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cmd/compile: missing combine constant store cases on amd64 #53324

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wdvxdr1123 opened this issue Jun 10, 2022 · 4 comments
Closed

cmd/compile: missing combine constant store cases on amd64 #53324

wdvxdr1123 opened this issue Jun 10, 2022 · 4 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. FeatureRequest FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
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@wdvxdr1123
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What version of Go are you using (go version)?

gotip

What did you do?

func a(b []byte, idx int) {
	binary.LittleEndian.PutUint64(b[idx:], 123)
}

What did you expect to see?

MOVQ $123, (AX)(DX*1)

What did you see instead?

MOVB $123, (AX)(DI*1)
MOVB $0, 1(AX)(DI*1)
MOVB $0, 2(AX)(DI*1)
MOVB $0, 3(AX)(DI*1)
MOVB $0, 4(AX)(DI*1)
MOVB $0, 5(AX)(DI*1)
MOVB $0, 6(AX)(DI*1)
MOVB $0, 7(AX)(DI*1)
@gopherbot
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Change https://go.dev/cl/411614 mentions this issue: cmd/compile: combine more constant stores on amd64

@cherrymui cherrymui added Performance NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Jun 10, 2022
@cherrymui cherrymui added this to the Unplanned milestone Jun 10, 2022
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jul 13, 2022
@klauspost
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I am also seeing this for non-constants, eg binary.LittleEndian.PutUint64(a, binary.LittleEndian.Uint64(b)) will for non-trivial cases outputs:

        MOVQ    (AX)(DX*1), DX
[...]
        MOVB    DL, (AX)(DI*1)
        MOVQ    DX, SI
        SHRQ    $8, DX
        MOVL    DX, 1(AX)(DI*1)
        MOVQ    SI, DX
        SHRQ    $40, SI
        MOVW    SI, 5(AX)(DI*1)
        SHRQ    $56, DX
        MOVB    DL, 7(AX)(DI*1)

Load is fine, but the store is split into an 8 byte write, a 32 bit write, a 16 bit write and finally an 8 bit write. Seems rather random.

@randall77
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@klauspost Yes, that doesn't look good. Should be fixable somehow.

@randall77 randall77 modified the milestones: Unplanned, Go1.20 Jul 28, 2022
@randall77
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I'm going to open a second issue for Klaus's example, as there's already a CL to fix the OP's case.

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Labels
compiler/runtime Issues related to the Go compiler and/or runtime. FeatureRequest FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
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