[llvm-dev] IndVarSimplify: getBackedgeTakenCount and Release vs Assert

Michael Kruse via llvm-dev
Wed, 05 Feb 2020 21:02:32 -0000

This is definitely not expected. `getBackedgeTakenCount` caches some
intermediate results (e.g. SCEV normalized forms) and its not unheard
of that these influence other computations. However, the results
should be the same whether its from the cache of freshly computed. It
is also possible that some pass does not correctly invalidate a
result.

Could you file a bug?

Michael

Am Mi., 5. Feb. 2020 um 11:46 Uhr schrieb Jeroen Dobbelaere via
llvm-dev <llvm-dev at lists.llvm.org>:
>
> Hi,
>
> I am investigating a difference in code generation between release and
assert builds of llvm.
> The culprit is IndVarSimplify that comes up with different behavior on the
same input:
>  in the assertion build, it does do an extra 'INDVARS: Rewriting loop
exit condition'
>
> After digging around, it seems that following change is the culprit:
>
> -----
> Author: Philip Reames <listmail at philipreames.com>  2019-08-01
03:16:08
> Committer: Philip Reames <listmail at philipreames.com>  2019-08-01
03:16:08
>
>     Fix a release-only build warning triggered by rL367485
>
>     llvm-svn: 367499
>
>
> [..]
>
>
> +#ifndef NDEBUG
> +  // Used below for a consistency check only
>    const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L);
> +#endif
> -----
>
> It seems that the 'SE->getBackedgeTakenCount(L)' call has
sideeffects.. Is that to be expected ?
> Is the correct solution then to always keep the call ?
>
> Thanks,
>
> Jeroen Dobbelaere
>
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