[LLVMdev] induction variable computation not preserving scev

[Nick Lewycky]

On 19 January 2011 13:01, Dan Gohman <gohman at apple.com> wrote:

>
> On Jan 18, 2011, at 12:32 AM, Nick Lewycky wrote:
>
> > Hi,
> >
> > I tracked down a bug in indvars where we weren't updating SCEV properly.
> The attached patch shows the fix to this bug with a testcase, but it also
> causes five new test failures.
>
> Indvars isn't restructuring the loop there, so it ideally shouldn't
> need to call forgetLoop().


Hmm? It certainly is, it's changing the condition on the branch instruction
that makes up the loop latch!

Offhand, is it possible that the problem
> here is the same as the one in PR8037?
>

No, that only shows up when you try to run "opt -analyze -indvars
-scalar-evolution" by causing a crash. With that patch in place, you can see
that "opt -analyze -indvars -scalar-evolution" produces different values for
the SCEVs than you get with "opt -indvars | opt -analyze -scalar-evolution"
which led me to add the forgetLoop here.


> > Would someone be willing to take a look at the failures and figure out
> why we're getting worse output once this patch is applied? If I had to
> hazard a guess, I'd say that indvars is probably emitting code that can't be
> analyzed by SCEV as easily as the original.
>
> It's certainly plausible. It's easy to loose track of things like
> overflow behavior when folding casts into arithmetic, for example.
>
> Dan
>
>
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