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Subject: Re: [PATCH] Make is_signed_type() simpler
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        jani.nikula@linux.intel.com, joonas.lahtinen@linux.intel.com,
        rodrigo.vivi@intel.com, intel-gfx@lists.freedesktop.org,
        rostedt@goodmis.org, mingo@redhat.com
References: <20190929200619.GA12851@avx2>
 <f99ca43d-1ba2-95fb-b90f-6706a06f8ce6@rasmusvillemoes.dk>
 <20190929210908.GA14456@avx2>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <91c5c649-9c9d-6c36-76c1-19208f9ad9a9@rasmusvillemoes.dk>
Date:   Mon, 30 Sep 2019 08:16:48 +0200
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On 29/09/2019 23.09, Alexey Dobriyan wrote:
> On Sun, Sep 29, 2019 at 10:21:48PM +0200, Rasmus Villemoes wrote:
>> On 29/09/2019 22.06, Alexey Dobriyan wrote:
>>> * Simply compare -1 with 0,
>>> * Drop unnecessary parenthesis sets
>>>
>>> -#define is_signed_type(type)       (((type)(-1)) < (type)1)
>>> +#define is_signed_type(type)       ((type)-1 < 0)
>>
>> NAK. I wrote it that way to avoid -Wtautological-compare when type is
>> unsigned.
> 
> Was is W=1?
> 
> godbolt doesn't show it with just -Wall
> 
> 	https://godbolt.org/z/kCA7mm
> 
> And the warning which found i915 case is -Wextra not -Wtautological-compare.
> 

Yeah, it's usually disabled/not enabled in the kernel. I did most of the
prototyping/testing in userspace with my default Cflags, and decided to
keep it this way in case somebody copy-pastes it to a code base that
does enable -Wtautological-compare. I see it's been copy-pasted to
tools/, who knows what they do. IMO, "it may be copy-pasted" would not
be valid reason for a transform the other way, but I really don't see a
reason for changing it now. Especially since it seems to require some
tree-wide adaptation.

Rasmus