Use -Wno-invalid-offsetof instead of dangerous offset_of hack #9563
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Summary: After facebook#9515 added a unique_ptr to Status, we see some warnings-as-error in some internal builds like this: ``` stderr: rocksdb/src/db/compaction/compaction_job.cc:2839:7: error: offset of on non-standard-layout type 'struct CompactionServiceResult' [-Werror,-Winvalid-offsetof] {offsetof(struct CompactionServiceResult, status), ^ ~~~~~~ ``` I see three potential solutions to resolving this: * Be more tricky about computing offsets, to bypass the warning, but this is basically invoking undefined behavior. * Migrate to something in place of offset, like a function mapping CompactionServiceResult* to Status* (for the `status` field). This might be required in the long term. * Use our new C++17 dependency to use offsetof in a well-defined way when the compiler allows it. From a comment on https://gist.github.com/graphitemaster/494f21190bb2c63c5516: A final note: in C++17, offsetof is conditionally supported, which means that you can use it on any type (not just standard layout types) and the compiler will error if it can't compile it correctly. That appears to be the best option if you can live with C++17 and don't need constexpr support. The C++17 semantics are confirmed on https://en.cppreference.com/w/cpp/types/offsetof, so we can suppress the warning as long as we accept that we might run into a compiler that rejects the code, and at that point we will find a solution, such as the more intrusive "migrate" solution above. Although this is currently only showing in our buck build, it will surely show up also with make and cmake, so I have updated those configurations as well. Test Plan: Tried out buck builds with platform009 and platform010 Reviewers: Subscribers: Tasks: Tags:
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@pdillinger has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator. |
riversand963
approved these changes
Feb 14, 2022
Makefile
Outdated
| @@ -501,6 +501,10 @@ endif | |||
| CFLAGS += $(C_WARNING_FLAGS) $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CCFLAGS) $(OPT) | |||
| CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverloaded-virtual -Wnon-virtual-dtor -Wno-missing-field-initializers | |||
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| # For offsetof to work on non-standard layout types, until some compiler | |||
| # completely rejects our usage of offsetof | |||
| CXXFLAGS += -Wno-expansion-to-defined | |||
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Any reference on why we need to make this change?
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copy-paste error, sorry
pdillinger
changed the title
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@pdillinger has updated the pull request. You must reimport the pull request before landing. |
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Requesting re-review because I expanded the scope |
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@pdillinger has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator. |
riversand963
approved these changes
Feb 15, 2022
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Summary: After #9515 added a unique_ptr to Status, we see some
warnings-as-error in some internal builds like this:
I see three potential solutions to resolving this:
functions removed in this change, inspired by
https://gist.github.com/graphitemaster/494f21190bb2c63c5516) However,
this construction is invoking undefined behavior that assumes consistent
layout with no compiler-introduced indirection. A compiler incompatible
with our assumptions will likely compile the code and exhibit undefined
behavior.
CompactionServiceResult* to Status* (for the
statusfield). This mightbe required in the long term.
when the compiler allows it. From a comment on
https://gist.github.com/graphitemaster/494f21190bb2c63c5516:
The C++17 semantics are confirmed on
https://en.cppreference.com/w/cpp/types/offsetof, so we can suppress the
warning as long as we accept that we might run into a compiler that
rejects the code, and at that point we will find a solution, such as
the more intrusive "migrate" solution above.
Although this is currently only showing in our buck build, it will
surely show up also with make and cmake, so I have updated those
configurations as well.
Also in the buck build, -Wno-expansion-to-defined does not appear to be
needed anymore (both current compiler configurations) so I
removed it.
Test Plan: Tried out buck builds with both current compiler configurations