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Re: [Qemu-devel] [PATCH] Enabled building of x86_64 code on Mac OS X

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH] Enabled building of x86_64 code on Mac OS X
Date: Tue, 27 Jan 2009 10:21:55 +0100
User-agent: Thunderbird (X11/20081112)

Anthony Liguori wrote:
> Paul Brook wrote:
>> I did say overall performance.
>> My experience with AMD hardware is that you generally get ~10%
>> overall improvement from LP64. Intel hardware (particularly the early
>> 64-bit cores) less so, but it's generally still a win.
>> I guess ILP32 long mode would probably increase that further, and
>> avoid the odd regressions. It's an awful lot of work to implement
>> though, especially on a target that isn't used to having lots of
>> incompatible variants.
> I expected a performance boost.  Since OS X only runs on Intel CPUs, I
> don't think it's dramatically worth it to get a few percent CPU
> improvement.  The reason I asked is that if we decided to drop Cocoa
> in favor of SDL, it would prevent the use of 64-bit.

The extra registers should speed up things quite a bit. While that's not
too much of a problem when emulating x86, which has few registers
anyway, emulating a ppc or x86_64 target should be faster on an x86_64
host. I haven't done measurements though.

As for the plus in code size on x86_64 - that doesn't really apply to
tcg, as that still generates 32-bit code when only 32-bit code is
necessary, which still looks the same on x86_64. Plus it does simplify
64-bit operations.

I would be very surprised if the improvement of emulating x86_64 on x86
vs. x86_64 is only ~10%.


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