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Duncan wrote: |
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> |
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> I was somewhat aware of that, but hadn't considered the effect on loops, |
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> and don't understand it enough to be able explain it as you did, nor enough |
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> to grok why if it's so much more efficient, gcc doesn't do it by default |
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> at least on archs sufficiently specified to know the instructions are |
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> there and that it makes sense. |
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- From my reading tree-vectorize attempts to use SIMD wherever possible |
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for parallel computation of arrays/etc. In theory that should almost |
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always be a net-benefit with few drawbacks. |
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The problem I understand is that it is sometimes a bit buggy - ie it |
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sometimes creates broken code. I think these issues have mostly been |
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fixed, but that would explain why it is not applied by default. |
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There is also an -ftree-vectorize-verbose=# parameter which generates |
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informational messages about why particular loops were or were not |
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vectorized. In theory this can help you develop more-easily-optimized code. |
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MMX/etc can tremendously improve program speed. Anytime you can do 4 |
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operations per cycle vs 1 you're going to improve throughput. |
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Otherwise, I agree with you as far as reducing memory footprint goes - |
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I've been running -Os for ages and I'm very happy with this. My RAM is |
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better applied to disk caching than storing unrolled loops in almost all |
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cases. I'm sure in niche cases the opposite is true, but the same |
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applies to -ffast-math and other dangerous optimizations. They should |
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probably be applied on a per-file basis by the developer, and not across |
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an entire build/system. |
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