Merge pull request #2758 from bnjbvr/revert-log

cranelift: Use a deferred display mechanism instead of `log_enabled!`

cranelift/codegen/src/lib.rs

@@ -97,6 +97,7 @@ mod inst_predicates;
 mod iterators;
 mod legalizer;
 mod licm;
+mod log;
 mod nan_canonicalization;
 mod partition_slice;
 mod postopt;

cranelift/codegen/src/log.rs

@@ -0,0 +1,39 @@
+//! This module implements deferred display helpers.
+//!
+//! These are particularly useful in logging contexts, where the maximum logging level filter might
+//! be enabled, but we don't want the arguments to be evaluated early:
+//!
+//! ```
+//! log::set_max_level(log::LevelFilter::max());
+//! fn expensive_calculation() -> String {
+//!   "a string that is very slow to generate".into()
+//! }
+//! log::debug!("{}", expensive_calculation());
+//! ```
+//!
+//! If the associated log implementation filters out log debug entries, the expensive calculation
+//! would have been spurious. In this case, we can wrap the expensive computation within an
+//! `DeferredDisplay`, so that the computation only happens when the actual `fmt` function is
+//! called.
+
+use core::fmt;
+
+pub(crate) struct DeferredDisplay<F>(F);
+
+impl<F: Fn() -> T, T: fmt::Display> DeferredDisplay<F> {
+    pub(crate) fn new(f: F) -> Self {
+        Self(f)
+    }
+}
+
+impl<F: Fn() -> T, T: fmt::Display> fmt::Display for DeferredDisplay<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0().fmt(f)
+    }
+}
+
+impl<F: Fn() -> T, T: fmt::Debug> fmt::Debug for DeferredDisplay<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0().fmt(f)
+    }
+}

cranelift/codegen/src/machinst/compile.rs

@@ -1,11 +1,12 @@
 //! Compilation backend pipeline: optimized IR to VCode / binemit.
 
 use crate::ir::Function;
+use crate::log::DeferredDisplay;
 use crate::machinst::*;
 use crate::settings;
 use crate::timing;
 
-use log::{debug, log_enabled, Level};
+use log::debug;
 use regalloc::{allocate_registers_with_opts, Algorithm, Options, PrettyPrint};
 
 /// Compile the given function down to VCode with allocated registers, ready
@@ -29,15 +30,12 @@ where
         lower.lower(b)?
     };
 
-    // Creating the vcode string representation may be costly for large functions, so don't do it
-    // if the Debug level hasn't been statically (through features) or dynamically (through
-    // RUST_LOG) enabled.
-    if log_enabled!(Level::Debug) {
-        debug!(
-            "vcode from lowering: \n{}",
-            vcode.show_rru(Some(b.reg_universe()))
-        );
-    }
+    // Creating the vcode string representation may be costly for large functions, so defer its
+    // rendering.
+    debug!(
+        "vcode from lowering: \n{}",
+        DeferredDisplay::new(|| vcode.show_rru(Some(b.reg_universe())))
+    );
 
     // Perform register allocation.
     let (run_checker, algorithm) = match vcode.flags().regalloc() {
@@ -106,12 +104,10 @@ where
         vcode.replace_insns_from_regalloc(result);
     }
 
-    if log_enabled!(Level::Debug) {
-        debug!(
-            "vcode after regalloc: final version:\n{}",
-            vcode.show_rru(Some(b.reg_universe()))
-        );
-    }
+    debug!(
+        "vcode after regalloc: final version:\n{}",
+        DeferredDisplay::new(|| vcode.show_rru(Some(b.reg_universe())))
+    );
 
     Ok(vcode)
 }