[PATCH] D73606: [X86] matchAdd: don't fold a large offset into a %rip relative address

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

`absl/hash/internal/hash.h` takes the address of a static variable `absl::hash_internal::CityHashState::kSeed`:

  ABSL_ATTRIBUTE_ALWAYS_INLINE static uint64_t Seed() {
    return static_cast<uint64_t>(reinterpret_cast<uintptr_t>(kSeed));
  }
  static const void* const kSeed;

With certain LTO optimizations, the address can be added to a large constant 1701208431. It is similar to `ret i64 add (i64 ptrtoint (i32* @foo to i64), i64 1701208431)`.
Before this patch, we could produce an instruction like `leaq kSeed+1701208431(%rip), %rax`. At the linking stage, it is an `R_X86_64_PC32` relocation overflow if `kSeed-rip+1701208431 >= 0x80000000`. This means kSeed-rip cannot be larger than 0x80000000-1701208431=0x1a999e91. This upper bound is not large, and some large programs can exceed the bound.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D73606/new/

https://reviews.llvm.org/D73606