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From: Florian Weimer <fweimer@redhat.com>
To: gcc@gcc.gnu.org
Subject: Minor C2X-induced ABI issue on powerpc64le
Date: Wed, 02 Mar 2022 16:03:11 +0100
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I've been following

  [RFC] Enabling -Wstrict-prototypes by default in C
  <https://discourse.llvm.org/t/rfc-enabling-wstrict-prototypes-by-default-in-c/60521/25>

and I've just realized that treating

  extern int foo();

as a prototype declaration (with the same ABI as extern "C" int foo();
in C++) introduces a minor ABI break if the caller is compiled in C2X
mode, but the definition is not.  Without the prototype (C18 mode and
earlier), the caller needs to create a parameter save area per the
powerpc64le ABI.  With the prototype (C2X mode), I would expect the
compiler to omit the parameter save area.  Unfortunately, this creates
an ABI quirk if the C2X status doesn't match between caller and function
implementation.

I believe GCC uses the parameter save area for general-purpose spilling,
so it's still used in a prototype-less function definition without any
parameters.  That's why this is a minor ABI break.

It's probably not much to worry about, but I'd like to point it out
nevertheless.

Thanks,
Florian