linux-kernel - Re: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO

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Message-ID: <CAHk-=wgBM7Swi7wmvi0M+sBFjzn-vEv1KzgM-hbJ4h3J3BR+UA@mail.gmail.com>
Date:   Thu, 26 Oct 2023 06:54:07 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Sebastian Reichel <sebastian.reichel@...labora.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        "kernel@...labora.com" <kernel@...labora.com>
Subject: Re: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO_OVERFLOW

On Wed, 25 Oct 2023 at 22:57, David Laight <David.Laight@...lab.com> wrote:
>
> Doesn't that version end up calling inline functions?
> So won't be usable in static initialisers - the same as statement functions.

David, please either read what I write, or actually test things out,
instead of just adding noise.

It is not a problem to have a _Generic() - or a
__builtin_chooise_expr() - that has an inline function that doesn't
get used. The compiler will remove it.

The problem is literally that you can't use "__auto_type", because
*STATEMENT EXPRESSIONS* do not work outside of function definitions,
because gcc won't even *parse* them, and just says

  error: braced-group within expression allowed only inside a function

exactly like I explained in the comment of the patch that does the _Generic().

Which is why my solution ended up using those inline functions, and
splitting manually by type, instead of an expression statement and
__auto_type.

Whether you then split into different expressions with _Generic() or
by using __builtin_choose_expr() is irrelevant, although once you do
"split by type", _Generic() ends up being much more convenient because
that's what it's designed for (and you don't end up trying to figure
out the size with sizeof() and typeof tricks).

               Linus