Switch default to new x86_64 backend.

This PR switches the default backend on x86, for both the
`cranelift-codegen` crate and for Wasmtime, to the new
(`MachInst`-style, `VCode`-based) backend that has been under
development and testing for some time now.

The old backend is still available by default in builds with the
`old-x86-backend` feature, or by requesting `BackendVariant::Legacy`
from the appropriate APIs.

As part of that switch, it adds some more runtime-configurable plumbing
to the testing infrastructure so that tests can be run using the
appropriate backend. `clif-util test` is now capable of parsing a
backend selector option from filetests and instantiating the correct
backend.

CI has been updated so that the old x86 backend continues to run its
tests, just as we used to run the new x64 backend separately.

At some point, we will remove the old x86 backend entirely, once we are
satisfied that the new backend has not caused any unforeseen issues and
we do not need to revert.

cranelift/codegen/src/isa/mod.rs

@@ -74,16 +74,15 @@ use thiserror::Error;
 #[cfg(feature = "riscv")]
 mod riscv;
 
-// N.B.: the old x86-64 backend (`x86`) and the new one (`x64`) can both be
-// included; if the new backend is included, then it is the default backend
-// returned for an x86-64 triple, but a specific option can request the old
-// backend. It is important to have the ability to instantiate *both* backends
-// in the same build so that we can do things like differential fuzzing between
-// backends, or perhaps offer a runtime configuration flag in the future.
+// N.B.: the old x86-64 backend (`x86`) and the new one (`x64`) are both
+// included whenever building with x86 support. The new backend is the default,
+// but the old can be requested with `BackendVariant::Legacy`. However, if this
+// crate is built with the `old-x86-backend` feature, then the old backend is
+// default instead.
 #[cfg(feature = "x86")]
 mod x86;
 
-#[cfg(feature = "x64")]
+#[cfg(feature = "x86")]
 mod x64;
 
 #[cfg(feature = "arm32")]
@@ -122,7 +121,7 @@ macro_rules! isa_builder {
 /// The "variant" for a given target. On one platform (x86-64), we have two
 /// backends, the "old" and "new" one; the new one is the default if included
 /// in the build configuration and not otherwise specified.
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug)]
 pub enum BackendVariant {
     /// Any backend available.
     Any,
@@ -149,13 +148,13 @@ pub fn lookup_variant(triple: Triple, variant: BackendVariant) -> Result<Builder
             isa_builder!(x86, (feature = "x86"), triple)
         }
         (Architecture::X86_64, BackendVariant::MachInst) => {
-            isa_builder!(x64, (feature = "x64"), triple)
+            isa_builder!(x64, (feature = "x86"), triple)
         }
-        #[cfg(feature = "x64")]
+        #[cfg(not(feature = "old-x86-backend"))]
         (Architecture::X86_64, BackendVariant::Any) => {
-            isa_builder!(x64, (feature = "x64"), triple)
+            isa_builder!(x64, (feature = "x86"), triple)
         }
-        #[cfg(not(feature = "x64"))]
+        #[cfg(feature = "old-x86-backend")]
         (Architecture::X86_64, BackendVariant::Any) => {
             isa_builder!(x86, (feature = "x86"), triple)
         }
@@ -277,7 +276,13 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     /// Get the ISA-dependent flag values that were used to make this trait object.
     fn isa_flags(&self) -> Vec<settings::Value>;
 
-    /// Hashes all flags, both ISA-independent and ISA-dependent, into the specified hasher.
+    /// Get the variant of this ISA (Legacy or MachInst).
+    fn variant(&self) -> BackendVariant {
+        BackendVariant::Legacy
+    }
+
+    /// Hashes all flags, both ISA-independent and ISA-specific, into the
+    /// specified hasher.
     fn hash_all_flags(&self, hasher: &mut dyn Hasher);
 
     /// Get the default calling convention of this target.