linux-kernel - Re: block: new gcc-5.1 warnings..

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Message-ID: <CA+55aFxHWVGQUF8282Eu3WRdqp5wQGwSfHmbQoTH_G+V1kLwFA@mail.gmail.com>
Date:	Wed, 27 May 2015 16:34:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: block: new gcc-5.1 warnings..

On Wed, May 27, 2015 at 4:18 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> So I do actually agree that
>
>     switch (boolean) {
>     case non-boolean:
>
> can very much be worth a warning. But then it's about type-safety
> issues, rather than about "you shouldn't use switch() with a boolean".

Btw, I'd actually like to see (possibly optionally) a warning for enum
types there too. Exactly because *type* based warnings very much make
sense, regardless of number of cases.

For example, try this:

    #include <stdbool.h>
    #include <stdio.h>

    enum a { one, two };

    int t(bool b, enum a e)
    {
        switch (b) {
        case true:
            printf("No arguments\n");
            /* fallthrough */
        case false:
            printf("\n");
        }

        switch (e) {
        case 0:
            printf("one");
            break;
        case two:
            printf("two");
            break;
        }
        return 0;
    }

and I'd argue that gcc-5.1 warns about TOTALLY THE WRONG THING.

It does that *stupid* warning:

    warning: switch condition has boolean value [-Wswitch-bool]

which is just idiotic and wrong.

The case statements are clearly boolean, there is absolutely nothing
wrong with that switch, and a compiler that warns about it is just
being f*cking moronic.

In contrast, that second switch() statement with the "case 0:" is
actually something that might well be worth warning for. I'd argue
that the code would clearly be more legible if it used "case one:"
instead.

So the new warning in gcc-5 seems to be just stupid. In general,
warnings that encourage you to write bad code are stupid. The above

    switch (boolean) {
    case true:

is *good* code, while the gcc documentation suggests that you should
cast it to "int" in order to avoid the warning, but anybody who
actually thinks that

    switch ((int)boolean) {
    switch 1:

is better, clearly has absolutely zero taste and is just objectively wrong.

Really. A warning where the very *documentation* tells you to do
stupid things is stupid.

              Linus
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