| Message ID | 1486058726-7262-1-git-send-email-ard.biesheuvel@linaro.org |
|---|---|
| State | New |
| Headers | show |
diff --git a/include/linux/log2.h b/include/linux/log2.h index fd7ff3d91e6a..d3fe63b12e96 100644 --- a/include/linux/log2.h +++ b/include/linux/log2.h @@ -203,6 +203,17 @@ unsigned long __rounddown_pow_of_two(unsigned long n) * ... and so on. */ -#define order_base_2(n) ilog2(roundup_pow_of_two(n)) +static inline __attribute_const__ +int __order_base_2(unsigned long n) +{ + return n > 1 ? ilog2(n - 1) + 1 : 0; +} +#define order_base_2(n) \ +( \ + __builtin_constant_p(n) ? ( \ + ((n) == 0 || (n) == 1) ? 0 : \ + ilog2((n) - 1) + 1) : \ + __order_base_2(n) \ +) #endif /* _LINUX_LOG2_H */
The function order_base_2() is defined (according to the comment block) as returning zero on input zero, but subsequently passes the input into roundup_pow_of_two(), which is explicitly undefined for input zero. This has gone unnoticed until now, but optimization passes in GCC 7 may produce constant folded function instances where a constant value of zero is passed into order_base_2(), resulting in link errors against the deliberately undefined '____ilog2_NaN'. So update order_base_2() to adhere to its own documented interface. Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> --- v2: make __order_base_2() return int to match the prototype of __ilog2_uXX() This is a followup to Will's message 'Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness' which can be found here: http://marc.info/?l=linux-kernel&m=147672952517795&w=2 include/linux/log2.h | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) -- 2.7.4