Wasmtime: allow using the experimental Cranelift x64 backend in cli;

This introduces two changes:

- first, a Cargo feature is added to make it possible to use the
Cranelift x64 backend directly from wasmtime's CLI.
- second, when passing a `cranelift-flags` parameter, and the given
parameter's name doesn't exist at the target-independent flag level, try
to set it as a target-dependent setting.

These two changes make it possible to try out the new x64 backend with:

    cargo run --features experimental_x64 -- run --cranelift-flags use_new_backend=true -- /path/to/a.wasm

Right now, this will fail because most opcodes required by the
trampolines are actually not implemented yet.

Cargo.toml

@@ -81,6 +81,10 @@ lightbeam = [
 jitdump = ["wasmtime/jitdump"]
 vtune = ["wasmtime/vtune"]
 
+# Try the experimental, work-in-progress new x86_64 backend. This is not stable
+# as of June 2020.
+experimental_x64 = ["wasmtime-jit/experimental_x64"]
+
 [badges]
 maintenance = { status = "actively-developed" }
 

cranelift/codegen/src/isa/mod.rs

@@ -150,6 +150,7 @@ pub enum LookupError {
 
 /// Builder for a `TargetIsa`.
 /// Modify the ISA-specific settings before creating the `TargetIsa` trait object with `finish`.
+#[derive(Clone)]
 pub struct Builder {
     triple: Triple,
     setup: settings::Builder,

crates/environ/src/data_structures.rs

@@ -10,11 +10,13 @@ pub mod ir {
 }
 
 pub mod settings {
-    pub use cranelift_codegen::settings::{builder, Builder, Configurable, Flags};
+    pub use cranelift_codegen::settings::{builder, Builder, Configurable, Flags, SetError};
 }
 
 pub mod isa {
-    pub use cranelift_codegen::isa::{unwind, CallConv, RegUnit, TargetFrontendConfig, TargetIsa};
+    pub use cranelift_codegen::isa::{
+        unwind, Builder, CallConv, RegUnit, TargetFrontendConfig, TargetIsa,
+    };
 }
 
 pub mod entity {

crates/jit/Cargo.toml

@@ -39,5 +39,9 @@ lightbeam = ["wasmtime-environ/lightbeam"]
 jitdump = ["wasmtime-profiling/jitdump"]
 vtune = ["wasmtime-profiling/vtune"]
 
+# Try the experimental, work-in-progress new x86_64 backend. This is not stable
+# as of June 2020.
+experimental_x64 = ["cranelift-codegen/x64"]
+
 [badges]
 maintenance = { status = "actively-developed" }

crates/wasmtime/src/runtime.rs

@@ -14,8 +14,8 @@ use std::path::Path;
 use std::rc::{Rc, Weak};
 use std::sync::Arc;
 use wasmparser::{OperatorValidatorConfig, ValidatingParserConfig};
-use wasmtime_environ::settings::{self, Configurable};
+use wasmtime_environ::settings::{self, Configurable, SetError};
-use wasmtime_environ::{ir, isa::TargetIsa, wasm, CacheConfig, Tunables};
+use wasmtime_environ::{ir, isa, isa::TargetIsa, wasm, CacheConfig, Tunables};
 use wasmtime_jit::{native, CompilationStrategy, Compiler};
 use wasmtime_profiling::{JitDumpAgent, NullProfilerAgent, ProfilingAgent, VTuneAgent};
 use wasmtime_runtime::{
@@ -36,6 +36,7 @@ use wasmtime_runtime::{
 #[derive(Clone)]
 pub struct Config {
     pub(crate) flags: settings::Builder,
+    pub(crate) isa_flags: isa::Builder,
     pub(crate) validating_config: ValidatingParserConfig,
     pub(crate) tunables: Tunables,
     pub(crate) strategy: CompilationStrategy,
@@ -94,6 +95,7 @@ impl Config {
                 },
             },
             flags,
+            isa_flags: native::builder(),
             strategy: CompilationStrategy::Auto,
             cache_config: CacheConfig::new_cache_disabled(),
             profiler: Arc::new(NullProfilerAgent),
@@ -377,7 +379,15 @@ impl Config {
     /// This method can fail if the flag's name does not exist, or the value is not appropriate for
     /// the flag type.
     pub unsafe fn cranelift_other_flag(&mut self, name: &str, value: &str) -> Result<&mut Self> {
-        self.flags.set(name, value)?;
+        if let Err(err) = self.flags.set(name, value) {
+            match err {
+                SetError::BadName(_) => {
+                    // Try the target-specific flags.
+                    self.isa_flags.set(name, value)?;
+                }
+                _ => bail!(err),
+            }
+        }
         Ok(self)
     }
 
@@ -600,7 +610,9 @@ impl Config {
     }
 
     pub(crate) fn target_isa(&self) -> Box<dyn TargetIsa> {
-        native::builder().finish(settings::Flags::new(self.flags.clone()))
+        self.isa_flags
+            .clone()
+            .finish(settings::Flags::new(self.flags.clone()))
     }
 
     fn build_compiler(&self) -> Compiler {