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I'm using 4.1.2. Does anyone have any ideas of the best places in my code to look? Experience with common causes? There are some ugly pointer casts (ie, d = (double) (* (float *) p), where p is pointer-to-int) that I'm working on eliminating, but no luck yet.
For what it's worth, -O0 is giving the correct answer. Thanks for any help.

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  • 3
    could you provide a minimum (non) working example ? Commented Jul 2, 2009 at 2:44
  • Are you compiling with -Wall and -Wextra? Commented Jul 2, 2009 at 2:50
  • Sorry, I'm working on it. Heavy going. Commented Jul 2, 2009 at 2:51
  • delta.tigris.org is an excellent tool for automated minimization of a test case. Commented Jul 2, 2009 at 18:27
  • The other thing that would lead to slightly different results with optimization is that the x86 FPU has more precision than specified, so unoptimized code that forces values to be written into the stack frame between operations will truncate the lower bits. Commented May 11, 2011 at 10:45

2 Answers 2

Reset to default
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I'd check for strict aliasing issues, as demonstrated here: http://www.cellperformance.com/mike_acton/2006/06/understanding_strict_aliasing.html

Without knowing exactly what your code does, the mention of "ugly pointer casts" make me suspect aliasing problems.

It would be helpful for you, and make it easier for us to answer, if you provided some code that demonstrated the issue.

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The post stackoverflow.com/questions/83962/… shows a similar problen
Compiling with Wall (suggested above) did it. The bad line is (p1 + dcb.ntrc_hdr + rNTW + t) = * (int *) &ftmp;
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Thanks everyone, -fno-strict-aliasing (suggested by several) solved my problem. Thanks for all your help.
Lesson learned: Always compile with warning flags.

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