[llvm] r221318 - Analysis: Make isSafeToSpeculativelyExecute fire less for divides

[Chandler Carruth]

On Sat, Nov 8, 2014 at 3:24 PM, Nick Lewycky <nicholas at mxc.ca> wrote:

> David Majnemer wrote:
>
>> It's a correctness issue.
>>
>> In general one cannot know whether or not a divide instruction is safe
>> to execute because isKnownNonZero returns true for poison.
>>
>> It is possible for the right hand side of the divide to always be poison
>> in a valid program so long as the divide cannot ever be executed.
>>
>
> Actually wait, how does that happen in practice? You need to have already
> had an integer overflow or something to trigger poison, right?
>

I think poison may be misleading... I think this is true for undef as well
unless you actually *transform* the divisor into some non-zero value so
that undef is collapsed to 1 for example. Otherwise something else can
collapse the undef to 0 invalidating the query. So divide is just not safe
to speculate with an undef (or poison?) divisor, even though such a divisor
satisfies "non-zero". Speculation of divide requires transforming the
divisor.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20141108/f1f67869/attachment.html>