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cmd/compile: reduce redundant register moves for regabi calls

Currently, if we have AX=a and BX=b, and we want to make a call
F(1, a, b), to move arguments into the desired registers it emits

	MOVQ AX, CX
	MOVL $1, AX // AX=1
	MOVQ BX, DX
	MOVQ CX, BX // BX=a
	MOVQ DX, CX // CX=b

This has a few redundant moves.

This is because we process inputs in order. First, allocate 1 to
AX, which kicks out a (in AX) to CX (a free register at the
moment). Then, allocate a to BX, which kicks out b (in BX) to DX.
Finally, put b to CX.

Notice that if we start with allocating CX=b, then BX=a, AX=1,
we will not have redundant moves. This CL reduces redundant moves
by allocating them in different order: First, for inpouts that are
already in place, keep them there. Then allocate free registers.
Then everything else.

                             before       after
cmd/compile binary size     23703888    23609680
            text size        8565899     8533291

(with regabiargs enabled.)

Change-Id: I69e1bdf745f2c90bb791f6d7c45b37384af1e874
Reviewed-on: https://go-review.googlesource.com/c/go/+/311371
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
This commit is contained in:
Cherry Zhang 2021-04-17 22:50:13 -04:00
parent b21e739f87
commit 6b8e3e2d06
2 changed files with 74 additions and 5 deletions

View File

@ -1377,12 +1377,58 @@ func (s *regAllocState) regalloc(f *Func) {
}
}
// Move arguments to registers. Process in an ordering defined
// by the register specification (most constrained first).
args = append(args[:0], v.Args...)
// Move arguments to registers.
// First, if an arg must be in a specific register and it is already
// in place, keep it.
args = append(args[:0], make([]*Value, len(v.Args))...)
for i, a := range v.Args {
if !s.values[a.ID].needReg {
args[i] = a
}
}
for _, i := range regspec.inputs {
mask := i.regs
if mask&s.values[args[i.idx].ID].regs == 0 {
if countRegs(mask) == 1 && mask&s.values[v.Args[i.idx].ID].regs != 0 {
args[i.idx] = s.allocValToReg(v.Args[i.idx], mask, true, v.Pos)
}
}
// Then, if an arg must be in a specific register and that
// register is free, allocate that one. Otherwise when processing
// another input we may kick a value into the free register, which
// then will be kicked out again.
// This is a common case for passing-in-register arguments for
// function calls.
for {
freed := false
for _, i := range regspec.inputs {
if args[i.idx] != nil {
continue // already allocated
}
mask := i.regs
if countRegs(mask) == 1 && mask&^s.used != 0 {
args[i.idx] = s.allocValToReg(v.Args[i.idx], mask, true, v.Pos)
// If the input is in other registers that will be clobbered by v,
// or the input is dead, free the registers. This may make room
// for other inputs.
oldregs := s.values[v.Args[i.idx].ID].regs
if oldregs&^regspec.clobbers == 0 || !s.liveAfterCurrentInstruction(v.Args[i.idx]) {
s.freeRegs(oldregs &^ mask &^ s.nospill)
freed = true
}
}
}
if !freed {
break
}
}
// Last, allocate remaining ones, in an ordering defined
// by the register specification (most constrained first).
for _, i := range regspec.inputs {
if args[i.idx] != nil {
continue // already allocated
}
mask := i.regs
if mask&s.values[v.Args[i.idx].ID].regs == 0 {
// Need a new register for the input.
mask &= s.allocatable
mask &^= s.nospill
@ -1401,7 +1447,7 @@ func (s *regAllocState) regalloc(f *Func) {
mask &^= desired.avoid
}
}
args[i.idx] = s.allocValToReg(args[i.idx], mask, true, v.Pos)
args[i.idx] = s.allocValToReg(v.Args[i.idx], mask, true, v.Pos)
}
// If the output clobbers the input register, make sure we have

View File

@ -0,0 +1,23 @@
// asmcheck
// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package codegen
//go:registerparams
func f1(a, b int) {
// amd64:"MOVQ\tBX, CX", "MOVQ\tAX, BX", "MOVL\t\\$1, AX", -"MOVQ\t.*DX"
g(1, a, b)
}
//go:registerparams
func f2(a, b int) {
// amd64:"MOVQ\tBX, AX", "MOVQ\t[AB]X, CX", -"MOVQ\t.*, BX"
g(b, b, b)
}
//go:noinline
//go:registerparams
func g(int, int, int) {}