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On Fri, May 17, 2013 at 9:39 PM, Ryan Hill <dirtyepic@g.o> wrote: |
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> On Wed, 15 May 2013 16:59:57 +0200 |
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> yac <yac@g.o> wrote: |
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> |
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>> Hi, |
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>> |
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>> I was recently investigating what cpu flags do I have and how does it |
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>> work. I have put what I have so far at [1]. |
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>> |
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>> So I thought I let you know in case someone wants to chip in. |
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>> |
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>> [1] https://github.com/yaccz/cufd |
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> |
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> I've seen gcc -Q --help=target giving false results before due to the way that |
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> options are parsed and some flags that set other flags not getting processed on |
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> the --help code path. The second option you list is better. |
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> |
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> -native doesn't set 3dnow, mmx, sse, sse2, sse3, ssse3 or sse4a. I'm guessing |
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> they're just handed through the -march setting. |
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Yes, they are. |
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> sys-apps/cpuid is awesome. |
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> |
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> MMX2/MMXEXT still confuses me. |
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SSE1 and /Enhanced/ 3DNow! added some extra MMX instructions. Some |
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(pshufw and pmulhuw particularly) turn out to be rather useful in |
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software compositing. I use them in the pixman MMX code. |
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See http://mattst88.com/programming/asmref/ (Sorry about the broken |
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search box. It works, you just don't see what you type) |
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Set Requires = Enhanced 3DNow!. The instructions that are listed as |
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'Enhanced 3DNow! or SSE1' are what are known as MMX2 or MMXEXT. |
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The particularly annoying thing about using them is that there's no |
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-mmmx2 or -mmmxext, but turning on -msse causes illegal instructions |
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to be generated for old AMD or Geode CPUs, while -m3dnow causes the |
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same problems for Intel CPUs. And, there's not actually even an |
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-m3dnowext flag (anymore?) anyway, so it's kind of a mess. |
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