linux-kernel - [PATCH] linux/find: ignore -Wtype-limits to reduce W=2 warnings by 34% tree-wide

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Date:   Wed, 27 Apr 2022 01:16:58 +0900
From:   Vincent Mailhol <mailhol.vincent@...adoo.fr>
To:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>
Subject: [PATCH] linux/find: ignore -Wtype-limits to reduce W=2 warnings by 34% tree-wide

find_first_bit(), find_first_and_bit(), find_first_and_bit() and
find_first_and_bit() all invokes GENMASK(size - 1, 0).

This triggers below warning when compiled with W=2.

| ./include/linux/find.h: In function 'find_first_bit':
| ./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)))
|       |                                    ^
| ./include/linux/build_bug.h:16:62: note: in definition of macro
| 'BUILD_BUG_ON_ZERO'
|    16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
|       |                                                              ^
| ./include/linux/bits.h:25:17: note: in expansion of macro '__is_constexpr'
|    25 |                 __is_constexpr((l) > (h)), (l) > (h), 0)))
|       |                 ^~~~~~~~~~~~~~
| ./include/linux/bits.h:38:10: note: in expansion of macro 'GENMASK_INPUT_CHECK'
|    38 |         (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
|       |          ^~~~~~~~~~~~~~~~~~~
| ./include/linux/find.h:119:45: note: in expansion of macro 'GENMASK'
|   119 |                 unsigned long val = *addr & GENMASK(size - 1, 0);
|       |                                             ^~~~~~~

linux/find.h being a widely used header file, above warning show up in
thousand of files which include this header (either directly on
indirectly).

Because it is a false positive, we just silence -Wtype-limits flag
locally to remove the spam. clang does not warn about it, so we just
apply the diag_ignore() directive to gcc.

By doing so, the warnings for a W=2 build are reduced by
34%. Benchmark was done with gcc 11.2.1 on Linux v5.17 x86_64
allyesconfig (except CONFIG_WERROR). Beforethe patch: 515496 warnings
and after: 340097.

For reference, past proposal to modify GENMASK_INPUT_CHECK() was
rejected in:
https://lore.kernel.org/all/20220304124416.1181029-1-mailhol.vincent@wanadoo.fr/

Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
 include/linux/find.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/find.h b/include/linux/find.h
index 5bb6db213bcb..efd4b3f7dd17 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -103,6 +103,10 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
 }
 #endif
 
+__diag_push();
+__diag_ignore(GCC, 8, "-Wtype-limits",
+	      "GENMASK(size - 1, 0) yields 'comparison of unsigned expression in < 0 is always false' which is OK");
+
 #ifndef find_first_bit
 /**
  * find_first_bit - find the first set bit in a memory region
@@ -193,6 +197,8 @@ unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
 }
 #endif
 
+__diag_pop(); /* ignore -Wtype-limits */
+
 /**
  * find_next_clump8 - find next 8-bit clump with set bits in a memory region
  * @clump: location to store copy of found clump
-- 
2.35.1