[llvm-dev] [VSXFMAMutate] OldFMAReg may be wrongly rewritten

Tim Shen via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 29 14:21:06 PST 2016


On Mon, Feb 22, 2016 at 1:06 PM Tim Shen <timshen at google.com> wrote:

> On Fri, Feb 19, 2016 at 5:10 PM Tim Shen <timshen at google.com> wrote:
>> I wonder if we can fix this by making the transformation simpler, that
>> is, instead of doing:
> I wrote a prototype (see attach) for this idea, it actually improves some
> of the test cases (e.g. fma-assoc.ll: test_FMADD_ASSOC1), but pessimize
> several other cases (e.g. test_FMADD_ASSOC_EXT1).
> I'm not sure what to do at this point, I have several options:
> 1) This pass simply omits the optimization for certain cases ("certain"
> needs to be defined).
> 2) This pass should never mutate anything out of the block, and may add a
> copy at the end of the block, if the value is live-out.
> 3) tune the transformation as I attempted, to make it simpler, working,
> and producing better result? It seems interesting to get the improvement as
> my prototype shown without hurting other test cases, but it needs a bit
> thinking and I'd like to fix the compiler crash soon.
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