[PATCH] D108661: The maximal representable alignment in LLVM IR is 1GiB, not 512MiB

[Craig Topper via Phabricator via cfe-commits]

craig.topper added a comment.

Wild speculation. This may be a historical artifact of LoadInst and StoreInst having their getAlignment() function written like this when this limit was created

  unsigned getAlignment() const {
    return (1 << (getSubclassDataFromInstruction() >> 1)) >> 1;
  }

Because "1" is signed int the left shift produced a signed int. The final right shift would be an arithmetic shift. So if the alignment was 1 << 30, the subclass data would be 31, the shift would produce INT_MIN, and the arithmetic right shift would produce 0xc0000000 which would be incorrect.

AllocaInst used 1U instead of 1 so would have been immune.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D108661/new/

https://reviews.llvm.org/D108661