On Tue, 29 Dec 2020, Richard Sandiford via Gcc wrote:
Any thoughts on what f should return in the following testcase, given the
usual GNU behaviour of treating signed >> as arithmetic shift right?
typedef int vs4 __attribute__((vector_size(16)));
typedef unsigned int vu4 __attribute__((vector_size(16)));
int
f (void)
{
vs4 x = { -1, -1, -1, -1 };
vu4 y = { 0, 0, 0, 0 };
return ((x + y) >> 1)[0];
}
The C frontend takes the type of x+y from the first operand, so x+y
is signed and f returns -1.
Symmetry is an important property of addition in C/C++.
The C++ frontend applies similar rules to x+y as it would to scalars,
with unsigned T having a higher rank than signed T, so x+y is unsigned
and f returns 0x7fffffff.
That looks like the most natural choice.
FWIW, Clang treats x+y as signed, so f returns -1 for both C and C++.
I think clang follows gcc and uses the type of the first operand.
--
Marc Glisse
