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cmd/compile: The bounds check can be removed in u%uint(x) for all x != 0 && x <= len(slice). #15079

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OneOfOne opened this issue Apr 3, 2016 · 6 comments
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cmd/compile: The bounds check can be removed in u%uint(x) for all x != 0 && x <= len(slice). #15079

OneOfOne opened this issue Apr 3, 2016 · 6 comments
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@OneOfOne
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OneOfOne commented Apr 3, 2016

Please answer these questions before submitting your issue. Thanks!

  1. What version of Go are you using (go version)?
    go version devel +29267c2 2016-04-03 01:50:04 +0000 linux/amd64
  2. What operating system and processor architecture are you using (go env)?
  3. What did you do?
    http://play.golang.org/p/gWCa1r06CI
  4. What did you expect to see?
    bb (and maybe aa)'s bounds checking removed
  5. What did you see instead?
        0x0013 00019 (blah.go:21)       MOVQ    $0, "".autotmp_6+6(SP)
        0x001c 00028 (blah.go:21)       MOVQ    $0, "".autotmp_6+8(SP)
        0x0025 00037 (blah.go:22)       MOVQ    $-3689348814741910323, AX
        0x002f 00047 (blah.go:22)       MOVQ    $32768, CX
        0x0036 00054 (blah.go:22)       MULQ    CX
        0x0039 00057 (blah.go:22)       SHRQ    $3, DX
        0x003d 00061 (blah.go:22)       LEAQ    (DX)(DX*4), AX
        0x0041 00065 (blah.go:22)       SHLQ    $1, AX
        0x0044 00068 (blah.go:22)       SUBQ    $32768, AX
        0x004a 00074 (blah.go:22)       NEGQ    AX
        0x004d 00077 (blah.go:22)       CMPQ    AX, $10
        0x0051 00081 (blah.go:22)       JCC     $0, 101
        0x0053 00083 (blah.go:21)       LEAQ    "".autotmp_6+6(SP), CX
        0x0058 00088 (blah.go:22)       MOVBLZX (CX)(AX*1), CX
        0x005c 00092 (blah.go:22)       MOVB    CL, "".~r0+24(FP)
        0x0060 00096 (blah.go:22)       ADDQ    $16, SP
        0x0064 00100 (blah.go:22)       RET
        0x0065 00101 (blah.go:22)       PCDATA  $0, $0
        0x0065 00101 (blah.go:22)       CALL    runtime.panicindex(SB)

I could've sworn there was a bug about this, but I can't find it
@randall77
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I would assert that this bug is about all this arithmetic not constant-folding away to nothing.

@cznic
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cznic commented Apr 3, 2016

Note: Wrt issue title: The bounds check can be removed in u%uint(x) for all x != 0 && x <= len(slice).

@OneOfOne OneOfOne changed the title cmd/compile: remove bounds checking for u%uint(len(slice)) cmd/compile: The bounds check can be removed in u%uint(x) for all x != 0 && x <= len(slice). Apr 3, 2016
@minux
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minux commented Apr 4, 2016 via email

@brtzsnr
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brtzsnr commented Apr 4, 2016

@brtzsnr

Also, len(slice) can be 0 which means division by zero.

@gopherbot
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CL https://golang.org/cl/21501 mentions this issue.

@gopherbot
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CL https://golang.org/cl/21502 mentions this issue.

gopherbot pushed a commit that referenced this issue Apr 4, 2016
@brtzsnr
cmd/compile: fold IsInBounds of modulo.
68325b5 
In b we only need the division by 0 check.

func b(i uint, v []byte) byte {
    return v[i%uint(len(v))]
}

Updates #15079.

Change-Id: Ic7491e677dd57cd6ba577efbce576dbb6e023cbd
Reviewed-on: https://go-review.googlesource.com/21502
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ahmed Waheed <oneofone@gmail.com>
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@brtzsnr @minux @cznic @OneOfOne @randall77 @gopherbot