c++/6914: -O2 and -O give different results for the same valid FP code

Tim Prince tprince@computer.org
Mon Jun 3 06:30:00 GMT 2002

On Monday 03 June 2002 06:09, Michael Veksler wrote:
> http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&
> So here are my conclusions (after reading the assembly, and running gdb
> on it):
> Floating point on x86 is done on 80 bit registers (IEEE's double
> extended type).
> GCC generates code to exploit all 80 bits during divide. It then spills
> the result
> to memory.
> The second divide is not spilled to memory, and the comparison is done
> between truncated value (which lost accuracy), and a completely accurate
> 80 bit
> value.
> According to http://www.validlab.com/goldberg/paper.ps this does not
> seem right.
> According to this paper, gcc should operate on double precision (64
> bit), or at
> least give the impression that it does so (to the outside viewer). But
> gcc operates
> on a mixed 80/64 bit setting, and that seems contradictory to IEEE spirit
> (and, probably, agains ISO C rules).
Among the remedies available would be 
a) set 53-bit rounding mode
b) choose -msse2, for appropriate targets

Tim Prince

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