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
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https://reviews.llvm.org/D108661/new/
https://reviews.llvm.org/D108661
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