RE: [PATCH 4/6] perf: Optimize get_recursion_context()

[David Laight <David.Laight@ACULAB.COM>]

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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