Re: [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions

From: Nick Desaulniers <ndesaulniers@google.com>
To: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	 Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org,  "H . Peter Anvin" <hpa@zytor.com>,
	Nathan Chancellor <nathan@kernel.org>, Tom Rix <trix@redhat.com>,
	 linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	 David Howells <dhowells@redhat.com>,
	Jan Beulich <JBeulich@suse.com>
Subject: Re: [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions
Date: Wed, 11 May 2022 17:28:06 -0700	[thread overview]
Message-ID: <CAKwvOdnnDaiJcV1gr9vV+ya-jWxx7+2KJNTDThyFctVDOgt9zQ@mail.gmail.com> (raw)
In-Reply-To: <20220512000340.1171988-2-mailhol.vincent@wanadoo.fr>

On Wed, May 11, 2022 at 5:04 PM Vincent Mailhol
<mailhol.vincent@wanadoo.fr> wrote:
>
> And finally, Nick Desaulniers pointed out in [2] that this also fixes
> a constant propagation missed-optimization in clang.
>
> [2] https://lore.kernel.org/all/CAKwvOdnH_gYv4qRN9pKY7jNTQK95xNeH1w1KZJJmvCkh8xJLBg@mail.gmail.com/

Regarding
https://github.com/llvm/llvm-project/issues/55394
it seems that functions with static linkage cannot be considered
library functions, so libcall optimization will not run on calls to
them. So the compiler might be able to do a better job for constants
if ffs() and friends indeed were not defined in a header as static
inline.  But that relies on the compiler knowing these tricks; I think
the kernel's approach is just fine (better in fact, because we should
inline these tiny functions, regardless of LTO), but like this series
shows, there may be room for improvement for other functions within
the kernel that are defined as static inline in headers that are
normally found in a libc.

So I no longer think there's a missed optimization here, but at this
point, it's not worth a respin of the series IMO to just let sleeping
dogs lie.

Unless the x86 maintainers wouldn't mind dropping that line and link
when applying?
-- 
Thanks,
~Nick Desaulniers