On 8/6/21 6:42 AM, Will Deacon wrote: > 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. ARC does too: There's LSR (Logical shift right) and ASR (Arithmetic Shift Right). So perhaps something to be done in the compiler. >> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx> >> --- >> 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@xxxxxxxxxx> > > 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.
