linux-kernel - Re: [sparc] Use of -fcall-used-* flags in Makefile?

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Date: Sun, 24 Mar 2024 09:53:53 +0100
From: Sam Ravnborg <sam@...nborg.org>
To: Koakuma <koachan@...tonmail.com>
Cc: "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
	"arnd@...db.de" <arnd@...db.de>
Subject: Re: [sparc] Use of -fcall-used-* flags in Makefile?

Hi Koakuma.

On Sat, Mar 23, 2024 at 04:37:27PM +0000, Koakuma wrote:
> Hello Sam,
> 
> Sam Ravnborg <sam@...nborg.org> wrote:
> 
> > Hi Koakuma,
> > Looking at https://github.com/gcc-mirror/gcc/blob/master/gcc/config/sparc/sparc.h
> > I read that:
> > 
> > On v9 systems:
> > g1,g5 are free to use as temporaries, and are free to use between calls
> > ...
> > g6-g7 are reserved for the operating system (or application in
> > embedded case).
> > 
> > Based on the above I would assume gcc do not change behaviour with or
> > without -fcall-used-g7.
> > [...]
> > For sparc32 the above file says:
> > 
> > g5 through g7 are reserved for the operating system.
> > 
> > So again - it looks like -fcall-used-g5 -fcall-used-g7 should have no
> > effect here and verification on a real target would be nice.
> > 
> > Sam
> 
> >From my understanding (and looking at the codegen results) those flags
> forces GCC to treat the named register as volatile, despite what the ABI
> says. However, I also believe that removing them wouldn't be harmful?
> 
> To quote my reasoning in the LLVM tracker:
> > omitting the flags shouldn't be harmful either - compilers will now
> > simply refuse to touch them, and any assembly code that happens
> > to touch them would still work like usual (because Linux' conventions
> > already treats them as volatile anyway).
> 
> But I am not entirely sure about it, that is why it'd be great if there's
> some explaination on why those flags were added in the first place.
> 
> > I do not have a sparc64 system at my hands - and for this qemu may not
> > cut it. But it would be super if someone with a working sparc64 target
> > could verify if the kernel could be built and works without
> > -fcall-used-g7.
> 
> I am currently running a build with those flags taken out on a T5120,
> and the kernel seems to be running okay for what I do (LLVM development),
> but I don't know if there are more comprehensive test suite for me
> to try on.

I tried to build a sparc32 kernel with the two -fcall-used-g5 -fcall-used-g7
flags dropped. Everything worked for me in qemu - but then I only booted
to a prompt and browsed around a little.

>From your explanation and our limited testing it seems likely the flags
can be dropped, but I cannot say for sure.

Unless others chime in maybe try to send in patches to drop the flags
from sparc32 and sparc64 and see how it goes.

	Sam