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/isa/arm32/inst/args.rs

@@ -2,7 +2,7 @@
 
 use crate::isa::arm32::inst::*;
 
-use regalloc::{RealRegUniverse, Reg};
+use regalloc::{PrettyPrint, RealRegUniverse, Reg};
 
 use std::string::String;
 
@@ -265,7 +265,7 @@ impl BranchTarget {
     }
 }
 
-impl ShowWithRRU for ShiftOpAndAmt {
+impl PrettyPrint for ShiftOpAndAmt {
     fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
         let op = match self.op() {
             ShiftOp::LSL => "lsl",
@@ -277,19 +277,19 @@ impl ShowWithRRU for ShiftOpAndAmt {
     }
 }
 
-impl ShowWithRRU for UImm8 {
+impl PrettyPrint for UImm8 {
     fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
         format!("#{}", self.value)
     }
 }
 
-impl ShowWithRRU for UImm12 {
+impl PrettyPrint for UImm12 {
     fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
         format!("#{}", self.value)
     }
 }
 
-impl ShowWithRRU for AMode {
+impl PrettyPrint for AMode {
     fn show_rru(&self, mb_rru: Option<&RealRegUniverse>) -> String {
         match self {
             &AMode::RegReg(rn, rm, imm2) => {
@@ -317,7 +317,7 @@ impl ShowWithRRU for AMode {
     }
 }
 
-impl ShowWithRRU for Cond {
+impl PrettyPrint for Cond {
     fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
         let mut s = format!("{:?}", self);
         s.make_ascii_lowercase();
@@ -325,7 +325,7 @@ impl ShowWithRRU for Cond {
     }
 }
 
-impl ShowWithRRU for BranchTarget {
+impl PrettyPrint for BranchTarget {
     fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
         match self {
             &BranchTarget::Label(label) => format!("label{:?}", label.get()),

cranelift/codegen/src/isa/arm32/inst/mod.rs

@@ -8,7 +8,7 @@ use crate::ir::{ExternalName, Opcode, SourceLoc, TrapCode, Type};
 use crate::machinst::*;
 use crate::{settings, CodegenError, CodegenResult};
 
-use regalloc::{RealRegUniverse, Reg, RegClass, SpillSlot, VirtualReg, Writable};
+use regalloc::{PrettyPrint, RealRegUniverse, Reg, RegClass, SpillSlot, VirtualReg, Writable};
 use regalloc::{RegUsageCollector, RegUsageMapper};
 
 use alloc::boxed::Box;
@@ -897,7 +897,7 @@ fn mem_finalize_for_show(
     (mem_str, mem)
 }
 
-impl ShowWithRRU for Inst {
+impl PrettyPrint for Inst {
     fn show_rru(&self, mb_rru: Option<&RealRegUniverse>) -> String {
         self.pretty_print(mb_rru, &mut EmitState::default())
     }