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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/102485] -Wa,-many no longer has any effect
Date: Wed, 23 Feb 2022 17:38:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102485

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #7 from Segher Boessenkool <segher at gcc dot gnu.org> ---
> GCC already passes -m<cpu> to the assembler though.

That mostly is historic.

> The justification for emitting the .machine directive is given as fixing a
> build breakage due to a build system that passes an incorrect -m<cpu> to =
the
> assembler.

Not really, no.  That is just one tiny part of the problem.  It is impossib=
le
to know what instruction sets we need ahead of time, and -many cannot work =
(and
*does not* work: there are quite a few mnemonics that encode to different i=
nsns
on different architecture versions (or for different CPUs), and we cannot k=
now
which is wanted, or which is preferred, ahead of time.

> *That* is the broken code (if any) that should have been fixed. But inste=
ad
> that is hacked around in a way that breaks working code that passes down
> -Wa,-many option as specified.

There is no working code that uses -many (accept by accident, if no problem
has hit you yet).

> The kernel builds with a base compatibility (say -mcpu=3Dpower4) and then=
 has
> certain code paths that are dynamically taken if running on newer CPUs wh=
ich
> use newer instructions with inline asm.
>=20
> This is an important usage and it's pervasive, it seems unreasonable to
> break it.  Adding .machine directives throughout inline asm for every
> instruction not in the base compatibility class is pretty horrible.

It is the only correct thing to do.=