inst);
         }
 
diff --git a/lib/entity/src/lib.rs b/lib/entity/src/lib.rs
index cb1c248db1..dd666c6ac7 100644
--- a/lib/entity/src/lib.rs
+++ b/lib/entity/src/lib.rs
@@ -31,7 +31,7 @@
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
-#![cfg_attr(feature = "std", warn(unstable_features))]
+#![cfg_attr(feature = "std", deny(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
diff --git a/lib/faerie/src/lib.rs b/lib/faerie/src/lib.rs
index 14397afbd0..078488a953 100644
--- a/lib/faerie/src/lib.rs
+++ b/lib/faerie/src/lib.rs
@@ -2,8 +2,8 @@
 //!
 //! Users of this module should not have to depend on faerie directly.
 
-#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
diff --git a/lib/filetests/Cargo.toml b/lib/filetests/Cargo.toml
index ac66ca660c..487675971f 100644
--- a/lib/filetests/Cargo.toml
+++ b/lib/filetests/Cargo.toml
@@ -11,5 +11,7 @@ publish = false
 [dependencies]
 cranelift-codegen = { path = "../codegen", version = "0.18.1" }
 cranelift-reader = { path = "../reader", version = "0.18.1" }
+file-per-thread-logger = "0.1.1"
 filecheck = "0.3.0"
 num_cpus = "1.8.0"
+log = "0.4.3"
diff --git a/lib/filetests/src/concurrent.rs b/lib/filetests/src/concurrent.rs
index f874e952f2..104967d757 100644
--- a/lib/filetests/src/concurrent.rs
+++ b/lib/filetests/src/concurrent.rs
@@ -3,7 +3,9 @@
 //! This module provides the `ConcurrentRunner` struct which uses a pool of threads to run tests
 //! concurrently.
 
+use cranelift_codegen::dbg::LOG_FILENAME_PREFIX;
 use cranelift_codegen::timing;
+use file_per_thread_logger;
 use num_cpus;
 use std::panic::catch_unwind;
 use std::path::{Path, PathBuf};
@@ -100,6 +102,7 @@ fn heartbeat_thread(replies: Sender<Reply>) -> thread::JoinHandle<()> {
     thread::Builder::new()
         .name("heartbeat".to_string())
         .spawn(move || {
+            file_per_thread_logger::initialize(LOG_FILENAME_PREFIX);
             while replies.send(Reply::Tick).is_ok() {
                 thread::sleep(Duration::from_secs(1));
             }
@@ -116,6 +119,7 @@ fn worker_thread(
     thread::Builder::new()
         .name(format!("worker #{}", thread_num))
         .spawn(move || {
+            file_per_thread_logger::initialize(LOG_FILENAME_PREFIX);
             loop {
                 // Lock the mutex only long enough to extract a request.
                 let Request(jobid, path) = match requests.lock().unwrap().recv() {
@@ -140,7 +144,7 @@ fn worker_thread(
                 });
 
                 if let Err(ref msg) = result {
-                    dbg!("FAIL: {}", msg);
+                    error!("FAIL: {}", msg);
                 }
 
                 replies.send(Reply::Done { jobid, result }).unwrap();
diff --git a/lib/filetests/src/lib.rs b/lib/filetests/src/lib.rs
index cdeac02df9..90565b5cb4 100644
--- a/lib/filetests/src/lib.rs
+++ b/lib/filetests/src/lib.rs
@@ -3,8 +3,8 @@
 //! This crate contains the main test driver as well as implementations of the
 //! available filetest commands.
 
-#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
 #![cfg_attr(feature = "cargo-clippy",
     allow(
           type_complexity,
@@ -18,11 +18,13 @@
     )
 )]
 
-#[macro_use(dbg)]
 extern crate cranelift_codegen;
 extern crate cranelift_reader;
+extern crate file_per_thread_logger;
 extern crate filecheck;
 extern crate num_cpus;
+#[macro_use]
+extern crate log;
 
 use cranelift_reader::TestCommand;
 use runner::TestRunner;
diff --git a/lib/filetests/src/runone.rs b/lib/filetests/src/runone.rs
index 8a28755003..b98f5f8cfb 100644
--- a/lib/filetests/src/runone.rs
+++ b/lib/filetests/src/runone.rs
@@ -29,7 +29,7 @@ fn read_to_string<P: AsRef<Path>>(path: P) -> io::Result<String> {
 /// If running this test causes a panic, it will propagate as normal.
 pub fn run(path: &Path) -> TestResult {
     let _tt = timing::process_file();
-    dbg!("---\nFile: {}", path.to_string_lossy());
+    info!("---\nFile: {}", path.to_string_lossy());
     let started = time::Instant::now();
     let buffer = read_to_string(path).map_err(|e| e.to_string())?;
     let testfile = parse_test(&buffer).map_err(|e| e.to_string())?;
@@ -122,7 +122,7 @@ fn run_one_test<'a>(
 ) -> SubtestResult<()> {
     let (test, flags, isa) = tuple;
     let name = format!("{}({})", test.name(), func.name);
-    dbg!("Test: {} {}", name, isa.map_or("-", TargetIsa::name));
+    info!("Test: {} {}", name, isa.map_or("-", TargetIsa::name));
 
     context.flags = flags;
     context.isa = isa;
diff --git a/lib/filetests/src/test_compile.rs b/lib/filetests/src/test_compile.rs
index 0e79bab429..1fb6d0e4c0 100644
--- a/lib/filetests/src/test_compile.rs
+++ b/lib/filetests/src/test_compile.rs
@@ -41,7 +41,7 @@ impl SubTest for TestCompile {
             .compile(isa)
             .map_err(|e| pretty_error(&comp_ctx.func, context.isa, e))?;
 
-        dbg!(
+        info!(
             "Generated {} bytes of code:\n{}",
             code_size,
             comp_ctx.func.display(isa)
diff --git a/lib/frontend/src/lib.rs b/lib/frontend/src/lib.rs
index be19ebc2be..7fca5ae122 100644
--- a/lib/frontend/src/lib.rs
+++ b/lib/frontend/src/lib.rs
@@ -129,7 +129,7 @@
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
-#![cfg_attr(feature = "std", warn(unstable_features))]
+#![cfg_attr(feature = "std", deny(unstable_features))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default))]
 #![cfg_attr(
     feature = "cargo-clippy",
diff --git a/lib/module/Cargo.toml b/lib/module/Cargo.toml
index d8a2bf9ccb..a137bcebd0 100644
--- a/lib/module/Cargo.toml
+++ b/lib/module/Cargo.toml
@@ -13,6 +13,7 @@ cranelift-codegen = { path = "../codegen", version = "0.18.1", default-features
 cranelift-entity = { path = "../entity", version = "0.18.1", default-features = false }
 hashmap_core = { version = "0.1.9", optional = true }
 failure = "0.1.1"
+log = { version = "0.4.3", default-features = false, features = ["release_max_level_warn"] }
 
 [features]
 default = ["std"]
diff --git a/lib/module/src/lib.rs b/lib/module/src/lib.rs
index 32b774824a..45e3bd683e 100644
--- a/lib/module/src/lib.rs
+++ b/lib/module/src/lib.rs
@@ -2,7 +2,7 @@
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
-#![cfg_attr(feature = "std", warn(unstable_features))]
+#![cfg_attr(feature = "std", deny(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
@@ -20,12 +20,13 @@
 #[cfg_attr(test, macro_use)]
 extern crate alloc;
 
-#[macro_use]
 extern crate cranelift_codegen;
 #[macro_use]
 extern crate cranelift_entity;
 #[macro_use]
 extern crate failure;
+#[macro_use]
+extern crate log;
 
 mod backend;
 mod data_context;
diff --git a/lib/module/src/module.rs b/lib/module/src/module.rs
index d5c616db66..133ad5af9f 100644
--- a/lib/module/src/module.rs
+++ b/lib/module/src/module.rs
@@ -467,7 +467,7 @@ where
     pub fn define_function(&mut self, func: FuncId, ctx: &mut Context) -> ModuleResult<()> {
         let compiled = {
             let code_size = ctx.compile(self.backend.isa()).map_err(|e| {
-                dbg!(
+                info!(
                     "defining function {}: {}",
                     func,
                     ctx.func.display(self.backend.isa())
diff --git a/lib/native/src/lib.rs b/lib/native/src/lib.rs
index 5439d743cc..3d4e4bac2c 100644
--- a/lib/native/src/lib.rs
+++ b/lib/native/src/lib.rs
@@ -1,8 +1,8 @@
 //! Performs autodetection of the host for the purposes of running
 //! Cranelift to generate code to run on the same machine.
 
-#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
diff --git a/lib/reader/src/lib.rs b/lib/reader/src/lib.rs
index edac07622f..2873a644ac 100644
--- a/lib/reader/src/lib.rs
+++ b/lib/reader/src/lib.rs
@@ -3,8 +3,8 @@
 //! The `cranelift_reader` library supports reading .clif files. This functionality is needed for
 //! testing Cranelift, but is not essential for a JIT compiler.
 
-#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
diff --git a/lib/serde/src/clif-json.rs b/lib/serde/src/clif-json.rs
index 659bc818a8..5fc8a1d469 100644
--- a/lib/serde/src/clif-json.rs
+++ b/lib/serde/src/clif-json.rs
@@ -1,7 +1,9 @@
 //! Utility for `cranelift_serde`.
 
-#![deny(trivial_numeric_casts)]
-#![warn(unused_import_braces, unstable_features, unused_extern_crates)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
+#![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
+#![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
     feature = "cargo-clippy",
     warn(
diff --git a/lib/simplejit/src/backend.rs b/lib/simplejit/src/backend.rs
index 0ae65a8a99..c867c0d825 100644
--- a/lib/simplejit/src/backend.rs
+++ b/lib/simplejit/src/backend.rs
@@ -10,6 +10,7 @@ use cranelift_module::{
 use cranelift_native;
 use libc;
 use memory::Memory;
+use std::collections::HashMap;
 use std::ffi::CString;
 use std::ptr;
 use target_lexicon::PointerWidth;
@@ -19,6 +20,7 @@ use winapi;
 /// A builder for `SimpleJITBackend`.
 pub struct SimpleJITBuilder {
     isa: Box<TargetIsa>,
+    symbols: HashMap<String, *const u8>,
 }
 
 impl SimpleJITBuilder {
@@ -40,7 +42,44 @@ impl SimpleJITBuilder {
     /// instead.
     pub fn with_isa(isa: Box<TargetIsa>) -> Self {
         debug_assert!(!isa.flags().is_pic(), "SimpleJIT requires non-PIC code");
-        Self { isa }
+        let symbols = HashMap::new();
+        Self { isa, symbols }
+    }
+
+    /// Define a symbol in the internal symbol table.
+    ///
+    /// The JIT will use the symbol table to resolve names that are declared,
+    /// but not defined, in the module being compiled.  A common example is
+    /// external functions.  With this method, functions and data can be exposed
+    /// to the code being compiled which are defined by the host.
+    ///
+    /// If a symbol is defined more than once, the most recent definition will
+    /// be retained.
+    ///
+    /// If the JIT fails to find a symbol in its internal table, it will fall
+    /// back to a platform-specific search (this typically involves searching
+    /// the current process for public symbols, followed by searching the
+    /// platform's C runtime).
+    pub fn symbol<'a, K>(&'a mut self, name: K, ptr: *const u8) -> &'a mut Self
+    where
+        K: Into<String>,
+    {
+        self.symbols.insert(name.into(), ptr);
+        self
+    }
+
+    /// Define multiple symbols in the internal symbol table.
+    ///
+    /// Using this is equivalent to calling `symbol` on each element.
+    pub fn symbols<'a, It, K>(&'a mut self, symbols: It) -> &'a mut Self
+    where
+        It: IntoIterator<Item = (K, *const u8)>,
+        K: Into<String>,
+    {
+        for (name, ptr) in symbols {
+            self.symbols.insert(name.into(), ptr);
+        }
+        self
     }
 }
 
@@ -48,6 +87,7 @@ impl SimpleJITBuilder {
 /// directly called and accessed.
 pub struct SimpleJITBackend {
     isa: Box<TargetIsa>,
+    symbols: HashMap<String, *const u8>,
     code_memory: Memory,
     readonly_memory: Memory,
     writable_memory: Memory,
@@ -73,6 +113,15 @@ pub struct SimpleJITCompiledData {
     relocs: Vec<RelocRecord>,
 }
 
+impl SimpleJITBackend {
+    fn lookup_symbol(&self, name: &str) -> *const u8 {
+        match self.symbols.get(name) {
+            Some(&ptr) => ptr,
+            None => lookup_with_dlsym(name),
+        }
+    }
+}
+
 impl<'simple_jit_backend> Backend for SimpleJITBackend {
     type Builder = SimpleJITBuilder;
 
@@ -96,6 +145,7 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
     fn new(builder: SimpleJITBuilder) -> Self {
         Self {
             isa: builder.isa,
+            symbols: builder.symbols,
             code_memory: Memory::new(),
             readonly_memory: Memory::new(),
             writable_memory: Memory::new(),
@@ -249,13 +299,13 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
                 let (def, name_str, _signature) = namespace.get_function_definition(&name);
                 match def {
                     Some(compiled) => compiled.code,
-                    None => lookup_with_dlsym(name_str),
+                    None => self.lookup_symbol(name_str),
                 }
             } else {
                 let (def, name_str, _writable) = namespace.get_data_definition(&name);
                 match def {
                     Some(compiled) => compiled.storage,
-                    None => lookup_with_dlsym(name_str),
+                    None => self.lookup_symbol(name_str),
                 }
             };
             // TODO: Handle overflow.
@@ -314,13 +364,13 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
                         let (def, name_str, _signature) = namespace.get_function_definition(&name);
                         match def {
                             Some(compiled) => compiled.code,
-                            None => lookup_with_dlsym(name_str),
+                            None => self.lookup_symbol(name_str),
                         }
                     } else {
                         let (def, name_str, _writable) = namespace.get_data_definition(&name);
                         match def {
                             Some(compiled) => compiled.storage,
-                            None => lookup_with_dlsym(name_str),
+                            None => self.lookup_symbol(name_str),
                         }
                     };
                     // TODO: Handle overflow.
diff --git a/lib/simplejit/src/lib.rs b/lib/simplejit/src/lib.rs
index 01dd0938f0..99297f33b0 100644
--- a/lib/simplejit/src/lib.rs
+++ b/lib/simplejit/src/lib.rs
@@ -1,7 +1,7 @@
 //! Top-level lib.rs for `cranelift_simplejit`.
 
-#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
diff --git a/lib/umbrella/src/lib.rs b/lib/umbrella/src/lib.rs
index ad241483e7..4faeabdd8a 100644
--- a/lib/umbrella/src/lib.rs
+++ b/lib/umbrella/src/lib.rs
@@ -1,7 +1,7 @@
 //! Cranelift umbrella crate, providing a convenient one-line dependency.
 
-#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates, unstable_features)]
+#![warn(unused_import_braces)]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
diff --git a/lib/wasm/Cargo.toml b/lib/wasm/Cargo.toml
index abc33510a1..4e40b5022d 100644
--- a/lib/wasm/Cargo.toml
+++ b/lib/wasm/Cargo.toml
@@ -17,6 +17,7 @@ hashmap_core = { version = "0.1.9", optional = true }
 failure = { version = "0.1.1", default-features = false, features = ["derive"] }
 failure_derive = { version = "0.1.1", default-features = false }
 target-lexicon = { version = "0.0.3", default-features = false }
+log = { version = "0.4.3", default-features = false, features = ["release_max_level_warn"] }
 
 [dev-dependencies]
 wabt = "0.4"
diff --git a/lib/wasm/src/environ/dummy.rs b/lib/wasm/src/environ/dummy.rs
index 21ee1d08a3..5077612529 100644
--- a/lib/wasm/src/environ/dummy.rs
+++ b/lib/wasm/src/environ/dummy.rs
@@ -1,4 +1,5 @@
-//! "Dummy" environment for testing wasm translation.
+//! "Dummy" implementations of `ModuleEnvironment` and `FuncEnvironment` for testing
+//! wasm translation.
 
 use cranelift_codegen::cursor::FuncCursor;
 use cranelift_codegen::ir::immediates::Imm64;
diff --git a/lib/wasm/src/func_translator.rs b/lib/wasm/src/func_translator.rs
index 8f764c102a..f9ab937ad4 100644
--- a/lib/wasm/src/func_translator.rs
+++ b/lib/wasm/src/func_translator.rs
@@ -67,7 +67,7 @@ impl FuncTranslator {
         environ: &mut FE,
     ) -> WasmResult<()> {
         let _tt = timing::wasm_translate_function();
-        dbg!(
+        info!(
             "translate({} bytes, {}{})",
             reader.bytes_remaining(),
             func.name,
@@ -265,7 +265,7 @@ mod tests {
         trans
             .translate(&BODY, &mut ctx.func, &mut runtime.func_env())
             .unwrap();
-        dbg!("{}", ctx.func.display(None));
+        debug!("{}", ctx.func.display(None));
         ctx.verify(runtime.func_env().flags()).unwrap();
     }
 
@@ -296,7 +296,7 @@ mod tests {
         trans
             .translate(&BODY, &mut ctx.func, &mut runtime.func_env())
             .unwrap();
-        dbg!("{}", ctx.func.display(None));
+        debug!("{}", ctx.func.display(None));
         ctx.verify(runtime.func_env().flags()).unwrap();
     }
 
@@ -335,7 +335,7 @@ mod tests {
         trans
             .translate(&BODY, &mut ctx.func, &mut runtime.func_env())
             .unwrap();
-        dbg!("{}", ctx.func.display(None));
+        debug!("{}", ctx.func.display(None));
         ctx.verify(runtime.func_env().flags()).unwrap();
     }
 }
diff --git a/lib/wasm/src/lib.rs b/lib/wasm/src/lib.rs
index 7606ebc0b4..a559ae941d 100644
--- a/lib/wasm/src/lib.rs
+++ b/lib/wasm/src/lib.rs
@@ -11,7 +11,7 @@
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
-#![cfg_attr(feature = "std", warn(unstable_features))]
+#![cfg_attr(feature = "std", deny(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
@@ -24,7 +24,6 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(not(feature = "std"), feature(alloc))]
 
-#[macro_use(dbg)]
 extern crate cranelift_codegen;
 #[macro_use]
 extern crate cranelift_entity;
@@ -36,6 +35,9 @@ extern crate failure;
 #[macro_use]
 extern crate failure_derive;
 
+#[macro_use]
+extern crate log;
+
 mod code_translator;
 mod environ;
 mod func_translator;