[clang] [compiler-rt] [UBSan] Add -fsanitize=uninitialized-read check (ISO C 6.3.2.1p2) (PR #207529)

[Thurston Dang via cfe-commits]

thurstond wrote:

> Cost is one alloca + one store per write + one load/branch per read, for candidate scalars only. Given -fsanitize=undefined is supposed to stay cheap, wanted to check before implementing: fine as part of the Undefined group, or would you rather this sit behind its own flag?

The allocas would make this check unusual by UBSan standards, let alone `-fsanitize=undefined`: "UndefinedBehaviorSanitizer (UBSan) is a fast undefined behavior detector. ... The checks have small runtime cost and no impact on address space layout" (https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html)

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The current implementation of this check is targeted towards ISO C23 6.3.2.1p2, which is not obvious from the check name (`-fsanitize=uninitialized-check`), and would potentially confuse users of other C or C++ dialects why the check doesn't work as expected. It would be better if the check implemented the semantics for the appropriate dialect per the `CGF.getLangOpts()`.

Users may also be confused by the difference between `-fsanitize=uninitialized-check` and `-fsanitize=memory` (arguably the former is a better description of what MemorySanitizer actually does).

Instead of an extra UBSan check, is it possible for the front-end to instead add appropriate annotations/attributes to make MemorySanitizer work correctly for this C23?



https://github.com/llvm/llvm-project/pull/207529