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/meta/src/lib.rs

@@ -25,7 +25,11 @@ pub fn isa_from_arch(arch: &str) -> Result<isa::Isa, String> {
 }
 
 /// Generates all the Rust source files used in Cranelift from the meta-language.
-pub fn generate(isas: &[isa::Isa], out_dir: &str) -> Result<(), error::Error> {
+pub fn generate(
+    old_backend_isas: &[isa::Isa],
+    new_backend_isas: &[isa::Isa],
+    out_dir: &str,
+) -> Result<(), error::Error> {
     // Create all the definitions:
     // - common definitions.
     let mut shared_defs = shared::define();
@@ -39,7 +43,7 @@ pub fn generate(isas: &[isa::Isa], out_dir: &str) -> Result<(), error::Error> {
     gen_types::generate("types.rs", &out_dir)?;
 
     // - per ISA definitions.
-    let isas = isa::define(isas, &mut shared_defs);
+    let target_isas = isa::define(old_backend_isas, &mut shared_defs);
 
     // At this point, all definitions are done.
     let all_formats = shared_defs.verify_instruction_formats();
@@ -53,9 +57,22 @@ pub fn generate(isas: &[isa::Isa], out_dir: &str) -> Result<(), error::Error> {
         &out_dir,
     )?;
 
-    gen_legalizer::generate(&isas, &shared_defs.transform_groups, "legalize", &out_dir)?;
+    let extra_legalization_groups: &[&'static str] = if !new_backend_isas.is_empty() {
+        // The new backend only requires the "expand" legalization group.
+        &["expand"]
+    } else {
+        &[]
+    };
 
-    for isa in isas {
+    gen_legalizer::generate(
+        &target_isas,
+        &shared_defs.transform_groups,
+        extra_legalization_groups,
+        "legalize",
+        &out_dir,
+    )?;
+
+    for isa in target_isas {
         gen_registers::generate(&isa, &format!("registers-{}.rs", isa.name), &out_dir)?;
 
         gen_settings::generate(
@@ -80,5 +97,28 @@ pub fn generate(isas: &[isa::Isa], out_dir: &str) -> Result<(), error::Error> {
         )?;
     }
 
+    for isa in new_backend_isas {
+        match isa {
+            isa::Isa::X86 => {
+                // If the old backend ISAs contained x86, this file has already been generated.
+                if old_backend_isas.iter().any(|isa| *isa == isa::Isa::X86) {
+                    continue;
+                }
+
+                let settings = crate::isa::x86::settings::define(&shared_defs.settings);
+                gen_settings::generate(
+                    &settings,
+                    gen_settings::ParentGroup::Shared,
+                    "settings-x86.rs",
+                    &out_dir,
+                )?;
+            }
+            isa::Isa::Arm64 => {
+                // aarch64 doesn't have platform-specific settings.
+            }
+            isa::Isa::Arm32 | isa::Isa::Riscv => todo!(),
+        }
+    }
+
     Ok(())
 }