linux-kernel - Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation

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Date:   Fri, 10 Mar 2017 19:41:33 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation

Christophe Leroy <christophe.leroy@....fr> writes:

> Help a bit the compiler to provide better code:
>
> unsigned int f(int i)
> {
> 	return 1 << (31 - i);
> }
>
> unsigned int g(int i)
> {
> 	return 0x80000000 >> i;
> }
>
> Disassembly of section .text:
>
> 00000000 <f>:
>    0:	20 63 00 1f 	subfic  r3,r3,31
>    4:	39 20 00 01 	li      r9,1
>    8:	7d 23 18 30 	slw     r3,r9,r3
>    c:	4e 80 00 20 	blr
>
> 00000010 <g>:
>   10:	3d 20 80 00 	lis     r9,-32768
>   14:	7d 23 1c 30 	srw     r3,r9,r3
>   18:	4e 80 00 20 	blr

Well yeah, it saves one instruction, but is it worth it? Are these gpio
routines in some hot path I don't know about?

cheers