linux-kernel - Re: [git pull] drm for 5.19-rc1

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Date:   Tue, 7 Jun 2022 11:15:06 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Dave Airlie <airlied@...il.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [git pull] drm for 5.19-rc1

On Tue, Jun 7, 2022 at 3:23 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> These header files are heavy users of large constants lacking the "U"
> suffix e.g.:
>
>     #define NB_ADAPTER_ID__SUBSYSTEM_ID_MASK 0xFFFF0000L

As Andreas says, this is not undefined behavior.

A hexadecimal integer constant will always get a type that fits the
actual value. So on a 32-bit architecture, because 0xFFFF0000 doesn't
fit in 'long', it will automatically become 'unsigned long'.

Now, a C compiler might still warn about such implicit type
conversions, but I'd be a bit surprised if any version of gcc actually
would do that, because this behavior for hex constants is *very*
traditional, and very common.

It's also true that the type of the constant - but not the value -
will be different on 32-bit and 64-bit architectures (ie on 64-bit, it
will be plain "long" and never extended to "unsigned long", because
the hex value obviously fits just fine).

I don't see any normal situation where that really matters, since any
normal use will have the same result.

The case you point to at

  https://lore.kernel.org/r/CAK8P3a0QrihBR_2FQ7uZ5w2JmLjv7czfrrarCMmJOhvNdJ3p9g@mail.gmail.com

is very different, because the constant "1" is always just a plain
signed "int". So when you do "(1 << 31)", that is now a signed integer
with the top bit set, and so it will have an actual negative value,
and that can cause various problems (when right-shifted, or when
compared to other values).

But hexadecimal constants can be signed types, but they never have
negative values.

                 Linus