David Benjamin adeae925dc [libc++] Make _LIBCPP_ASSUME usable when it is appropriate (#91801) ...

libc++ turned off _LIBCPP_ASSUME because turning every debug assert into
__builtin_assume tripped [1]. However, this means we can't use _LIBCPP_ASSUME
when there is a clear optimization intent. See [2] for discussion of a place 
where _LIBCPP_ASSUME would be valuable.

This patch fixes this by not undefining the definition of _LIBCPP_ASSUME and
making sure that we don't attempt to `_LIBCPP_ASSSUME` every assertion in
the library.

[1]: https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609
[2]: https://github.com/llvm/llvm-project/pull/78929#issuecomment-1936582711

2024-09-17 11:22:27 -04:00

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