linux-kernel - Re: Compiler warning about comparing result of !!test

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Date:	Mon, 27 Apr 2015 16:25:01 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	David Howells <dhowells@...hat.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: Compiler warning about comparing result of !!test_bit() to an
 integer

On Mon, 27 Apr 2015, David Howells wrote:

> Hi Jiri,
> 
> This line in hid-input.c:
> 
> 	if (usage->type == EV_KEY && !!test_bit(usage->code, input->key) != value)
> 		input_event(input, EV_MSC, MSC_SCAN, usage->hid);
> 
> gives:
> 
> ../drivers/hid/hid-input.c: In function 'hidinput_hid_event':
> ../drivers/hid/hid-input.c:1160:167: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
> 
> with gcc-5.1.  Given that 'value' doesn't appear to be a boolean, is this correct?

gcc-5 became somehow very picky about '!!' usages. I recall quite some 
internal discussions with our gcc guys, which resulted in upstream gcc 
bugzillas being filled about gcc warning in completely legitimate cases.

> 
> This was introduced by commit c01d50d181f074a60bf3ed54eb055ce1679afb98.

Gah, that was long time ago! :)

This is because we want to handle autorepeat (i.e. value 2) correctly. Now 
that you ask, it seems to me that we actually should be doing

	!!test_bit(usage->code, input->key) != !!value

to be really exact (i.e. so that we really can't have oscilating state).

I'll think about it a little bit more, and eventually probably fix it this 
way with your Reported-by: if you are OK with that.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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