Re: Compiler warning about comparing result of !!test_bit() to an integer

From: Jiri Kosina
Date: Mon Apr 27 2015 - 10:25:09 EST

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.


Jiri Kosina
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