On Mon, 23 Nov 2015, Arnd Bergmann wrote:
> 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.
But here it must have returned true, and is_power_of_2() returned true
as well (which implies that __base is not zero), ans somehow aving an
additional __base != 0 test changes the outcome. There is a correctness
issue beyond __builtin_constant_p it seems.
> Note that llvm will always return false for __builtin_constant_p on
> non-pointer arguments, which breaks a lot of optimizations.
If llvm is able to optimize this case on its own then we won't need all
this contraption.
Nicolas
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