Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH 4/6] perf: Optimize get_recursion_context() | Date | Sat, 31 Oct 2020 13:18:27 +0000 |
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From: David Laight > Sent: 31 October 2020 12:12 > ... > The gcc 7.5.0 I have handy probably generates the best code for: > > unsigned char q_2(unsigned int pc) > { > unsigned char rctx = 0; > > rctx += !!(pc & (NMI_MASK)); > rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK)); > rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)); > > return rctx; > } > > 0000000000000000 <q_2>: > 0: f7 c7 00 00 f0 00 test $0xf00000,%edi # clock 0 > 6: 0f 95 c0 setne %al # clock 1 > 9: f7 c7 00 00 ff 00 test $0xff0000,%edi # clock 0 > f: 0f 95 c2 setne %dl # clock 1 > 12: 01 c2 add %eax,%edx # clock 2 > 14: 81 e7 00 01 ff 00 and $0xff0100,%edi > 1a: 0f 95 c0 setne %al > 1d: 01 d0 add %edx,%eax # clock 3 > 1f: c3 retq > > I doubt that is beatable.
I lied, you should be able to get: test $0xff0000,%edi # clock 0 setne %al # clock 1 test $0xff0100,%edi # clock 0 setne %dl # clock 1 add $fffff000,%edi adc %dl, %al # clock 2
But I suspect getting it from the compiler might be hard!
David
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