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Subject[PATCH v2] linux/bits.h: GENMASK_INPUT_CHECK: reduce W=2 noise by 31% treewide
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This patch silences a -Wtypes-limits warning in GENMASK_INPUT_CHECK()
which is accountable for 31% of all warnings when compiling with W=2.

Indeed, GENMASK_INPUT_CHECK() will generate some warnings at W=2 level
if invoked with an unsigned integer and zero. For example, this:

| #include <linux/bits.h>
| unsigned int foo(unsigned int bar)
| { return GENMASK(bar, 0); }

would yield 30 lines of warning. Extract:

| In file included from ./include/linux/bits.h:22,
| from ./foo.c:1:
| foo.c: In function 'foo':
| ./include/linux/bits.h:25:36: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits]
| 25 | __is_constexpr((l) > (h)), (l) > (h), 0)))
| | ^

This pattern is harmless (false positive) and for that reason,
-Wtypes-limits was moved to W=2. c.f. [1].

However because it occurs in header files (example: find_first_bit()
from linux/find.h [2]), GENMASK_INPUT_CHECK() is accountable for
roughly 31% (164714/532484) of all W=2 warnings for an
allyesconfig. This is an issue because that noise makes it harder to
triage and find relevant W=2 warnings.

Reference (using gcc 11.2, linux v5.17-rc6 on x86_64):

| $ make allyesconfig
| $ sed -i '/CONFIG_WERROR/d' .config
| $ make W=2 -j8 2> kernel_w2.log > /dev/null
| $ grep "\./include/linux/bits\.h:.*: warning" kernel_w2\.log | wc -l
| 164714
| $ grep ": warning: " kernel_w2.log | wc -l
| 532484

In this patch, we silence this warning by:

* replacing the comparison > by and logical and && in the first
argument of __builtin_choose_expr().

* casting the high bit of the mask to a signed integer in the second
argument of __builtin_choose_expr().

[1] https://lore.kernel.org/lkml/20200708190756.16810-1-rikard.falkeborn@gmail.com/
[2] https://elixir.bootlin.com/linux/v5.17-rc6/source/include/linux/find.h#L119

Link: https://lore.kernel.org/lkml/cover.1590017578.git.syednwaris@gmail.com/
Link: https://lore.kernel.org/lkml/20220304124416.1181029-1-mailhol.vincent@wanadoo.fr/
Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of GENMASK inputs")
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
* Changelog *

v1 -> v2:

* Rewrote the commit message to make it less verbose
* Add in CC all people from:
https://lore.kernel.org/lkml/cover.1590017578.git.syednwaris@gmail.com/
---
include/linux/bits.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bits.h b/include/linux/bits.h
index 87d112650dfb..542e9a8985b1 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -22,7 +22,7 @@
#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)))
+ __is_constexpr((h)) && __is_constexpr((l)), (l) > (int)(h), 0)))
#else
/*
* BUILD_BUG_ON_ZERO is not available in h files included from asm files,
--
2.34.1
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