cmd/compile: some codes in DSE rely on the order of values

[fengyoulin]

Go version

go version devel go1.24-3ca78afb3b Mon Nov 18 04:56:52 2024 +0000 linux/amd64

Output of go env in your module/workspace:

GOAMD64='v1'
GOARCH='amd64'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOOS='linux'
GOVERSION='devel go1.24-3ca78afb3b Mon Nov 18 04:56:52 2024 +0000'

What did you do?

Compile the following code, the first time with no -gcflags, the second with -gcflags='-d=ssa/check/on', then compare the assembly code.

func foo(v uint64) (b [8]byte) {
    b[0] = byte(v)
    b[1] = byte(v >> 8)
    b[2] = byte(v >> 16)
    b[3] = byte(v >> 24)
    b[4] = byte(v >> 32)
    b[5] = byte(v >> 40)
    b[6] = byte(v >> 48)
    b[7] = byte(v >> 56)
    return b
}

What did you see happen?

The assembly code generated without -gcflags:

  foo.go:5              0x464fc0                88442408                MOVB AL, 0x8(SP)
  foo.go:6              0x464fc4                4889c1                  MOVQ AX, CX
  foo.go:6              0x464fc7                48c1e808                SHRQ $0x8, AX
  foo.go:6              0x464fcb                88442409                MOVB AL, 0x9(SP)
  foo.go:7              0x464fcf                4889c8                  MOVQ CX, AX
  foo.go:7              0x464fd2                48c1e910                SHRQ $0x10, CX
  foo.go:7              0x464fd6                884c240a                MOVB CL, 0xa(SP)
  foo.go:8              0x464fda                4889c1                  MOVQ AX, CX
  foo.go:8              0x464fdd                48c1e818                SHRQ $0x18, AX
  foo.go:8              0x464fe1                8844240b                MOVB AL, 0xb(SP)
  foo.go:9              0x464fe5                4889c8                  MOVQ CX, AX
  foo.go:9              0x464fe8                48c1e920                SHRQ $0x20, CX
  foo.go:9              0x464fec                884c240c                MOVB CL, 0xc(SP)
  foo.go:10             0x464ff0                4889c1                  MOVQ AX, CX
  foo.go:10             0x464ff3                48c1e828                SHRQ $0x28, AX
  foo.go:10             0x464ff7                8844240d                MOVB AL, 0xd(SP)
  foo.go:11             0x464ffb                4889c8                  MOVQ CX, AX
  foo.go:11             0x464ffe                48c1e930                SHRQ $0x30, CX
  foo.go:11             0x465002                884c240e                MOVB CL, 0xe(SP)
  foo.go:12             0x465006                48c1e838                SHRQ $0x38, AX
  foo.go:12             0x46500a                8844240f                MOVB AL, 0xf(SP)
  foo.go:13             0x46500e                c3                      RET

And the assembly code generated with -gcflags='-d=ssa/check/on':

  foo.go:4              0x464fc0                48c744240800000000      MOVQ $0x0, 0x8(SP) // the redundant zeroing
  foo.go:5              0x464fc9                88442408                MOVB AL, 0x8(SP)
  foo.go:6              0x464fcd                4889c1                  MOVQ AX, CX
  foo.go:6              0x464fd0                48c1e808                SHRQ $0x8, AX
  foo.go:6              0x464fd4                88442409                MOVB AL, 0x9(SP)
  foo.go:7              0x464fd8                4889c8                  MOVQ CX, AX
  foo.go:7              0x464fdb                48c1e910                SHRQ $0x10, CX
  foo.go:7              0x464fdf                884c240a                MOVB CL, 0xa(SP)
  foo.go:8              0x464fe3                4889c1                  MOVQ AX, CX
  foo.go:8              0x464fe6                48c1e818                SHRQ $0x18, AX
  foo.go:8              0x464fea                8844240b                MOVB AL, 0xb(SP)
  foo.go:9              0x464fee                4889c8                  MOVQ CX, AX
  foo.go:9              0x464ff1                48c1e920                SHRQ $0x20, CX
  foo.go:9              0x464ff5                884c240c                MOVB CL, 0xc(SP)
  foo.go:10             0x464ff9                4889c1                  MOVQ AX, CX
  foo.go:10             0x464ffc                48c1e828                SHRQ $0x28, AX
  foo.go:10             0x465000                8844240d                MOVB AL, 0xd(SP)
  foo.go:11             0x465004                4889c8                  MOVQ CX, AX
  foo.go:11             0x465007                48c1e930                SHRQ $0x30, CX
  foo.go:11             0x46500b                884c240e                MOVB CL, 0xe(SP)
  foo.go:12             0x46500f                48c1e838                SHRQ $0x38, AX
  foo.go:12             0x465013                8844240f                MOVB AL, 0xf(SP)
  foo.go:13             0x465017                c3                      RET

It seems that the redundant zeroing cannot be removed when compiling with -gcflags='-d=ssa/check/on'.

What did you expect to see?

The redundant zeroing can be removed even when compiling with -gcflags='-d=ssa/check/on'.

[gabyhelp]

Related Issues

• cmd/compile: double zeroing and unnecessary copying/stack use #67957 (closed)
• cmd/compile: eliminate redundant zeroing after lower pass #47107
• cmd/compile: record which instructions clobber flags #12054 (closed)
• cmd/compile: unnecessary zeroing of register on arm #8474 (closed)
• cmd/compile: less optimized AMD64 code #24537
• cmd/compile: unneccesary 0 check in division #45928 (closed)
• cmd/compile: elide unnecessary copies #54613
• cmd/compile: -ssa=0 generates incorrect code for append(s,s) #17039 (closed)
• cmd/compile: flagalloc doesn't delete dead instructions #65039
• cmd/compile: missed optimization when using verbose array initialization #54612

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[fengyoulin]

The sample code comes from issue 47107, which should have been fixed by CL 578376, and only have the problem when compiling with -gcflags='-d=ssa/check/on'.

[gopherbot]

Change https://go.dev/cl/629016 mentions this issue: cmd/compile: do not treat OpLocalAddr as load in DSE

[dmitshur]

CC @golang/compiler.