linux-kernel - Re: [PATCH] [v3] arm64/io: add constant-argument check

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Date: Wed, 5 Jun 2024 08:56:56 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mark Rutland <mark.rutland@....com>
Cc: Arnd Bergmann <arnd@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Baoquan He <bhe@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH] [v3] arm64/io: add constant-argument check

On Wed, Jun 05, 2024 at 10:15:38AM +0100, Mark Rutland wrote:
> On Tue, Jun 04, 2024 at 10:59:57PM +0200, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > In some configurations __const_iowrite32_copy() does not get inlined
> > and gcc runs into the BUILD_BUG():
> > 
> > In file included from <command-line>:
> > In function '__const_memcpy_toio_aligned32',
> >     inlined from '__const_iowrite32_copy' at arch/arm64/include/asm/io.h:203:3,
> >     inlined from '__const_iowrite32_copy' at arch/arm64/include/asm/io.h:199:20:
> > include/linux/compiler_types.h:487:45: error: call to '__compiletime_assert_538' declared with attribute error: BUILD_BUG failed
> >   487 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> >       |                                             ^
> > include/linux/compiler_types.h:468:25: note: in definition of macro '__compiletime_assert'
> >   468 |                         prefix ## suffix();                             \
> >       |                         ^~~~~~
> > include/linux/compiler_types.h:487:9: note: in expansion of macro '_compiletime_assert'
> >   487 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> >       |         ^~~~~~~~~~~~~~~~~~~
> > include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
> >    39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> >       |                                     ^~~~~~~~~~~~~~~~~~
> > include/linux/build_bug.h:59:21: note: in expansion of macro 'BUILD_BUG_ON_MSG'
> >    59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
> >       |                     ^~~~~~~~~~~~~~~~
> > arch/arm64/include/asm/io.h:193:17: note: in expansion of macro 'BUILD_BUG'
> >   193 |                 BUILD_BUG();
> >       |                 ^~~~~~~~~
> > 
> > Move the check for constant arguments into the inline function to ensure
> > it is still constant if the compiler decides against inlining it, and
> > mark them as __always_inline to override the logic that sometimes leads
> > to the compiler not producing the simplified output.
> > 
> > Note that either the __always_inline annotation or the check for a
> > constant value are sufficient here, but combining the two looks cleaner
> > as it also avoids the macro. With clang-8 and older, the macro was still
> > needed, but all versions of gcc and clang can reliably perform constant
> > folding here.
> > 
> > Fixes: ead79118dae6 ("arm64/io: Provide a WC friendly __iowriteXX_copy()")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> I have a trivial nit below, but either way this looks good to me, so
> regardless of that:
> 
> Reviewed-by: Mark Rutland <mark.rutland@....com>

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Still codegens what I expect on clang-17 at least, agree with Mark's
note

Thanks,
Jason