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doc: mention xmm clobber macros usage

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Originally committed as revision 25364 to svn://svn.ffmpeg.org/ffmpeg/trunk
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Ramiro Polla 2010-10-06 02:46:22 +00:00
parent 326bf69acc
commit 2344dc6bb6
1 changed files with 6 additions and 0 deletions
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doc/optimization.txt
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@ -172,6 +172,12 @@ yasm), you do this by using:
cglobal functon_name, num_args, num_regs, num_xmm_regs
In inline asm, you specify clobbered registers at the end of your asm:
__asm__(".." ::: "%eax").
If gcc is not set to support sse (-msse) it will not accept xmm registers
in the clobber list. For that we use two macros to declare the clobbers.
XMM_CLOBBERS should be used when there are other clobbers, for example:
__asm__(".." ::: XMM_CLOBBERS("xmm0",) "eax");
and XMM_CLOBBERS_ONLY should be used when the only clobbers are xmm registers:
__asm__(".." :: XMM_CLOBBERS_ONLY("xmm0"));
Do not expect a compiler to maintain values in your registers between separate
(inline) asm code blocks. It is not required to. For example, this is bad: