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[dev.ssa] cmd/compile/internal/ssa: simplify boolean phis
* Decreases the generated code slightly. * Similar to phiopt pass from gcc, except it only handles booleans. Handling Eq/Neq had no impact on the generated code. name old time/op new time/op delta Template 453ms ± 4% 451ms ± 4% ~ (p=0.468 n=24+24) GoTypes 1.55s ± 1% 1.55s ± 2% ~ (p=0.287 n=24+25) Compiler 6.53s ± 2% 6.56s ± 1% +0.46% (p=0.050 n=23+23) MakeBash 45.8s ± 2% 45.7s ± 2% ~ (p=0.866 n=24+25) name old text-bytes new text-bytes delta HelloSize 676k ± 0% 676k ± 0% ~ (all samples are equal) CmdGoSize 8.07M ± 0% 8.07M ± 0% -0.03% (p=0.000 n=25+25) Change-Id: Ia62477b7554127958a14cb27f85849b095d63663 Reviewed-on: https://go-review.googlesource.com/20090 Reviewed-by: Keith Randall <khr@golang.org> Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro> TryBot-Result: Gobot Gobot <gobot@golang.org>
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@ -164,6 +164,7 @@ var passes = [...]pass{
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{name: "zero arg cse", fn: zcse, required: true}, // required to merge OpSB values
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{name: "opt deadcode", fn: deadcode}, // remove any blocks orphaned during opt
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{name: "generic cse", fn: cse},
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{name: "phiopt", fn: phiopt},
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{name: "nilcheckelim", fn: nilcheckelim},
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{name: "prove", fn: prove},
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{name: "generic deadcode", fn: deadcode},
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86
src/cmd/compile/internal/ssa/phiopt.go
Normal file
86
src/cmd/compile/internal/ssa/phiopt.go
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@ -0,0 +1,86 @@
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package ssa
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// phiopt eliminates boolean Phis based on the previous if.
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//
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// Main use case is to transform:
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// x := false
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// if b {
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// x = true
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// }
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// into x = b.
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//
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// In SSA code this appears as
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//
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// b0
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// If b -> b1 b2
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// b1
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// Plain -> b2
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// b2
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// x = (OpPhi (ConstBool [true]) (ConstBool [false]))
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//
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// In this case we can replace x with a copy of b.
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func phiopt(f *Func) {
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for _, b := range f.Blocks {
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if len(b.Preds) != 2 || len(b.Values) == 0 {
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continue
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}
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pb0, b0 := b, b.Preds[0]
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for b0.Kind != BlockIf && len(b0.Preds) == 1 {
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pb0, b0 = b0, b0.Preds[0]
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}
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if b0.Kind != BlockIf {
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continue
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}
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pb1, b1 := b, b.Preds[1]
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for b1.Kind != BlockIf && len(b1.Preds) == 1 {
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pb1, b1 = b1, b1.Preds[0]
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}
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if b1 != b0 {
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continue
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}
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// b0 is the if block giving the boolean value.
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var reverse bool
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if b0.Succs[0] == pb0 && b0.Succs[1] == pb1 {
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reverse = false
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} else if b0.Succs[0] == pb1 && b0.Succs[1] == pb0 {
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reverse = true
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} else {
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b.Fatalf("invalid predecessors\n")
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}
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for _, v := range b.Values {
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if v.Op != OpPhi || !v.Type.IsBoolean() || v.Args[0].Op != OpConstBool || v.Args[1].Op != OpConstBool {
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continue
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}
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ok, isCopy := false, false
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if v.Args[0].AuxInt == 1 && v.Args[1].AuxInt == 0 {
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ok, isCopy = true, !reverse
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} else if v.Args[0].AuxInt == 0 && v.Args[1].AuxInt == 1 {
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ok, isCopy = true, reverse
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}
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// (Phi (ConstBool [x]) (ConstBool [x])) is already handled by opt / phielim.
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if ok && isCopy {
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if f.pass.debug > 0 {
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f.Config.Warnl(int(b.Line), "converted OpPhi to OpCopy")
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}
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v.reset(OpCopy)
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v.AddArg(b0.Control)
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continue
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}
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if ok && !isCopy {
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if f.pass.debug > 0 {
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f.Config.Warnl(int(b.Line), "converted OpPhi to OpNot")
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}
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v.reset(OpNot)
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v.AddArg(b0.Control)
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continue
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}
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}
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}
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}
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43
test/phiopt.go
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43
test/phiopt.go
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@ -0,0 +1,43 @@
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// +build amd64
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// errorcheck -0 -d=ssa/phiopt/debug=3
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package main
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func f0(a bool) bool {
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x := false
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if a {
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x = true
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} else {
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x = false
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}
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return x // ERROR "converted OpPhi to OpCopy$"
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}
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func f1(a bool) bool {
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x := false
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if a {
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x = false
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} else {
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x = true
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}
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return x // ERROR "converted OpPhi to OpNot$"
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}
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func f2(a, b int) bool {
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x := true
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if a == b {
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x = false
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}
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return x // ERROR "converted OpPhi to OpNot$"
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}
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func f3(a, b int) bool {
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x := false
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if a == b {
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x = true
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}
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return x // ERROR "converted OpPhi to OpCopy$"
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}
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func main() {
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}
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