Add a work-in-progress backend for x86_64 using the new instruction selection;

Most of the work is credited to Julian Seward.

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Chris Fallin <cfallin@mozilla.com>

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

@@ -0,0 +1,92 @@
+//! X86_64-bit Instruction Set Architecture.
+
+use alloc::boxed::Box;
+
+use regalloc::RealRegUniverse;
+use target_lexicon::Triple;
+
+use crate::ir::Function;
+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::isa::x64::inst::regs::create_reg_universe_systemv;
+
+mod abi;
+mod inst;
+mod lower;
+
+/// An X64 backend.
+pub(crate) struct X64Backend {
+    triple: Triple,
+    flags: Flags,
+}
+
+impl X64Backend {
+    /// Create a new X64 backend with the given (shared) flags.
+    fn new_with_flags(triple: Triple, flags: Flags) -> Self {
+        Self { triple, flags }
+    }
+
+    fn compile_vcode(&self, func: &Function, flags: Flags) -> CodegenResult<VCode<inst::Inst>> {
+        // This performs lowering to VCode, register-allocates the code, computes
+        // block layout and finalizes branches. The result is ready for binary emission.
+        let abi = Box::new(abi::X64ABIBody::new(&func, flags));
+        compile::compile::<Self>(&func, self, abi)
+    }
+}
+
+impl MachBackend for X64Backend {
+    fn compile_function(
+        &self,
+        func: &Function,
+        want_disasm: bool,
+    ) -> CodegenResult<MachCompileResult> {
+        let flags = self.flags();
+        let vcode = self.compile_vcode(func, flags.clone())?;
+        let sections = vcode.emit();
+        let frame_size = vcode.frame_size();
+
+        let disasm = if want_disasm {
+            Some(vcode.show_rru(Some(&create_reg_universe_systemv(flags))))
+        } else {
+            None
+        };
+
+        Ok(MachCompileResult {
+            sections,
+            frame_size,
+            disasm,
+        })
+    }
+
+    fn flags(&self) -> &Flags {
+        &self.flags
+    }
+
+    fn name(&self) -> &'static str {
+        "x64"
+    }
+
+    fn triple(&self) -> Triple {
+        self.triple.clone()
+    }
+
+    fn reg_universe(&self) -> RealRegUniverse {
+        create_reg_universe_systemv(&self.flags)
+    }
+}
+
+/// Create a new `isa::Builder`.
+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))
+        },
+    }
+}