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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>
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@ -1377,12 +1377,58 @@ func (s *regAllocState) regalloc(f *Func) {
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}
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}
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// Move arguments to registers. Process in an ordering defined
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// by the register specification (most constrained first).
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args = append(args[:0], v.Args...)
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// Move arguments to registers.
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// First, if an arg must be in a specific register and it is already
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// in place, keep it.
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args = append(args[:0], make([]*Value, len(v.Args))...)
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for i, a := range v.Args {
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if !s.values[a.ID].needReg {
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args[i] = a
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}
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}
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for _, i := range regspec.inputs {
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mask := i.regs
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if mask&s.values[args[i.idx].ID].regs == 0 {
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if countRegs(mask) == 1 && mask&s.values[v.Args[i.idx].ID].regs != 0 {
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args[i.idx] = s.allocValToReg(v.Args[i.idx], mask, true, v.Pos)
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}
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}
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// Then, if an arg must be in a specific register and that
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// register is free, allocate that one. Otherwise when processing
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// another input we may kick a value into the free register, which
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// then will be kicked out again.
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// This is a common case for passing-in-register arguments for
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// function calls.
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for {
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freed := false
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for _, i := range regspec.inputs {
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if args[i.idx] != nil {
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continue // already allocated
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}
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mask := i.regs
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if countRegs(mask) == 1 && mask&^s.used != 0 {
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args[i.idx] = s.allocValToReg(v.Args[i.idx], mask, true, v.Pos)
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// If the input is in other registers that will be clobbered by v,
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// or the input is dead, free the registers. This may make room
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// for other inputs.
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oldregs := s.values[v.Args[i.idx].ID].regs
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if oldregs&^regspec.clobbers == 0 || !s.liveAfterCurrentInstruction(v.Args[i.idx]) {
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s.freeRegs(oldregs &^ mask &^ s.nospill)
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freed = true
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}
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}
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}
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if !freed {
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break
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}
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}
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// Last, allocate remaining ones, in an ordering defined
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// by the register specification (most constrained first).
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for _, i := range regspec.inputs {
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if args[i.idx] != nil {
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continue // already allocated
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}
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mask := i.regs
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if mask&s.values[v.Args[i.idx].ID].regs == 0 {
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// Need a new register for the input.
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mask &= s.allocatable
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mask &^= s.nospill
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@ -1401,7 +1447,7 @@ func (s *regAllocState) regalloc(f *Func) {
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mask &^= desired.avoid
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}
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}
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args[i.idx] = s.allocValToReg(args[i.idx], mask, true, v.Pos)
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args[i.idx] = s.allocValToReg(v.Args[i.idx], mask, true, v.Pos)
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}
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// If the output clobbers the input register, make sure we have
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23
test/codegen/regabi_regalloc.go
Normal file
23
test/codegen/regabi_regalloc.go
Normal file
@ -0,0 +1,23 @@
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// asmcheck
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// Copyright 2021 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package codegen
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//go:registerparams
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func f1(a, b int) {
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// amd64:"MOVQ\tBX, CX", "MOVQ\tAX, BX", "MOVL\t\\$1, AX", -"MOVQ\t.*DX"
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g(1, a, b)
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}
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//go:registerparams
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func f2(a, b int) {
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// amd64:"MOVQ\tBX, AX", "MOVQ\t[AB]X, CX", -"MOVQ\t.*, BX"
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g(b, b, b)
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}
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//go:noinline
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//go:registerparams
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func g(int, int, int) {}
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