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RE: possible compiler optimization error
> From: email@example.com On Behalf Of Brian Dessent
> Sent: Thursday, June 28, 2007 1:53 PM
> To: firstname.lastname@example.org
> Subject: Re: possible compiler optimization error
Thanks for looking at it. I am in unfamiliar water here.
> Try with -ffloat-store. Or if you have a sse2 capable
> machine, set the
> appropriate -march= and use -mfpmath=sse. Both of these attempt to
> bypass problems caused by the excess precision of 80 bit double on
> i387. If they fix the problem, it's a bug in your code, not
> anything to do with the compiler.
-mfpmath=sse didn't work but -msse did. Here are some new findings...
-ffloat-store -O2 passes
-march=i686 -O2 fails
-march=i686 -sse -O2 fails
-march=i686 -sse2 -O2 passes
> It looks like you limit the precision in the
> output in your printfs to 15 places, but then you don't understand why
> comparison operators don't compare the same... that is very
> telling, in that you don't understand the excess precision problem.
> course they look the same if you limit the precision! But that's not
> comparison operators work, as they operate on the raw 80 bit values.
I do realize that they may in fact differ way out there beyond 15
What I don't understand is how two numbers pass a ==, then fail a >=,
then pass a >= unless (after compiler optimizations) the second and
third comparisons are actually comparing copies of these numbers which
aren't "bit-exact" copies.
Is this what you're saying might be happening and what -ffloat-store is
supposed to resolve?
If so, that makes sense and I can accept that.
> If you want a definitive answer then you need to provide a standalone
> testcase that compiles. Sample code taken out of context
> that can't be compiled is significantly less useful.
I really want to but it is a huge program and I am afraid that if I
create a chopped down example I can't guarantee that the same
optimizations will happen.
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