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Re: i386 floating point expertise needed

On Wed, Nov 29, 2017 at 06:54:27PM -0500, Terry Moore wrote:
> > We need someone with i386 floating point knowledge (and I mean, deep down
> > low, what every instruction is supposed to do, and how it is supposed to
> > be used, including all the surrounding support flags words and whatever)
> > to have a look at this, and work out what the problem is.
> > I have not been following, but I do know the 386 from years ago.
> I used to have such knowledge. Unfortunately, I don't know Unix-style 
> 386 assembly, having gained that knowledge in the x86 commercial
> world; but I don't think that's needed.
> The problem is that the 386 does computations with 80-bit numbers, not
> 64-bit.
> The compiler has evidently optimized out a store that would have forced the 
> truncation. 

If this is the cause, it should go away with gcc -ffloat-store.  This
should be easy to confirm...

I though GCC always stored both operands before a comparison.  I'm not
surprised the subtraction test, however, fails.


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