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ImErina
n00b
Joined: 10 Feb 2022
Posts: 51
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 Posted: Fri Jun 16, 2023 9:15 am Post subject: Can ccache reduce llvm compiling? |
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| When installing llvm, clang, compiler-rt, libclc and so, a lot of things are compiled several times. With in ccache.conf, can it reduce the times of compilation between these packages? |
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ImErina
n00b
Joined: 10 Feb 2022
Posts: 51
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| Posted: Sat Jun 17, 2023 12:36 am Post subject: |
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A quick test shows that is it possibly cannot.
Here's what ccache says after building llvm, clang, libclc.
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Cacheable calls: 5855 / 6446 (90.83%)
Hits: 212 / 5855 ( 3.62%)
Direct: 79 / 212 (37.26%)
Preprocessed: 133 / 212 (62.74%)
Misses: 5643 / 5855 (96.38%)
Uncacheable calls: 591 / 6446 ( 9.17%)
Local storage:
Cache size (GB): 0.3 / 25.0 ( 1.22%)
Hits: 212 / 5855 ( 3.62%)
Misses: 5643 / 5855 (96.38%)
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So how exactly does ccache works? What does it take into account for generating a hash? |
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Hu
Administrator
Joined: 06 Mar 2007
Posts: 22451
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| Posted: Sat Jun 17, 2023 12:54 am Post subject: |
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According to How ccache works (ccache manual): | Code: | The detection is done by hashing different kinds of information that should be unique for the compilation and then using the hash sum to identify the cached output.
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The following information is always included in the hash:
the extension used by the compiler for a file with preprocessor output (normally .i for C code and .ii for C++ code)
the compiler’s size and modification time (or other compiler-specific information specified by compiler_check)
the name of the compiler
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In the preprocessor mode, the hash is formed of the common information and:
the preprocessor output from running the compiler with -E
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I omitted the parts unlikely to be relevant to your question, but you can see them at the linked document. |
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Ginta
Apprentice
Joined: 17 Feb 2006
Posts: 152
Location: Spain
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| Posted: Sat Jun 17, 2023 1:08 pm Post subject: |
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Ccache is proven to be useful only on rebuilds (ebuild testing, emerge -e world, manual reinstall, etc) but not on new installs.
Also, some USE changes alter internal constants so it renders the cache invalid again.
So unless you do a lot of same or nearly same scenario recompiles ccache is not helping.
_________________
Some people should use emerge brain on theirselves... |
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