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cmd/compile: fix bounds check regressions from Go 1.6 #14721

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rsc opened this issue Mar 8, 2016 · 4 comments
Closed

cmd/compile: fix bounds check regressions from Go 1.6 #14721

rsc opened this issue Mar 8, 2016 · 4 comments
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Go1.7Early

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@rsc
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rsc commented Mar 8, 2016

The old compiler knows that

var x [16]byte
z := x[i&15]
z := x[i%16]
...

does not need bounds checks. We need to make sure that the new SSA compiler knows this too. Today it seems not to, in certain cases.

Per discussion today.
@rsc rsc added this to the Go1.7Early milestone Mar 8, 2016
@randall77
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Note that [i%16] can only skip the bounds check if i is unsigned (or can otherwise be proved >= 0).

@brtzsnr
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brtzsnr commented Mar 8, 2016

The first one can be done with rewrites easily. Something like.

(IsInBounds (And64 (Const64 [c]) x) (Const64 [d])) && c < d -> (ConstBool [1]).

Is something more general than this you have in mind?

@rsc
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rsc commented Mar 8, 2016 via email

@gopherbot
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CL https://golang.org/cl/20482 mentions this issue.

@golang golang locked and limited conversation to collaborators Mar 13, 2017
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