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papu
l33t
Joined: 25 Jan 2008
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 Posted: Sun Aug 18, 2024 9:01 pm Post subject: Does gcc support thinlto? |
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hi,
I am not sure if gcc have thinLTO like clang has,
if yes , are there same level of performance between gcc and clang with thin?
COMMON_FLAGS="-O2 -pipe -march=native -flto=thin"
COMMON_FLAGS="-O2 -pipe -march=native -flto"
thanks 
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krumpf
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Joined: 15 Jul 2018
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| Posted: Sun Aug 18, 2024 9:12 pm Post subject: |
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From GCC flto documentation :
| Quote: | -flto[=n]
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program.
[…]
Use -flto=auto to use GNU make’s job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. |
Afaik, there's no thinlto in gcc, it's specific to clang.
If you want to get better lto performance when using gcc, try to install and setup the mold linker.
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Hu
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| Posted: Sun Aug 18, 2024 9:21 pm Post subject: |
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| gcc supports -ffat-lto-objects and -fno-fat-lto-objects, where the latter would seem to be the "thin" variant. Per info gcc, no-fat is default on targets with linker plugin support. |
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papu
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| Posted: Sun Aug 18, 2024 9:30 pm Post subject: |
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| krumpf wrote: | From GCC flto documentation :
| Quote: | -flto[=n]
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program.
[…]
Use -flto=auto to use GNU make’s job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. |
Afaik, there's no thinlto in gcc, it's specific to clang.
If you want to get better lto performance when using gcc, try to install and setup the mold linker. |
ok, thanks!
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Perfect Gentleman
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Joined: 18 May 2014
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| Posted: Mon Aug 19, 2024 12:15 am Post subject: |
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| afaik, mold doesn't support LTO itself, it translates LTO to bfd/lld. So there is no any speed improvement with mold when using LTO. |
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sam_
Developer
Joined: 14 Aug 2020
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| Posted: Mon Aug 19, 2024 12:45 am Post subject: |
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mold has plugin support and can handle gcc LTO by itself. Curiously, the maintainer of mold is the same person who rejected such plugin support for lld a few years ago.
You can easily verify this by running:
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echo 'int main() { __builtin_printf("hello\n"); }' | gcc -x c - -flto -fuse-ld=mold -Wl,-v -Wl,--trace
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sam_
Developer
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| Posted: Mon Aug 19, 2024 12:51 am Post subject: |
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| Hu wrote: | | gcc supports -ffat-lto-objects and -fno-fat-lto-objects, where the latter would seem to be the "thin" variant. Per info gcc, no-fat is default on targets with linker plugin support. |
Yes, this matches my understanding.
Now, one question is whether GCC could implement the same style of partitioning as Clang to speed things up, but there are trade-offs associated with that (namely worse optimisation). There are other differences in how they implement LTO (the amount of information to stream, representing that, and so on) but it's not about thin-vs-not. GCC is also working on incremental LTO, not that it's much use for ebuilds. |
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