From: Jakub Jelinek <jakub@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: paulmck@kernel.org, linux-toolchains@vger.kernel.org,
peterz@infradead.org, hpa@zytor.com, rostedt@goodmis.org,
gregkh@linuxfoundation.org, keescook@chromium.org
Subject: Re: A few proposals from the C standards committee
Date: Tue, 23 Jan 2024 21:35:46 +0100 [thread overview]
Message-ID: <ZbAjIga031shxM5B@tucnak> (raw)
In-Reply-To: <CAHk-=wgykfH7dP3rYmrBuVu0qbdFu37eKyyBrdzcshUZErPj-g@mail.gmail.com>
On Tue, Jan 23, 2024 at 12:20:14PM -0800, Linus Torvalds wrote:
> And no, __builtin_unreachable() is not it either, because it again has
> the same issue as "assert()" - in *practice* compilers can use it as a
> hint, but that's an incidental result, not part of a documented "this
> is how you specify a known range"
>
> So yes, I can do things like
>
> if (a < 0) __builtin_unreachable();
>
> and it will generate the *code* that I want, but it sure as hell isn't
> some standard C syntax.
C++23 has [[assume (condition)]]; for this (see https://wg21.link/p1774r8)
and GCC supports it also as [[gnu::assume (condition)]] and
__attribute__((assume (condition)));, both in C (the former only in C23)
and C++. Side-effects in condition aren't evaluated, so it has
different behavior from if (!(condition)) __builtin_unreachable ();
Jakub
next prev parent reply other threads:[~2024-01-23 20:36 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-23 16:46 A few proposals from the C standards committee Paul E. McKenney
2024-01-23 18:58 ` Linus Torvalds
2024-01-23 20:00 ` Paul E. McKenney
2024-01-23 20:20 ` Linus Torvalds
2024-01-23 20:35 ` Jakub Jelinek [this message]
2024-01-23 20:43 ` Linus Torvalds
2024-01-23 20:46 ` H. Peter Anvin
2024-01-24 13:46 ` Paul E. McKenney
2024-01-25 13:00 ` Paul E. McKenney
2024-01-24 13:16 ` Paul E. McKenney
2024-01-23 20:44 ` H. Peter Anvin
2024-01-24 12:52 ` Paul E. McKenney
2024-01-23 20:39 ` Linus Torvalds
2024-01-23 22:35 ` Martin Uecker
2024-01-23 20:16 ` H. Peter Anvin
2024-01-23 20:24 ` Linus Torvalds
2024-01-24 14:58 ` Paul E. McKenney
2024-01-25 12:52 ` Paul E. McKenney
2024-01-23 22:39 ` Kees Cook
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