https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91680
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |jakub at gcc dot gnu.org
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is not that easy. For C99 1 << 31 is UB, but for C89 it is unclear, and I
believe in C++ it is well defined, similarly in the middle-end it is well
defined.
So, while 1 << x is a power of 2, possibly negative, (unsigned long long) (1 <<
x)
is only power of two if x is not 31, otherwise it is -2147483648ULL.
Guess for 256 / q we could still optimize it because 256 / -2147483648ULL == 0
== 256 >> 31, but we can't do that for arbitrary dividend (e.g. variable),
where we can't prove that it makes no difference.
