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Am Donnerstag 02 August 2007 23:36 schrieb Alexander Skwar: |
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> · Florian Philipp <f.philipp@××××××.de>: |
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> > You see, they are not compatible and even if some code works I wouldn't |
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> > bet multimedia apps will perform well. |
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> > |
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> > With -mtune the instruction set stays the same. It is just "rearranged". |
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> |
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> Hm. Allright. When using just -mtune (ie. without -march), the |
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> docs at |
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> http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/i386-and-x86_002d64-Options.htm |
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>l |
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> |
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> say: |
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> | While picking a specific cpu-type will schedule things appropriately |
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> | for that particular chip, the compiler will not generate any code that |
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> | does not run on the i386 without the -march=cpu-type option being used. |
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> |
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> If -mtune=athlon-xp is used, code is generated which may make |
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> use of 3dNOW!. 3dNOW! is, of course, not to be found on 386 :) |
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> If the instruction set stays the same, code generated with |
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> -mtune=athlon-xp would not be executable on 386 machines, if |
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> I understand you correctly. |
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> |
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> Hm. With -mtune, the set of available instructions (ie. |
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> stuff like 3dNOW!, I suppose?) is NOT changed from the default |
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> of i386, is it? Or what does "Tune to cpu-type everything applicable |
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> about the generated code, except for the ABI and the set of available |
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> instructions." mean - especially note the "except for [...] the set of |
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> available instructions" part. |
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> |
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> So with "-mtune=pentium-m -march=athlon-xp" I'm making the compiler |
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> generate code which is "ordered" the way it's best for pentium-m |
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> machines while allowing it to use athlon-xp instruction set? Is |
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> that what I'm doing? |
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> |
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> If so, then it seems you're right - code will run, but maybe not |
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> so well. |
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> |
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> Is that understanding correct? If so, then I really should think |
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> twice about using "-mtune=pentium-m -march=athlon-xp", shouldn't |
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> I? |
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> |
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> Curious, |
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> |
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> Alexander Skwar |
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> -- |
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|
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At least that's how I understand the issue. At the moment I've got two ideas |
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to solve your problem: |
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|
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1. set march to an inferior target (pentium-3 and pentium-3m seem okay: mmx |
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and sse) and mtune for one (or even both?) of them |
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2. set march to one of them and disable incompatible instruction sets with |
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options like -mno-sse2 or -mno-3dnow |
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