[PATCH] D72742: Don't assume promotable integers are zero/sign-extended already in x86-64 ABI.

Wed Jan 15 04:47:58 PST 2020

emilio added a comment.

The relevant discussion is here <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92821#c2>. From the "Parameter Passing" section in https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf, there's no mention of sign-extension requirements for arguments.

> The only related thing is
>  "When a value of type _Bool is returned or passed in a register or on the stack,
>  bit 0 contains the truth value and bits 1 to 7 shall be zero."
>  with a footnote:
>  "Other bits are left unspecified, hence the consumer side of those values can rely on it being 0 or 1 when truncated to 8 bit."

which says that _Bool has only significant low 8 bits and the rest is unspecified.

Then Michael Matz (one of the editors of that document) clarifies below:

> Yes, we (intentionally) haven't required any extensions to happen for arguments

or return values smaller than 64bit (e.g. we haven't even specified that arguments <= 32bit would be zero-extended in the high bits, as would have been
natural with the instruction set).  If LLVM relies on that it would be a bug.

There's more information on that bug and related ones, but I think the above should be enough :)

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72742/new/

https://reviews.llvm.org/D72742