cmd/compile: s[:0] has unnecessary comparison #14872

randall77 · 2016-03-18T22:53:11Z

func f(s []int) []int {
    return s[:0]
}

compiles to

    0x0000 00000 (slice0.go:4)  MOVQ    "".s+24(FP), CX
    0x0005 00005 (slice0.go:4)  CMPQ    CX, $0
    0x0009 00009 (slice0.go:4)  JEQ $0, 11
    0x000b 00011 (slice0.go:4)  MOVQ    "".s+8(FP), DX
    0x0010 00016 (slice0.go:4)  MOVQ    DX, "".~r1+32(FP)
    0x0015 00021 (slice0.go:4)  MOVQ    $0, "".~r1+40(FP)
    0x001e 00030 (slice0.go:4)  MOVQ    CX, "".~r1+48(FP)

The comparison is unnecessary, [:0] is always legal. And the branch doesn't go anywhere.

Might be a dup of #14849

@brtzsnr

The text was updated successfully, but these errors were encountered:

brtzsnr · 2016-03-20T20:49:41Z

Looks like a dup of #14849, but let me know if you find more of these cases. The generated code is now

v22 00074 (a.go:23) MOVQ "".s(FP), AX
v10 00075 (a.go:23) MOVQ AX, "".~r1+24(FP)
v13 00076 (a.go:23) MOVQ $0, "".~r1+32(FP)
v21 00077 (a.go:23) MOVQ "".s+16(FP), AX
v27 00078 (a.go:23) MOVQ AX, "".~r1+40(FP)

bradfitz changed the title ~~s[:0] has unnecessary comparison~~ cmd/compile: s[:0] has unnecessary comparison Mar 18, 2016

ianlancetaylor added this to the Go1.7 milestone Mar 19, 2016

randall77 closed this as completed Mar 20, 2016

golang locked and limited conversation to collaborators Mar 20, 2017

gopherbot added the FrozenDueToAge label Mar 20, 2017

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cmd/compile: s[:0] has unnecessary comparison #14872

cmd/compile: s[:0] has unnecessary comparison #14872

randall77 commented Mar 18, 2016

brtzsnr commented Mar 20, 2016

cmd/compile: s[:0] has unnecessary comparison #14872

cmd/compile: s[:0] has unnecessary comparison #14872

Comments

randall77 commented Mar 18, 2016

brtzsnr commented Mar 20, 2016