machinst: allow passing constant information to the instruction emitter;

A new associated type Info is added to MachInstEmit, which is the
immutable counterpart to State. It can't easily be constructed from an
ABICallee, since it would require adding an associated type to the
latter, and making so leaks the associated type in a lot of places in
the code base and makes the code harder to read. Instead, the EmitInfo
state can simply be passed to the `Vcode::emit` function directly.

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

@@ -1,5 +1,7 @@
 //! X86_64-bit Instruction Set Architecture.
 
+use self::inst::EmitInfo;
+
 use super::TargetIsa;
 use crate::ir::{condcodes::IntCC, Function};
 use crate::isa::x64::{inst::regs::create_reg_universe_systemv, settings as x64_settings};
@@ -20,7 +22,7 @@ mod settings;
 pub(crate) struct X64Backend {
     triple: Triple,
     flags: Flags,
-    _x64_flags: x64_settings::Flags,
+    x64_flags: x64_settings::Flags,
     reg_universe: RealRegUniverse,
 }
 
@@ -31,7 +33,7 @@ impl X64Backend {
         Self {
             triple,
             flags,
-            _x64_flags: x64_flags,
+            x64_flags,
             reg_universe,
         }
     }
@@ -39,8 +41,9 @@ impl X64Backend {
     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 emit_info = EmitInfo::new(flags.clone(), self.x64_flags.clone());
         let abi = Box::new(abi::X64ABICallee::new(&func, flags)?);
-        compile::compile::<Self>(&func, self, abi)
+        compile::compile::<Self>(&func, self, abi, emit_info)
     }
 }
 
@@ -52,6 +55,7 @@ impl MachBackend for X64Backend {
     ) -> CodegenResult<MachCompileResult> {
         let flags = self.flags();
         let vcode = self.compile_vcode(func, flags.clone())?;
+
         let buffer = vcode.emit();
         let buffer = buffer.finish();
         let frame_size = vcode.frame_size();