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cmd/compile: remove bounds checking for sub-slices #14905

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OneOfOne opened this issue Mar 22, 2016 · 6 comments
Closed

cmd/compile: remove bounds checking for sub-slices #14905

OneOfOne opened this issue Mar 22, 2016 · 6 comments
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@OneOfOne
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Please answer these questions before submitting your issue. Thanks!

  1. What version of Go are you using (go version)?
    go version devel +259b7ed 2016-03-22 00:18:31 +0000 linux/amd64
  2. What operating system and processor architecture are you using (go env)?
    Linux Ava 4.4.5-1-ARCH #1 SMP PREEMPT Thu Mar 10 07:38:19 CET 2016 x86_64 GNU/Linux
  3. What did you do?
func a(in []byte) uint64 {
    return uint64(in[0]) | uint64(in[1])<<8 | uint64(in[2])<<16 | uint64(in[3])<<24 |
        uint64(in[4])<<32 | uint64(in[5])<<40 | uint64(in[6])<<48 | uint64(in[7])<<56
}

func b(p []byte) uint64 {
        p = p[:16]
    return a(p[8:8])
}
  1. What did you expect to see?
    No check for p[8:8]
  2. What did you see instead?
        0x001a 00026 (blah.go:10)       CMPQ    CX, $8
        0x001e 00030 (blah.go:10)       JCS     $0, 39

On a side note, by inlining that function by hand, some of my code is now faster than using unsafe, so there's that.

Amazing job with the SSA branch guys.
@ianlancetaylor ianlancetaylor added this to the Unplanned milestone Mar 22, 2016
@ianlancetaylor
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CC @randall77 @dr2chase

@randall77
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I think you mean p[8:16], not p[8:8]. p[8:8] is guaranteed to panic when given to a.

But I think your general issue is still there. p[8:16] looks ok, but p[7:15] has the same extra comparison in there that p[8:8] has. After we do cap(p)>=16, we know to get rid of another cap(p)>=16, but not a cap(p)>=15.

@brtzsnr

@brtzsnr
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brtzsnr commented Mar 22, 2016

It should be p = p[:16:len(p)] otherwise you extend p over the original length. This is binary.BigEndian.Uint64(), right? Why not use that instead, we might be able to do something faster than your example.

@brtzsnr
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brtzsnr commented Mar 22, 2016

There is some more opportunity here to optimize:

    v9 = Const64 <int> [16]
    v8 = SliceLen <int> v7
    v17 = IsSliceInBounds <bool> v9 v8

So we have v8 >= v9. Later

    v25 = Const64 <int> [8]
    v35 = Eq64 <bool> v25 v8

v35 cannot be true. Related to #14900.

gopherbot pushed a commit that referenced this issue Mar 22, 2016
@brtzsnr
cmd/compile: simplify SliceCap when it equals SliceLen
e61db31 
Shows up occassionally, especially after p = p[:8:len(p)]

Updates #14905

Change-Id: Iab35ef2eac57817e6a10c6aaeeb84709e8021641
Reviewed-on: https://go-review.googlesource.com/21025
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
@brtzsnr
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brtzsnr commented Apr 1, 2016

I think this is fixed after https://go-review.googlesource.com/#/c/21008/

@OneOfOne
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OneOfOne commented Apr 2, 2016

Yep, I just confirmed, closing.

@OneOfOne OneOfOne closed this as completed Apr 2, 2016
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@brtzsnr @OneOfOne @ianlancetaylor @randall77 @gopherbot