linux-kernel - [RESEND PATCH v3 2/2] linux/bits.h: simplify GENMASK_INPUT

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Message-ID: <20241112190840.601378-6-mailhol.vincent@wanadoo.fr>
Date: Wed, 13 Nov 2024 04:08:40 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
	Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Cc: linux-kernel@...r.kernel.org,
	linux-sparse@...r.kernel.org,
	Rikard Falkeborn <rikard.falkeborn@...il.com>,
	Vincent Mailhol <mailhol.vincent@...adoo.fr>
Subject: [RESEND PATCH v3 2/2] linux/bits.h: simplify GENMASK_INPUT_CHECK()

In GENMASK_INPUT_CHECK(),

  __builtin_choose_expr(__is_constexpr((l) > (h)), (l) > (h), 0)

is the exact expansion of:

  _statically_true((l) > (h))

Apply _statically_true() to simplify GENMASK_INPUT_CHECK().

Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
This change passes the unit tests from CONFIG_BITS_TEST, including the
extra negative tests provided under #ifdef TEST_GENMASK_FAILURES [1].

[1] commit 6d511020e13d ("lib/test_bits.c: add tests of GENMASK")
Link: https://git.kernel.org/torvalds/c/6d511020e13d

** Changelog **

v2 -> v3:

   - split the single patch into a series of two patches.

   - add greater explanation of why _statically_true() is needed in
     addition of the existing statically_true(). Explain the pros and
     cons of both.

   - use __builtin_choose_expr() in _statically_true(). The
     _statically_true() of the v2 works perfectly fine when used in
     conjunction with BUILD_BUG_ON_ZERO() but fails if used in arrays
     or in static_assert()

   - update the patch description accordingly

Link: https://lore.kernel.org/all/20241111164743.339117-2-mailhol.vincent@wanadoo.fr/

v1 -> v2:

   - introduce _statically_true(), taking inspiration from
     statically_true() as introduced in commit 22f546873149 ("minmax:
     improve macro expansion and type checking").

Link: https://lore.kernel.org/all/20240609073513.256179-1-mailhol.vincent@wanadoo.fr/
---
 include/linux/bits.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/bits.h b/include/linux/bits.h
index 60044b608817..01713e1eda56 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -20,9 +20,8 @@
  */
 #if !defined(__ASSEMBLY__)
 #include <linux/build_bug.h>
-#define GENMASK_INPUT_CHECK(h, l) \
-	(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
-		__is_constexpr((l) > (h)), (l) > (h), 0)))
+#include <linux/compiler.h>
+#define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(_statically_true((l) > (h)))
 #else
 /*
  * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
-- 
2.45.2