Re: Strange problems with data-tests.el

[Paul Eggert]

Eli Zaretskii wrote:
> This all looks like a compiler bug

Yes and no. In this area, GCC deliberately doesn't conform to the C standard 
unless you use an option like -fexcess-precision=standard that causes GCC to 
conform but often generate worse code.

The odd results you observed can occur when a C 'double' variable spontaneously 
changes value, e.g., when an 80-bit floating-point register is spilled to a 
64-bit memory location. I reproduced the problem on Fedora 28 and fixed it by 
installing the attached patch into master; please give it a try.

I suspect that it's only bad luck that exposed the problem, that is, that the 
recent bignum or misc changes triggered the bug only because they caused GCC to 
optimize in a different way. In theory if we're unlucky the bug could recur, 
since GCC doesn't make promises in this area. However, I changed arithcompare in 
such a way that GCC would have to be pretty perverse to cause the problem to 
resurface, and similarly for other C compilers.

0001-Port-better-to-x86-fexcess-precision-fast.patch
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