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cmd/compile: for small objects, use constants directly instead of copying from statictmp #21561

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randall77 opened this Issue Aug 22, 2017 · 1 comment

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randall77 commented Aug 22, 2017

func f() []int {
	return []int{7}
}

generates:

0x001d 00029 (tmp1.go:3)  LEAQ    type.[1]int(SB), AX
0x0024 00036 (tmp1.go:4)  MOVQ    AX, (SP)
0x0028 00040 (tmp1.go:4)  CALL    runtime.newobject(SB)
0x002d 00045 (tmp1.go:4)  MOVQ    8(SP), AX
0x0032 00050 (tmp1.go:4)  MOVQ    "".statictmp_0(SB), CX
0x0039 00057 (tmp1.go:4)  MOVQ    CX, (AX)

Note that the 7 is written by loading from a readonly global and writing to the slice. We should just use a

MOVQ $7, (AX)

for the last 2 lines.
Up to some small constant in size, emitting the constants explicitly instead of copying them from a statictmp is better.

@randall77 randall77 added this to the Go1.10 milestone Aug 22, 2017

@josharian

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josharian commented Aug 23, 2017

We do this a few places already. IIRC, the cutoff we have used for "up to some small constant in size" is whether it is SSA-able.

@bradfitz bradfitz modified the milestones: Go1.10, Go1.11 Nov 28, 2017

@bradfitz bradfitz modified the milestones: Go1.11, Unplanned May 18, 2018

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