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Re: [Qemu-devel] [patch] make qemu work with GCC 4

From: Michael Matz
Subject: Re: [Qemu-devel] [patch] make qemu work with GCC 4
Date: Thu, 30 Aug 2007 18:52:19 +0200 (CEST)


On Wed, 29 Aug 2007, Michael Matz wrote:

> > >        I solved that by placing one of the T[012] operands into memory
> > >        for HOST_I386, thereby freeing one reg.  Here's some justification
> > >        of why that doesn't really cost performance: with three free regs
> > >        GCC is already spilling like mad in the snippets, we just trade one
> > >        of those memory accesses (to stack) with one other mem access to
> > >        the cpu_state structure, which will be in cache.
> > 
> > Do you have any evidence to support this claim?
> Not really, only an apple and orange comparison.  A 10000 iteration 
> tests/sha1 run in the same Linux image, with -no-kqemu, on host and target 
> i386:  time ./sha1
> with qemu-0.8.2 (compiled by gcc 3.3-hammer): 7.92 seconds
> with qemu-0.9.0-cvs (gcc4.1 compiled, with the patch): 8.15 seconds
> I'll try to get a better comparison.

So, I've now compared our 0.9.0 package, once without patch compiled by 
3.3-hammer, and once with patch and compiled by gcc 4.2:

gcc33 compiled: 7.81 seconds (i.e. a bit faster than 0.8.2 was)
gcc42 compiled: 8.07 seconds

I.e. 3% slower.


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