linux-kernel - Re: [PATCH v4 1/2] compiler.h: add const

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Message-ID: <Z3LnNBWn8dHZIo7E@yury-ThinkPad>
Date: Mon, 30 Dec 2024 10:32:20 -0800
From: Yury Norov <yury.norov@...il.com>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
	linux-kernel@...r.kernel.org, linux-sparse@...r.kernel.org,
	Rikard Falkeborn <rikard.falkeborn@...il.com>
Subject: Re: [PATCH v4 1/2] compiler.h: add const_true()

On Wed, Nov 13, 2024 at 10:53:55AM -0800, Yury Norov wrote:
> On Thu, Nov 14, 2024 at 02:18:32AM +0900, Vincent Mailhol wrote:
> > __builtin_constant_p() is known for not always being able to produce
> > constant expression [1] which led to the introduction of
> > __is_constexpr() [2]. Because of its dependency on
> > __builtin_constant_p(), statically_true() suffers from the same
> > issues.
> > 
> > For example:
> > 
> >   void foo(int a)
> >   {
> >   	 /* fail on GCC */
> >   	BUILD_BUG_ON_ZERO(statically_true(a));
> > 
> >   	 /* fail on both clang and GCC */
> >   	static char arr[statically_true(a) ? 1 : 2];
> >   }
> > 
> > For the same reasons why __is_constexpr() was created to cover
> > __builtin_constant_p() edge cases, __is_constexpr() can be used to
> > resolve statically_true() limitations.
> > 
> > Note that, somehow, GCC is not always able to fold this:
> > 
> >   __is_constexpr(x) && (x)
> > 
> > It is OK in BUILD_BUG_ON_ZERO() but not in array declarations nor in
> > static_assert():
> > 
> >   void bar(int a)
> >   {
> >   	/* success */
> >   	BUILD_BUG_ON_ZERO(__is_constexpr(a) && (a));
> > 
> >   	/* fail on GCC */
> >   	static char arr[__is_constexpr(a) && (a) ? 1 : 2];
> > 
> >   	/* fail on GCC */
> >   	static_assert(__is_constexpr(a) && (a));
> >   }
> > 
> > Encapsulating the expression in a __builtin_choose_expr() switch
> > resolves all these failed tests.
> > 
> > Define a new const_true() macro which, by making use of the
> > __builtin_choose_expr() and __is_constexpr(x) combo, always produces a
> > constant expression.
> > 
> > It should be noted that statically_true() is the only one able to fold
> > tautologic expressions in which at least one on the operands is not a
> > constant expression. For example:
> > 
> >   statically_true(true || var)
> >   statically_true(var == var)
> >   statically_true(var * 0 + 1)
> >   statically_true(!(var * 8 % 4))
> > 
> > always evaluates to true, whereas all of these would be false under
> > const_true() if var is not a constant expression [3].
> > 
> > For this reason, usage of const_true() be should the exception.
> > Reflect in the documentation that const_true() is less powerful and
> > that statically_true() is the overall preferred solution.
> > 
> > [1] __builtin_constant_p cannot resolve to const when optimizing
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
> > 
> > [2] commit 3c8ba0d61d04 ("kernel.h: Retain constant expression output for max()/min()")
> > Link: https://git.kernel.org/torvalds/c/3c8ba0d61d04
> > 
> > [3] https://godbolt.org/z/c61PMxqbK
> > 
> > Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> 
> For the series:
> 
> Reviewed-by: Yury Norov <yury.norov@...il.com>
> 
> If no objections, I'll move it with my tree.

This is already in my branch, but there was a discussion after I pulled
it. Can you guys tell me what is your conclusion on that? Should I
keep it in the branch, or drop?

Thanks,
Yury