machinst x64: Only use the feature flag to enable the x64 new backend;

Before this patch, running the x64 new backend would require both
compiling with --features experimental_x64 and running with
`use_new_backend`.

This patches changes this behavior so that the runtime flag is not
needed anymore: using the feature flag will enforce usage of the new
backend everywhere, making using and testing it much simpler:

    cargo run --features experimental_x64 ;; other CLI options/flags

This also gives a hint at what the meta language generation would look
like after switching to the new backend.

Compiling only with the x64 codegen flag gives a nice compile time speedup.

cranelift/codegen/src/isa/mod.rs

@@ -121,7 +121,11 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
     match triple.architecture {
         Architecture::Riscv32 | Architecture::Riscv64 => isa_builder!(riscv, "riscv", triple),
         Architecture::I386 | Architecture::I586 | Architecture::I686 | Architecture::X86_64 => {
-            isa_builder!(x86, "x86", triple)
+            if cfg!(feature = "x64") {
+                isa_builder!(x64, "x64", triple)
+            } else {
+                isa_builder!(x86, "x86", triple)
+            }
         }
         Architecture::Arm { .. } => isa_builder!(arm32, "arm32", triple),
         Architecture::Aarch64 { .. } => isa_builder!(aarch64, "arm64", triple),

cranelift/codegen/src/isa/x64/mod.rs

@@ -11,28 +11,33 @@ use crate::isa::Builder as IsaBuilder;
 use crate::machinst::pretty_print::ShowWithRRU;
 use crate::machinst::{compile, MachBackend, MachCompileResult, TargetIsaAdapter, VCode};
 use crate::result::CodegenResult;
-use crate::settings::{self, Flags};
+use crate::settings::{self as shared_settings, Flags};
 
-use crate::isa::x64::inst::regs::create_reg_universe_systemv;
+use crate::isa::x64::{inst::regs::create_reg_universe_systemv, settings as x64_settings};
+
+use super::TargetIsa;
 
 mod abi;
 mod inst;
 mod lower;
+mod settings;
 
 /// An X64 backend.
 pub(crate) struct X64Backend {
     triple: Triple,
     flags: Flags,
+    _x64_flags: x64_settings::Flags,
     reg_universe: RealRegUniverse,
 }
 
 impl X64Backend {
     /// Create a new X64 backend with the given (shared) flags.
-    fn new_with_flags(triple: Triple, flags: Flags) -> Self {
+    fn new_with_flags(triple: Triple, flags: Flags, x64_flags: x64_settings::Flags) -> Self {
         let reg_universe = create_reg_universe_systemv(&flags);
         Self {
             triple,
             flags,
+            _x64_flags: x64_flags,
             reg_universe,
         }
     }
@@ -103,10 +108,17 @@ impl MachBackend for X64Backend {
 pub(crate) fn isa_builder(triple: Triple) -> IsaBuilder {
     IsaBuilder {
         triple,
-        setup: settings::builder(),
-        constructor: |triple: Triple, flags: Flags, _arch_flag_builder: settings::Builder| {
-            let backend = X64Backend::new_with_flags(triple, flags);
-            Box::new(TargetIsaAdapter::new(backend))
-        },
+        setup: x64_settings::builder(),
+        constructor: isa_constructor,
     }
 }
+
+fn isa_constructor(
+    triple: Triple,
+    shared_flags: Flags,
+    builder: shared_settings::Builder,
+) -> Box<dyn TargetIsa> {
+    let isa_flags = x64_settings::Flags::new(&shared_flags, builder);
+    let backend = X64Backend::new_with_flags(triple, shared_flags, isa_flags);
+    Box::new(TargetIsaAdapter::new(backend))
+}

cranelift/codegen/src/isa/x64/settings.rs

@@ -0,0 +1,9 @@
+//! x86 Settings.
+
+use crate::settings::{self, detail, Builder};
+use core::fmt;
+
+// Include code generated by `cranelift-codegen/meta/src/gen_settings.rs:`. This file contains a
+// public `Flags` struct with an impl for all of the settings defined in
+// `cranelift-codegen/meta/src/isa/x86/settings.rs`.
+include!(concat!(env!("OUT_DIR"), "/settings-x86.rs"));

cranelift/codegen/src/isa/x86/mod.rs

@@ -57,20 +57,12 @@ fn isa_constructor(
 
     let isa_flags = settings::Flags::new(&shared_flags, builder);
 
-    if isa_flags.use_new_backend() {
-        #[cfg(not(feature = "x64"))]
-        panic!("new backend x86 support not included by cargo features!");
-
-        #[cfg(feature = "x64")]
-        super::x64::isa_builder(triple).finish(shared_flags)
-    } else {
-        Box::new(Isa {
-            triple,
-            isa_flags,
-            shared_flags,
-            cpumode: level1,
-        })
-    }
+    Box::new(Isa {
+        triple,
+        isa_flags,
+        shared_flags,
+        cpumode: level1,
+    })
 }
 
 impl TargetIsa for Isa {