[llvm-dev] beneficial optimization of undef examples needed

John Regehr via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 16 15:19:09 PDT 2017

I'll repeat that open-ended requests would that end up generating lots 
of work for other people probably aren't going to get great results here.


On 6/16/17 4:03 PM, Peter Lawrence via llvm-dev wrote:
> All,
>      These discussions seem to be based on the premise that there is a
> need for the compiler to exploit undefined behavior for performance
> optimization reasons.
> So far the only beneficial optimization I am aware of that relies on some
> form of “undefined” is Dan Gohman’s original project for LP64 targets of
> promoting i32 induction variables to i64 and hoisting sign-extension out
> of the loop.
> But “undef” / “poison” never appears in either the original or the transformed
> IR for these types of loops, instead properties of “+nsw” are used to
> justify the transformation.  The transformation does not just fall out because
> we’ve done a good job at defining “undef” / “poison” IR nodes.
> So I’d like to see some concrete examples of where the compiler can
> do useful optimization based on “undef” / “poison” appearing explicitly
> In the IR,  finding some would surely advance this discussion.
> Peter Lawrence.
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