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Subject: [Bug tree-optimization/55875] [4.8 Regression] IVopts caused miscompilation
Date: Fri, 04 Jan 2013 12:24:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55875

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-01-04 12:24:12 UTC ---
ssa name _12
  type long unsigned int
  base 4294967295
  step 1
ssa name _13
  type long unsigned int
  base 34359738360
  step 8
ssa name _14
  type const struct A *
  base a_4 + 34359738360
  step 8
  base object (void *) a_4

all look wrong to me, it ignores the wrapping in the narrower type.

OT, I wonder why number of iterations analysis doesn't figure out smaller
number:
Analyzing # of iterations of loop 1
  exit condition [1, + , 1] < (unsigned int) n_5
  bounds on difference of bases: 0 ... 4294967294
  result:
    # of iterations (unsigned int) n_5 + 4294967295, bounded by 4294967294
The loop condition is:
  i_18 = i_21 + 1;
  if (i_18 < _20)
and _20 is:
  _20 = (unsigned int) n_5;
where
  short unsigned int n;
thus, I'd thought we should figure out from that that i_18 will be at most
0xffff (highest possible n_5 is 0xffff).
But that is 4.9 material likely, while the ivopts? bug is a severe 4.8 issue.