[C++] PCG breaks compilation caching #35596
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pitrou
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Also warn/error out if non-deterministic compilation macros such as `__TIME__` are used (with gcc and clang at least).
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Interesting, I also have #34715 open which could be a big improvement for our caching situation. |
pitrou
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### Rationale for this change The PCG headers use the `__DATE__` and `__TIME__` macros, which makes builds non-deterministic: imneme/pcg-cpp#59 Deterministic builds are useful for a number of reasons. One is security audits of binary artifacts, another (that directly affects us) is making compilation caching as efficient as possible. Preprocessor-based compilation caching breaks when time-dependent macros are used. ### What changes are included in this PR? Remove the `struct static_arbitrary_seed` construct from PCG. Also, enable a gcc/clang warning that detects the use of non-deterministic preprocessor macros. ### Are these changes tested? Yes, by the additional warning (which is turned into an error in "checkin" warnings mode). ### Are there any user-facing changes? No. * Closes: #35596 Authored-by: Antoine Pitrou <antoine@python.org> Signed-off-by: Antoine Pitrou <antoine@python.org>
rtpsw
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…e#35597) ### Rationale for this change The PCG headers use the `__DATE__` and `__TIME__` macros, which makes builds non-deterministic: imneme/pcg-cpp#59 Deterministic builds are useful for a number of reasons. One is security audits of binary artifacts, another (that directly affects us) is making compilation caching as efficient as possible. Preprocessor-based compilation caching breaks when time-dependent macros are used. ### What changes are included in this PR? Remove the `struct static_arbitrary_seed` construct from PCG. Also, enable a gcc/clang warning that detects the use of non-deterministic preprocessor macros. ### Are these changes tested? Yes, by the additional warning (which is turned into an error in "checkin" warnings mode). ### Are there any user-facing changes? No. * Closes: apache#35596 Authored-by: Antoine Pitrou <antoine@python.org> Signed-off-by: Antoine Pitrou <antoine@python.org>
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Describe the bug, including details regarding any error messages, version, and platform.
Noticing that I sometimes got unexpected slow rebuilds, I tried to dive a bit into ccache debug output and found out that by default the PCG headers use the
__DATE__and__TIME__macros, which makes builds non-deterministic:imneme/pcg-cpp#59
We should perhaps simply remove PCG's
struct static_arbitrary_seed, which would make compilation caching more efficient. @benibus @bkietz @assignUserComponent(s)
C++
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