possible compiler optimization error

[Mike Marchywka]

Send some disassembled code fragments- it should be pretty clear.
Or, you can probably cast  and dump as hex/bin and see what is going on.








>From: "Frederich, Eric P21322" <eric.frederich@siemens.com>
>To: <cygwin@cygwin.com>
>Subject: RE: possible compiler optimization error
>Date: Thu, 28 Jun 2007 14:28:16 -0400
>
> > From: cygwin-owner@cygwin.com On Behalf Of Brian Dessent
> > Sent: Thursday, June 28, 2007 1:53 PM
> > To: cygwin@cygwin.com
> > 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.
>Of
> > course they look the same if you limit the precision!  But that's not
>how the
> > 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
>decimal places.
>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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