[Bug target/44606] Wrong SPE floating point during computation

[Kyle dot D dot Moffett at boeing dot com]

------- Comment #5 from Kyle dot D dot Moffett at boeing dot com  2010-08-31 14:03 -------
Created an attachment (id=21605)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21605&action=view)
Further stripped testcase with problematic section identified

Ok, I've spent a bit more time fiddling with this testcase, and I believe I can
display exactly where the bug happens.  In the attached "test.c" file, you can
see a section like this:

  saved_r = total_r;
  saved_g = total_g;
  saved_b = total_b;
  Minify(2*x + 10, 15.0);
  save2_r = total_r;
  save2_g = total_g;
  save2_b = total_b;

The "Minify()" macro is supposed to add nonzero values to total_[rgb] but when
compiled with -O2 (or -O1 and a few extra optimizations) the values of
save2_[rgb] are the same as those of saved_[rgb].

I'm also attaching a Makefile I used while testing this program.  In the
Makefile I enable -O1 and then turn off every optimization pass that is not
strictly necessary to reproduce the bug.  The Makefile simply builds 2 copies
of the program, one with -O0 and one with -O2, then runs them and compares
their output.

Some kind of loop optimization or unrolling seems to be involved.  The specific
optimizations which are mandatory for the bug to show up are below, if any of
these is turned off the bug seems to go away:
  -fdce
  -fguess-branch-probability
  -fschedule-insns
  -ftree-ch
  -ftree-dominator-opts
  -ftree-loop-optimize

Cheers,
Kyle Moffett


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44606