Messages in this thread Patch in this message | | | From | Vincent Mailhol <> | Subject | [PATCH] linux/bits.h: fix -Wtype-limits warnings in GENMASK_INPUT_CHECK() | Date | Fri, 4 Mar 2022 21:44:16 +0900 |
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When compiling with -Wtype-limits (activated for example with make W=2), GENMASK_INPUT_CHECK() will generate some warnings if invoked with an unsigned integer and zero.
For example, this:
| #include <linux/bits.h> | u32 foo(u32 bar) | { return GENMASK(bar, 0); }
would yield:
| 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))) | | ^ | ./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)) | | ^~~~~~~~~~~~~~~~~~~ | foo.c:16:10: note: in expansion of macro 'GENMASK' | 16 | { return GENMASK(bar, 0); } | | ^~~~~~~ | ./include/linux/bits.h:25:48: 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:38:10: note: in expansion of macro 'GENMASK_INPUT_CHECK' | 38 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) | | ^~~~~~~~~~~~~~~~~~~ | foo.c:16:10: note: in expansion of macro 'GENMASK' | 16 | { return GENMASK(bar, 0); } | | ^~~~~~~
This pattern is harmless but because it occurs in header files (example find_first_bit() from linux/find.h [1]) and because of the include hell, the macro GENMASK_INPUT_CHECK() is accountable for 31% (164714/532484) of all warnings when compiling all modules at W=2 level.
Reference (using gcc 11.2 and linux v5.17-rc6):
| $ 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
This heavily pollutes the output of make W=2 and make it painful to find other relevant issues.
In this patch, we silent 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().
By doing so, 31% of W=2 warnings get removed.
[1] https://elixir.bootlin.com/linux/v5.17-rc6/source/include/linux/find.h#L119
Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of GENMASK inputs") Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr> --- 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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