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cmd/compile: redundant jumps #14277

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brtzsnr opened this issue Feb 9, 2016 · 5 comments
Closed

cmd/compile: redundant jumps #14277

brtzsnr opened this issue Feb 9, 2016 · 5 comments
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Go1.7

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@brtzsnr
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brtzsnr commented Feb 9, 2016

dev.ssa: The following code

// gostress -seed 31876
package main
import "fmt"
var b2i = map[bool]int{true:1}
func main() {
fmt.Println(f1_ssa(false, 0, 1, ))
}
func f1_ssa(a1 bool, a2 uint8, a3 int64, ) int {
switch {} // prevent inlining
a3 += a3 << 1 // int64
v1 := (a3 & a3) + (a3 | a3) + (a3 ^ (a3 - a3)) << ((1 * (2 + 3)) | 1) // int64
_ = v1
a2--
v2 := (a2 != a2) || (a1 && a1) || a1 || (v1 == v1) // bool
_ = v2
a2--
v2 = v2 // bool
v3 := (v1 * v1) ^ (a3 >> 1) * (a3 << (a2 | a2)) * v1 // int64
_ = v3
return 0 + (int(1) >> (2 - 1) * 2)
}

results in a lot of redundant jumps. After the trim pass:

b3:
v1 = InitMem <mem>
v9 = VarDef <mem> {~r3} v1
v2 = SP <uintptr> : SP
v10 = MOVQstoreconst <mem> {~r3} v2 v9
v85 = Arg <bool> {a1} : a1[bool]
v32 = LoadReg <bool> v85 : AX
v50 = TESTB <flags> v32 v32
NE v50 → b4 b4
b4: ← b3 b3
NE v50 → b10 b8
b10: ← b8 b8 b4
v52 = VarDef <mem> {~r3} v10
v53 = MOVQstoreconst <mem> {~r3} v2 v52
Ret v53
b8: ← b4
NE v50 → b10 b10

and the code generated

00070 (f.go:8)  TEXT    "".f1_ssa(SB), $0
00071 (f.go:8)  FUNCDATA    $0, "".gcargs·2(SB)
00072 (f.go:8)  FUNCDATA    $1, "".gclocals·3(SB)
00073 (f.go:8)  TYPE    "".a1(FP)type.bool, $1
00074 (f.go:8)  TYPE    "".a2+1(FP)type.uint8, $1
00075 (f.go:8)  TYPE    "".a3+8(FP)type.int64, $8
00076 (f.go:8)  TYPE    "".~r3+16(FP)type.int, $8
00077 (f.go:8)  TYPE    "".v1(SP)type.int64, $8
00078 (f.go:8)  TYPE    "".v2(SP)type.bool, $1
00079 (f.go:8)  TYPE    "".v3(SP)type.int64, $8
00080 (f.go:8)  TYPE "".autotmp_0006(SP)type.int64, $8
00081 (f.go:8)  TYPE "".autotmp_0007(SP)type.uint8, $1
00082 (f.go:8)  TYPE "".autotmp_0008(SP)type.uint8, $1
v9
00083 (f.go:8)  VARDEF  "".~r3+16(FP)
v10
00084 (f.go:8)  MOVQ    $0, "".~r3+16(FP)
v32
00085 (f.go:8)  MOVB    "".a1(FP), AX
v50
00086 (f.go:8)  TESTB   AX, AX
b3
00087 (f.go:14) JEQ 88
b4
00088 (f.go:14) JEQ 92
v52
00089 (f.go:20) VARDEF  "".~r3+16(FP)
v53
00090 (f.go:20) MOVQ    $0, "".~r3+16(FP)
b10
00091 (f.go:20) RET
b8
00092 (f.go:14) JNE 89
b8
00093 (f.go:14) JMP 89
00094 (<unknown line number>)   END

For example JEQ 92 branch cannot be taken.
@ianlancetaylor ianlancetaylor added this to the Go1.7 milestone Feb 9, 2016
@ianlancetaylor
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CC @randall77

@gopherbot
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CL https://golang.org/cl/19414 mentions this issue.

@gopherbot
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CL https://golang.org/cl/19464 mentions this issue.

gopherbot pushed a commit that referenced this issue Feb 17, 2016
@brtzsnr
[dev.ssa] cmd/compile/internal/ssa: transform degenerate control blocks
c67cac0 
* In cases where we end up with empty branches like in
if a then jmp b else jmp b;
the flow can be replaced by a; jmp b.

The following functions is optimized as follows:
func f(a bool, x int) int {
        v := 0
        if a {
                v = -1
        } else {
                v = -1
        }
        return x | v
}

Before this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:362)  MOVQ  $0, "".~r2+16(FP)
02821 (arith_ssa.go:362)  MOVB  "".a(FP), AX
02822 (arith_ssa.go:362)  TESTB AX, AX
02823 (arith_ssa.go:364)  JEQ 2824
02824 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02825 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02826 (arith_ssa.go:369)  RET

After this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02821 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02822 (arith_ssa.go:369)  RET

Updates #14277

Change-Id: Ibe7d284f43406c704903632a4fcf2a4a64059686
Reviewed-on: https://go-review.googlesource.com/19464
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
@brtzsnr
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brtzsnr commented Feb 17, 2016

Not closing because a few cases are not handled yet. I'm looking for more examples.

@brtzsnr
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brtzsnr commented Apr 1, 2016

Fixed now. The generated code looks very nice:

        a.go:20 0x401110        48c744241800000000      MOVQ $0x0, 0x18(SP)
        a.go:20 0x401119        c3                      RET

@brtzsnr brtzsnr closed this as completed Apr 1, 2016
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