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Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000
- From: Duncan Sands <duncan dot sands at math dot u-psud dot fr>
- To: gcc at gcc dot gnu dot org
- Cc: Vladimir Makarov <vmakarov at redhat dot com>, Chris Lattner <clattner at apple dot com>, Evan Cheng <evan dot cheng at apple dot com>
- Date: Wed, 13 May 2009 13:51:00 +0200
- Subject: Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000
- References: <4A098018.1030900@redhat.com> <4A09BA64.1010601@redhat.com> <4A09CD2A.7050805@redhat.com>
Hi,
> Sorry, I missed to mention that I used an additional option -mpc64 for
> 32-bit GCC4.4. It is not possible to generate SPECFP2000 expected
> results by GCC4.4 without this option. LLVM does not support this
> option. And this option can significantly improve the performance. So
> 32-bit comparison of SPECFP2000 should be taken with a grain of salt.
what does -mpc64 do exactly? The gcc docs say:
`-mpc64' rounds the the significands of results of floating-point operations to 53 bits (double precision)
Does this mean that a rounding operation is performed after each fp
operation, or that optimizations are permitted that don't result in
accurate extended double precision values as long as they are correct
to 53 bits, or something else?
The LLVM code generators have an option called -limit-float-precision:
-limit-float-precision=<uint> - Generate low-precision inline sequences for some float libcalls
I'm not sure what it does exactly, but perhaps it is similar to -mpc64?
Ciao,
Duncan.