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

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Date:   Tue, 21 Mar 2017 16:04:40 +0100
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation



Le 10/03/2017 à 16:41, Segher Boessenkool a écrit :
> On Fri, Mar 10, 2017 at 03:41:23PM +0100, Christophe LEROY wrote:
>>>>>> gpio_get() and gpio_set() are used extensively by some GPIO based
>>>>>> drivers like SPI, NAND, so it may be worth it as it doesn't impair
>>>>>> readability (if anyone prefers, we could write  (1 << 31) >> i  instead
>>>>>> of  0x80000000 >> i )
>>>>> 1 << 31 is undefined behaviour, of course.
>>>> Shall it be 1U << 31 ?
>>> Sure, that works.  "1 << (31 - i)" is most readable (but it doesn't yet
>>> generate the code you want).
>> Euh .... I'm a bit lost. Do you mean the form we have today is the
>> driver is wrong ?
> Heh, yes.  But is't okay with GCC, so don't worry about it.
>
> The point is that "0x80000000 >> i" is less readable.
>
>
FYI, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80131

Christophe