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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/63986] [5 Regression][SH] gcc.target/sh/pr51244-15.c failures
Date: Thu, 20 Nov 2014 13:13:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63986

--- Comment #5 from ppalka at gcc dot gnu.org ---
(In reply to Richard Biener from comment #3)
> Ok, now already existing forwprop code gets fed with
> 
>   <bb 2>:
>   _3 = a_2(D) == 0;
>   x_4 = (char) _3;
>   _7 = ~_3;
>   _8 = (int) _7;
>   MEM[(int *)d_5(D) + 8B] = _8;
>   if (x_4 != 0)
> 
> where we now in the first forwprop pass identified the opportunity
> to use ~_3 instead of x_4 == 0 thus x_4 is now no longer multi-use.
> This makes us optimize if (x_4 != 0) to if (_3 != 0) which we
> re-optimize in fold_gimple_cond now to '_3' and then of course
> if (_3 != 0) (err, and we return "changed"....) which means we
> now propagate _again_ via forward_propagate_into_gimple_cond
> which now specifically allows aggressive forwarding of compares,
> bypassing single-use restrictions.  See
> 
> 2014-11-16  Patrick Palka  <ppalka@gcc.gnu.org>
> 
>         PR middle-end/63790
>         * tree-ssa-forwprop.c (forward_propagate_into_comparison_1):
>         Always combine comparisons or conversions from booleans.
> 
> thus me fixing my "mistake" does not help anymore.

I worried that such an issue might pop up...

If you think my patch should be reverted, note that there is a more benign
version of the patch here
https://gcc.gnu.org/ml/gcc-patches/2014-11/msg00963.html (which still fixes the
test case I was originally interested in fixing).