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On Wednesday 27 September 2006 22:59, Daniel Iliev <danny@××××××××.com> |
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wrote about 'Re: [gentoo-amd64] First Impressions': |
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> Dan Pasanen wrote: |
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> > Ok, im not too sure about this, but i was wondering, i have dual core |
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> > turion64's should that affect if im running -O2 or -O3? currently im |
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> > running -O2. and if i did change it to -O3, what, if anything should |
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> > i recompile? |
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Dual core or not won't affect your CFLAGS much. Worry mostly about the |
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size of your L1 cache, compared to processors of the same architecture |
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being sold today. If yours is fairly large, go with -O3; If undersized, |
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go with -Os; Otherwise, -O2 will serve you well. |
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> I thing it is reasonable to doubt that a program compiled with |
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> "-O3" would greatly outperform the same program compiled with "-O2". |
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This is a reasonable doubt, for reasons already mentioned. |
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> The |
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> same logic is applicable about stability. I have to admit that my logic |
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> here is a plain arithmetic of type: "1 lemon - 10 cents, 10 lemons - 100 |
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> cents". |
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Well, optimization techniques are quite a bit more varied than lemons. |
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In particular, if you could only choose one optimization technique from the |
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gcc info pages there is probably a "correct" choice that 90% of the gcc |
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developers would agree on. And guess what? They probably put that one |
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in -O1. :) |
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By the time you get to -O3, you are really just instructing the compiler on |
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what tradeoffs to make. finline-functions and funswitch-loops are code |
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size vs. execution path length tradeoffs and fgcse-after-reload is a [lots |
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of] compilation time vs. [a little] (code size AND execution path length) |
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tradeoff. |
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Again, the performance gained (or lost) from a given compiler optimization |
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option can vary based on program and processor. If you don't feel you can |
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make a good decision based on reading the gcc info pages, don't include |
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the option in your CFLAGS and just go with -O2 or -O3. |
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|
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-- |
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"If there's one thing we've established over the years, |
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it's that the vast majority of our users don't have the slightest |
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clue what's best for them in terms of package stability." |
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-- Gentoo Developer Ciaran McCreesh |
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