[PATCH] D141020: [X86][ABI] Clobber %RAX for preserve_allcc and preserve_mostcc CCs

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

Having a register be both preserved and used to store a return value is obviously a contradiction.  However, it's consistent to say that a register is preserved if and only if it is not used to return a value.  (Not sure off the top of my head what changes are required to make that work correctly.)

The C calling convention uses RAX/RDX/XMM0/XMM1 to return values, and LLVM additionally allows using RCX/XMM2/XMM3 for certain IR signatures.  So I don't think just removing RAX from the list does the right thing.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D141020/new/

https://reviews.llvm.org/D141020