Fangrui Song ce80c80dca [Hashing] Use a non-deterministic seed if LLVM_ENABLE_ABI_BREAKING_CHECKS ...

Hashing.h provides hash_value/hash_combine/hash_combine_range, which are
primarily used by `DenseMap<StringRef, X>`

Users shouldn't rely on specific hash values due to size_t differences
on 32-bit/64-bit platforms and potential algorithm changes.
`set_fixed_execution_hash_seed` is provided but it has never been used.

In LLVM_ENABLE_ABI_BREAKING_CHECKS builds, take the the address of a
static storage duration variable as the seed like
absl/hash/internal/hash.h `kSeed`. (See https://reviews.llvm.org/D93931
for workaround for older Clang. Mach-O x86-64 forces PIC, so absl's
`__apple_build_version__` check is unnecessary.)

LLVM_ENABLE_ABI_BREAKING_CHECKS defaults to `WITH_ASSERTS` and is
enabled in an assertion build.

In a non-assertion build, `get_execution_seed` returns the fixed value
regardless of `NDEBUG`. Removing a variable load yields noticeable
size/performance improvement.

A few users relying on the iteration order of `DenseMap<StringRef, X>`
have been fixed (e.g., f8f4235612 c025bd1fdb 89e8e63f47
86eb6bf671 eb8d036565 0ea6b8e476 58d7a6e0e6 8ea31db272
592abf29f9 664497557a).
From my experience fixing [`StringMap`](https://discourse.llvm.org/t/reverse-iteration-bots/72224)
iteration order issues, the scale of issues is similar.

Pull Request: https://github.com/llvm/llvm-project/pull/96282

2024-06-28 15:30:19 -07:00

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