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On Sun, 20 Apr 2014 21:14:51 -0600 |
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Ryan Hill <rhill@g.o> wrote: |
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|
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> Hey all, |
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> |
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> As more and more packages are starting to add LTO flags automatically through |
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> their build systems, I thought I'd point out a couple things: |
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> |
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> - LTO utterly destroys debug info. Flags like -g are incompatible with LTO. |
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> |
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> - LTO causes .GCC.command.line sections to be discarded, which means your |
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> package will always be QA flagged as ignoring CFLAGS. |
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> |
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> - LTO takes a _lot_ of memory. That memory is required on the host arch. |
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> Distcc doesn't help things here, because linking happens locally. Consider |
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> all the archs your package is built on, and if they all routinely have |
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> multiple GBs of memory installed. |
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> |
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> - LTO in 4.7 is still fairly buggy. There are no plans to fix it. 4.8 is |
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> better, but 4.9 moves to a different model, so bugs in 4.8 probably won't be |
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> fixed, especially regarding memory usage. |
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> |
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> - I'm happy to backport patches to fix LTO problems if they're available, but |
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> you'll generally have to do the legwork. And like I said, most aren't going |
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> to be backportable. |
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> |
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> Please take these things into consideration when deciding whether or not this |
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> feature is worth it. |
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> |
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> Thanks. |
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> |
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> |
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One thing I forgot to mention - LTO can also have detrimental effect on certain |
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architectures. On some (eg. ppc), performance can actually be degraded due to |
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increased register pressure. On others like alpha it's questionable if it'll |
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even work at all... |
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|
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|
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-- |
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Ryan Hill psn: dirtyepic_sk |
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gcc-porting/toolchain/wxwidgets @ gentoo.org |
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47C3 6D62 4864 0E49 8E9E 7F92 ED38 BD49 957A 8463 |
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