On 8/6/21 6:42 AM, Will Deacon wrote:

On Thu, Aug 05, 2021 at 12:14:08PM -0700, Vineet Gupta wrote:

quoted

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.

ARC does too: There's LSR (Logical shift right) and ASR (Arithmetic 
Shift Right).
So perhaps something to be done in the compiler.

quoted

Signed-off-by: Vineet Gupta <redacted>
---
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@kernel.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.

OK I'll give it a try too.