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Am 02.05.2015 um 13:25 schrieb Nikos Chantziaras: |
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> On 02/05/15 14:19, Volker Armin Hemmann wrote: |
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>> Am 02.05.2015 um 07:04 schrieb Nikos Chantziaras: |
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>>> On 01/05/15 10:44, Andrew Savchenko wrote: |
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>>>> On Fri, 1 May 2015 05:09:51 +0000 (UTC) Martin Vaeth wrote: |
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>>>>> Andrew Savchenko <bircoph@g.o> wrote: |
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>>>>>> |
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>>>>>> That's why kernel makes sure that no floating point instructions |
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>>>>>> sneaks in using CFLAGS, you may see a lot of -mno-${intrucion_set} |
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>>>>>> flags when running make -V. |
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>>>>> |
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>>>>> So it should be sufficient that the kernel does not use "float" |
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>>>>> or "double", shouldn't it? |
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>>>> |
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>>>> No. Optimizer paths may be very unobvious, i.e. I'll not be |
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>>>> surprised if under some conditions vectorizer may use float |
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>>>> instructions for int code. |
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>>> |
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>>> The kernel uses -O2 and several -march variants (e.g. -march=core2). |
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>>> Several other options are used to prevent GCC from generating |
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>>> unsuitable code. |
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>>> |
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>>> Specifying another -march variant does not affect the optimizer |
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>>> though. It only affects the code generator. If you don't modify the |
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>>> other CFLAGS and only change -march, you will not get FP instructions |
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>>> unless you use FP in the code. |
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>>> |
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>>> Also, I'd be very interested to see *any* optimization that would |
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>>> somehow transform integer code to FP code (note that SIMD is not FP |
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>>> and is perfectly fine in the kernel.) In fact, optimizers tend to |
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>>> transform FP into SIMD, at least on x86 (and other architectures that |
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>>> have fast SIMD instructions.) If I inspect the generated assembly from |
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>>> GCC or Clang, I cannot find FP anywhere, even for code using "float" |
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>>> and "double" operations. They get converted to SIMD on modern CPUs |
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>>> (unless you specify a compiler flag that tells it to use the FPU, for |
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>>> example if you need 80-bit extended precision, which is supported by |
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>>> the x86 FPU.) |
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>>> |
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>>> |
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>>> |
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>> |
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>> http://www.agner.org/optimize/calling_conventions.pdf |
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> |
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> Not sure what you're trying to say. |
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> |
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> |
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> |
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that simd is not save in kernel if not carefully guarded. |
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Really people, just don't fuck around with the cflags. |
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