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Re: Do C++ signed types have modulo semantics?
- From: Steven Bosscher <stevenb at suse dot de>
- To: Ulrich Weigand <uweigand at de dot ibm dot com>
- Cc: gcc at gcc dot gnu dot org, Robert Dewar <dewar at adacore dot com>, Mark Mitchell <mark at codesourcery dot com>, Michael Veksler <VEKSLER at il dot ibm dot com>, Paul Koning <pkoning at equallogic dot com>, gdr at integrable-solutions dot net, nathan at codesourcery dot com
- Date: Wed, 29 Jun 2005 18:46:34 +0200
- Subject: Re: Do C++ signed types have modulo semantics?
- References: <200506281202.j5SC27s7024579@53v30g15.boeblingen.de.ibm.com> <200506281409.02246.stevenb@suse.de>
On Tuesday 28 June 2005 14:09, Steven Bosscher wrote:
> On Tuesday 28 June 2005 14:02, Ulrich Weigand wrote:
> > Steven Bosscher wrote:
> > > Anyway, I've started a SPEC run with "-O2" vs. "-O2 -fwrapv". Let's
> > > see how big the damage would be ;-)
> >
> > Please make sure to include a 64-bit target, where it actually makes any
> > difference. (I recall performance degradations of 20-30% in some
> > SPECfp cases from getting induction variable reduction wrong ...)
>
> Yeah, I'm testing on an AMD64 box, both 64 bits and 32 bits.
And the numbers are, only those tests that build in both cases,
left is base == "-O2", right is peak == "-O2 -fwrapv:
32-bits 64-bits
164.gzip 733 733 819 820
175.vpr 703 707 718 719
176.gcc 886 892 977 955
181.mcf 527 527 415 414
186.crafty 877 893 1345 1351
253.perlbmk 941 944 971 975
254.gap 769 759 784 782
255.vortex 1094 1086 1153 1122
256.bzip2 708 707 786 782
300.twolf 1037 1030 834 830
168.wupwise 762 755 865 829
171.swim 695 679 696 699
172.mgrid 395 394 741 562
173.applu 590 588 693 656
177.mesa 701 693 1055 1058
179.art 479 484 930 912
183.equake 825 834 840 808
188.ammp 716 723 877 862
200.sixtrack 446 456 434 414
Note that (for unknown reasons) peak is always ~.5% higher than base on
this tester even if you compare identical compilers. So 1% wins are not
really interesting.
What is interesting is the higher score for crafty with -fwrapv for the
32-bits case. The rest is in the noise for 32-bits. For 64-bits, gcc
itself takes a hit and so do vortex and all the SPECfp benchmarks. See
especially mgrid.
Gr.
Steven