Fix AArch64 ABI to respect half-caller-save, half-callee-save vec regs.

This PR updates the AArch64 ABI implementation so that it (i) properly
respects that v8-v15 inclusive have callee-save lower halves, and
caller-save upper halves, by conservatively approximating (to full
registers) in the appropriate directions when generating prologue
caller-saves and when informing the regalloc of clobbered regs across
callsites.

In order to prevent saving all of these vector registers in the prologue
of every non-leaf function due to the above approximation, this also
makes use of a new regalloc.rs feature to exclude call instructions'
writes from the clobber set returned by register allocation. This is
safe whenever the caller and callee have the same ABI (because anything
the callee could clobber, the caller is allowed to clobber as well
without saving it in the prologue).

Fixes #2254.

cranelift/codegen/src/machinst/abi_impl.rs

@@ -347,6 +347,8 @@ pub trait ABIMachineSpec {
         loc: SourceLoc,
         opcode: ir::Opcode,
         tmp: Writable<Reg>,
+        callee_conv: isa::CallConv,
+        callee_conv: isa::CallConv,
     ) -> SmallVec<[(InstIsSafepoint, Self::I); 2]>;
 
     /// Get the number of spillslots required for the given register-class and
@@ -359,8 +361,9 @@ pub trait ABIMachineSpec {
     /// Get the "nominal SP to FP" offset from an instruction-emission state.
     fn get_nominal_sp_to_fp(s: &<Self::I as MachInstEmit>::State) -> i64;
 
-    /// Get all caller-save registers.
-    fn get_caller_saves(call_conv: isa::CallConv) -> Vec<Writable<Reg>>;
+    /// Get all caller-save registers, that is, registers that we expect
+    /// not to be saved across a call to a callee with the given ABI.
+    fn get_regs_clobbered_by_call(call_conv_of_callee: isa::CallConv) -> Vec<Writable<Reg>>;
 }
 
 /// ABI information shared between body (callee) and caller.
@@ -682,6 +685,10 @@ impl<M: ABIMachineSpec> ABICallee for ABICalleeImpl<M> {
         &self.flags
     }
 
+    fn call_conv(&self) -> isa::CallConv {
+        self.sig.call_conv
+    }
+
     fn liveins(&self) -> Set<RealReg> {
         let mut set: Set<RealReg> = Set::empty();
         for &arg in &self.sig.args {
@@ -1040,7 +1047,7 @@ fn abisig_to_uses_and_defs<M: ABIMachineSpec>(sig: &ABISig) -> (Vec<Reg>, Vec<Wr
     }
 
     // Compute defs: all retval regs, and all caller-save (clobbered) regs.
-    let mut defs = M::get_caller_saves(sig.call_conv);
+    let mut defs = M::get_regs_clobbered_by_call(sig.call_conv);
     for ret in &sig.rets {
         match ret {
             &ABIArg::Reg(reg, ..) => defs.push(Writable::from_reg(reg.to_reg())),
@@ -1063,8 +1070,10 @@ pub struct ABICallerImpl<M: ABIMachineSpec> {
     dest: CallDest,
     /// Location of callsite.
     loc: ir::SourceLoc,
-    /// Actuall call opcode; used to distinguish various types of calls.
+    /// Actual call opcode; used to distinguish various types of calls.
     opcode: ir::Opcode,
+    /// Caller's calling convention.
+    caller_conv: isa::CallConv,
 
     _mach: PhantomData<M>,
 }
@@ -1085,6 +1094,7 @@ impl<M: ABIMachineSpec> ABICallerImpl<M> {
         extname: &ir::ExternalName,
         dist: RelocDistance,
         loc: ir::SourceLoc,
+        caller_conv: isa::CallConv,
     ) -> CodegenResult<ABICallerImpl<M>> {
         let sig = ABISig::from_func_sig::<M>(sig)?;
         let (uses, defs) = abisig_to_uses_and_defs::<M>(&sig);
@@ -1095,6 +1105,7 @@ impl<M: ABIMachineSpec> ABICallerImpl<M> {
             dest: CallDest::ExtName(extname.clone(), dist),
             loc,
             opcode: ir::Opcode::Call,
+            caller_conv,
             _mach: PhantomData,
         })
     }
@@ -1106,6 +1117,7 @@ impl<M: ABIMachineSpec> ABICallerImpl<M> {
         ptr: Reg,
         loc: ir::SourceLoc,
         opcode: ir::Opcode,
+        caller_conv: isa::CallConv,
     ) -> CodegenResult<ABICallerImpl<M>> {
         let sig = ABISig::from_func_sig::<M>(sig)?;
         let (uses, defs) = abisig_to_uses_and_defs::<M>(&sig);
@@ -1116,6 +1128,7 @@ impl<M: ABIMachineSpec> ABICallerImpl<M> {
             dest: CallDest::Reg(ptr),
             loc,
             opcode,
+            caller_conv,
             _mach: PhantomData,
         })
     }
@@ -1255,8 +1268,17 @@ impl<M: ABIMachineSpec> ABICaller for ABICallerImpl<M> {
             self.emit_copy_reg_to_arg(ctx, i, rd.to_reg());
         }
         let tmp = ctx.alloc_tmp(word_rc, word_type);
-        for (is_safepoint, inst) in
-            M::gen_call(&self.dest, uses, defs, self.loc, self.opcode, tmp).into_iter()
+        for (is_safepoint, inst) in M::gen_call(
+            &self.dest,
+            uses,
+            defs,
+            self.loc,
+            self.opcode,
+            tmp,
+            self.sig.call_conv,
+            self.caller_conv,
+        )
+        .into_iter()
         {
             match is_safepoint {
                 InstIsSafepoint::Yes => ctx.emit_safepoint(inst),