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[dev.ssa] cmd/compile: add SSA pass to move values closer to uses
Even this very simple, restricted initial implementation helps.
While running make.bash, it moves 84437 values
to new, closer homes.
As a concrete example:
func f_ssa(i, j int, b bool) int {
if !b {
return 0
}
return i + j
}
It cuts off one stack slot and two instructions:
Before:
"".f_ssa t=1 size=96 value=0 args=0x20 locals=0x18
0x0000 00000 (x.go:3) TEXT "".f_ssa(SB), $24-32
0x0000 00000 (x.go:3) SUBQ $24, SP
0x0004 00004 (x.go:3) FUNCDATA $0, "".gcargs·0(SB)
0x0004 00004 (x.go:3) FUNCDATA $1, "".gclocals·1(SB)
0x0004 00004 (x.go:5) MOVQ $0, AX
0x0006 00006 (x.go:3) MOVQ 32(SP), CX
0x000b 00011 (x.go:3) MOVQ 40(SP), DX
0x0010 00016 (x.go:3) LEAQ 48(SP), BX
0x0015 00021 (x.go:3) MOVB (BX), BPB
0x0018 00024 (x.go:3) MOVQ $0, SI
0x001a 00026 (x.go:3) MOVQ SI, 56(SP)
0x001f 00031 (x.go:3) TESTB BPB, BPB
0x0022 00034 (x.go:5) MOVQ AX, (SP)
0x0026 00038 (x.go:3) MOVQ CX, 8(SP)
0x002b 00043 (x.go:3) MOVQ DX, 16(SP)
0x0030 00048 (x.go:4) JEQ 74
0x0032 00050 (x.go:3) MOVQ 8(SP), AX
0x0037 00055 (x.go:3) MOVQ 16(SP), CX
0x003c 00060 (x.go:7) LEAQ (AX)(CX*1), DX
0x0040 00064 (x.go:7) MOVQ DX, 56(SP)
0x0045 00069 (x.go:3) ADDQ $24, SP
0x0049 00073 (x.go:3) RET
0x004a 00074 (x.go:5) MOVQ (SP), AX
0x004e 00078 (x.go:5) MOVQ AX, 56(SP)
0x0053 00083 (x.go:3) JMP 69
After:
"".f_ssa t=1 size=80 value=0 args=0x20 locals=0x10
0x0000 00000 (x.go:3) TEXT "".f_ssa(SB), $16-32
0x0000 00000 (x.go:3) SUBQ $16, SP
0x0004 00004 (x.go:3) FUNCDATA $0, "".gcargs·0(SB)
0x0004 00004 (x.go:3) FUNCDATA $1, "".gclocals·1(SB)
0x0004 00004 (x.go:3) MOVQ 32(SP), AX
0x0009 00009 (x.go:3) MOVQ 24(SP), CX
0x000e 00014 (x.go:3) LEAQ 40(SP), DX
0x0013 00019 (x.go:3) MOVB (DX), BL
0x0015 00021 (x.go:3) MOVQ $0, BP
0x0017 00023 (x.go:3) MOVQ BP, 48(SP)
0x001c 00028 (x.go:3) TESTB BL, BL
0x001e 00030 (x.go:3) MOVQ AX, (SP)
0x0022 00034 (x.go:3) MOVQ CX, 8(SP)
0x0027 00039 (x.go:4) JEQ 64
0x0029 00041 (x.go:3) MOVQ 8(SP), AX
0x002e 00046 (x.go:3) MOVQ (SP), CX
0x0032 00050 (x.go:7) LEAQ (AX)(CX*1), DX
0x0036 00054 (x.go:7) MOVQ DX, 48(SP)
0x003b 00059 (x.go:3) ADDQ $16, SP
0x003f 00063 (x.go:3) RET
0x0040 00064 (x.go:5) MOVQ $0, AX
0x0042 00066 (x.go:5) MOVQ AX, 48(SP)
0x0047 00071 (x.go:3) JMP 59
Of course, the old backend is still well ahead:
"".f_ssa t=1 size=48 value=0 args=0x20 locals=0x0
0x0000 00000 (x.go:3) TEXT "".f_ssa(SB), $0-32
0x0000 00000 (x.go:3) NOP
0x0000 00000 (x.go:3) NOP
0x0000 00000 (x.go:3) FUNCDATA $0, gclocals·a8eabfc4a4514ed6b3b0c61e9680e440(SB)
0x0000 00000 (x.go:3) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (x.go:4) CMPB "".b+24(FP), $0
0x0005 00005 (x.go:4) JNE 17
0x0007 00007 (x.go:5) MOVQ $0, "".~r3+32(FP)
0x0010 00016 (x.go:5) RET
0x0011 00017 (x.go:7) MOVQ "".i+8(FP), BX
0x0016 00022 (x.go:7) MOVQ "".j+16(FP), BP
0x001b 00027 (x.go:7) ADDQ BP, BX
0x001e 00030 (x.go:7) MOVQ BX, "".~r3+32(FP)
0x0023 00035 (x.go:7) RET
Some regalloc improvements should help considerably.
Change-Id: I95bb5dd83e56afd70ae4e983f1d32dffd0c3d46a
Reviewed-on: https://go-review.googlesource.com/13142
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Josh Bleecher Snyder
parent
d1c15a0e3e
commit
f91ff1a509
@ -65,6 +65,7 @@ var passes = [...]pass{
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{"generic deadcode", deadcode},
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{"dse", dse},
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{"fuse", fuse},
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{"tighten", tighten}, // move values closer to their uses
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{"lower", lower},
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{"lowered cse", cse},
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{"lowered deadcode", deadcode},
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@ -94,6 +95,11 @@ var passOrder = [...]constraint{
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{"nilcheckelim", "generic deadcode"},
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// nilcheckelim generates sequences of plain basic blocks
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{"nilcheckelim", "fuse"},
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// tighten should happen before lowering to avoid splitting naturally paired instructions such as CMP/SET
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{"tighten", "lower"},
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// tighten will be most effective when as many values have been removed as possible
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{"generic deadcode", "tighten"},
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{"generic cse", "tighten"},
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// don't layout blocks until critical edges have been removed
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{"critical", "layout"},
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// regalloc requires the removal of all critical edges
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70
src/cmd/compile/internal/ssa/tighten.go
Normal file
70
src/cmd/compile/internal/ssa/tighten.go
Normal file
@ -0,0 +1,70 @@
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// Copyright 2015 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 ssa
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// tighten moves Values closer to the Blocks in which they are used.
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// This can reduce the amount of register spilling required,
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// if it doesn't also create more live values.
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// For now, it handles only the trivial case in which a
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// Value with one or fewer args is only used in a single Block.
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// TODO: Do something smarter.
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// A Value can be moved to any block that
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// dominates all blocks in which it is used.
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// Figure out when that will be an improvement.
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func tighten(f *Func) {
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// For each value, the number of blocks in which it is used.
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uses := make([]int, f.NumValues())
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// For each value, one block in which that value is used.
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home := make([]*Block, f.NumValues())
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changed := true
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for changed {
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changed = false
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// Reset uses
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for i := range uses {
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uses[i] = 0
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}
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// No need to reset home; any relevant values will be written anew anyway
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for _, b := range f.Blocks {
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for _, v := range b.Values {
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for _, w := range v.Args {
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uses[w.ID]++
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home[w.ID] = b
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}
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}
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if b.Control != nil {
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uses[b.Control.ID]++
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home[b.Control.ID] = b
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}
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}
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for _, b := range f.Blocks {
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for i := 0; i < len(b.Values); i++ {
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v := b.Values[i]
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if v.Op == OpPhi {
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continue
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}
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if uses[v.ID] == 1 && home[v.ID] != b && len(v.Args) < 2 {
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// v is used in exactly one block, and it is not b.
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// Furthermore, it takes at most one input,
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// so moving it will not increase the
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// number of live values anywhere.
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// Move v to that block.
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c := home[v.ID]
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c.Values = append(c.Values, v)
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v.Block = c
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last := len(b.Values) - 1
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b.Values[i] = b.Values[last]
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b.Values[last] = nil
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b.Values = b.Values[:last]
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changed = true
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}
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}
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}
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}
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}
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