linux-kernel - Re: [RFC] bitops/non-atomic: make @nr unsigned to avoid any DIV

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Date:   Fri, 6 Aug 2021 14:42:45 +0100
From:   Will Deacon <will@...nel.org>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org
Subject: Re: [RFC] bitops/non-atomic: make @nr unsigned to avoid any DIV

On Thu, Aug 05, 2021 at 12:14:08PM -0700, Vineet Gupta wrote:
> signed math causes generation of costlier instructions such as DIV when
> they could be done by barrerl shifter.
> 
> Worse part is this is not caught by things like bloat-o-meter since
> instruction length / symbols are typically same size.
> 
> e.g.
> 
> stock (signed math)
> __________________
> 
> 919b4614 <test_taint>:
> 919b4614:	div	r2,r0,0x20
>                 ^^^
> 919b4618:	add2	r2,0x920f6050,r2
> 919b4620:	ld_s	r2,[r2,0]
> 919b4622:	lsr	r0,r2,r0
> 919b4626:	j_s.d	[blink]
> 919b4628:	bmsk_s	r0,r0,0
> 919b462a:	nop_s
> 
> (patched) unsigned math
> __________________
> 
> 919b4614 <test_taint>:
> 919b4614:	lsr	r2,r0,0x5  @nr/32
>                 ^^^
> 919b4618:	add2	r2,0x920f6050,r2
> 919b4620:	ld_s	r2,[r2,0]
> 919b4622:	lsr	r0,r2,r0     #test_bit()
> 919b4626:	j_s.d	[blink]
> 919b4628:	bmsk_s	r0,r0,0
> 919b462a:	nop_s

Just FYI, but on arm64 the existing codegen is alright as we have both
arithmetic and logical shifts.

> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> ---
> This is an RFC for feeback, I understand this impacts every arch,
> but as of now it is only buld/run tested on ARC.
> ---
> ---
>  include/asm-generic/bitops/non-atomic.h | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)

Acked-by: Will Deacon <will@...nel.org>

We should really move test_bit() into the atomic header, but I failed to fix
the resulting include mess last time I tried that.

Will