[PATCH] Fix PR33340, SSA corruption due to FRE

Richard Guenther richard.guenther@gmail.com
Sat Sep 8 13:06:00 GMT 2007


On 9/8/07, Daniel Berlin <dberlin@dberlin.org> wrote:
> On 9/7/07, Richard Guenther <rguenther@suse.de> wrote:
> >
> > FRE replaces a load by a SSA_NAME that occurs in an abnormal PHI.
> > Fixed by not value-numbering anything as such SSA_NAME.
> >
> > Danny, does this make sense?
> >
>
> Well, it does have the value of the abnormal ssa name.  It is simply
> the case that we can't do any replacements with that ssa name.
>
> Thus, i'd rather block replacement than value numbering.

I thought about this as well, but for

  b_2 = a_1(ab);
  c_3 = b_2;

we would then value number c_3 as a_1(ab) and prevent FRE of things
involving b_2 as well as a_1(ab).  Which is what we see in the testcase
for the PR.  We have

  tmp_4 = a_2(ab);
  store = tmp_4;
  ...
  x_2 = store;

with the patch we replace the load by tmp_4, but if we value number tmp_4
as a_2(ab) we would try to replace it by a_2(ab) instead and thus reject it.

?

Thanks,
Richard.



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