[Bug target/102485] -Wa,-many no longer has any effect

["npiggin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102485

--- Comment #8 from Nicholas Piggin <npiggin at gmail dot com> ---
(In reply to Segher Boessenkool from comment #7)
> > GCC already passes -m<cpu> to the assembler though.
> 
> That mostly is historic.

So? I was pointing out the compiler already tells the assembler what
instruction set to use without the .machine directive.

> 
> > 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 really the justification for emitting the .machine directive as
provided in the changelog of the commit which introduced the change.

> That is just one tiny part of the problem.  It is impossible
> 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
> insns
> on different architecture versions (or for different CPUs), and we cannot
> know
> which is wanted, or which is preferred, ahead of time.

I understand the problems with -many, but it can and does work for real
software. E.g., Linux kernel as of before this change. It's not -many I'm
wedded to though, it's any ability to fix this sanely because of the .machine
directive.

The kernel should would change to using a specific CPU, e.g., -mcpu=power4
-Wa,-mpower10 and deal with the very rare few clashing mnemonics (e.g., tlbie)
specially with the .machine directive when an older one is required.

> 
> > *That* is the broken code (if any) that should have been fixed. But instead
> > 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).

I'll reword. "Instead that is hacked around in a way that breaks working code
that passes down the -Wa,-m<cpu> option as specified."

> 
> > The kernel builds with a base compatibility (say -mcpu=power4) and then has
> > certain code paths that are dynamically taken if running on newer CPUs which
> > use newer instructions with inline asm.
> > 
> > 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.

It's not. Not passing .machine and passing -mcpu is just as correct. With the
added bonus that it allows software to use a superset of instructions in such
cases. And even the great bonus that existing "broken" code that uses -many
will continue to work.

The correct way to deal with this is not to break this, it is to add a warning
to -many for some period to binutils to give code a chance to migrate. I'm all
for removing -many, and that is the right way to do it. By deprecating -many
and providing warnings. Not by hacking around it in the compiler that breaks
things.