On Monday 23 November 2015 11:04:33 Nicolas Pitre wrote:
>
> OK... I'm able to "fix" the build with:
>
> diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
> index 163f77999e..d246c4c801 100644
> --- a/include/asm-generic/div64.h
> +++ b/include/asm-generic/div64.h
> @@ -206,7 +206,7 @@ extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
> uint32_t __rem; \
> (void)(((typeof((n)) *)0) == ((uint64_t *)0)); \
> if (__builtin_constant_p(__base) && \
> - is_power_of_2(__base)) { \
> + is_power_of_2(__base) && __base != 0) { \
> __rem = (n) & (__base - 1); \
> (n) >>= ilog2(__base); \
> } else if (__div64_const32_is_OK && \
>
> What doesn't make sense to me is the fact that is_power_of_2() is
> defined as:
>
> static inline __attribute__((const))
> bool is_power_of_2(unsigned long n)
> {
> return (n != 0 && ((n & (n - 1)) == 0));
> }
>
> So the test for zero is already in there.
>
> And adding BUILD_BUG_ON(__builtin_constant_p(__base) && __base == 0)
> before the if doesn't trig either.
I've seen similarly messed up situations with PROFILE_ALL_BRANCHES
before, I think it's got something to do with how __builtin_constant_p()
is used inside of the __trace_if() macro, and how gcc sometimes falls
back to treating variables as not-really-constant based on context.
To gcc, __builtin_constant_p is just best-effort, and they don't care
about returning false sometimes if they catch most cases in practice.
Note that llvm will always return false for __builtin_constant_p on
non-pointer arguments, which breaks a lot of optimizations.
Arnd
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