On Thu, 18 Jul 2024, Richard Biener wrote:
> The following adds support for vector conditionals in C. The support
> was nearly there already but c_objc_common_truthvalue_conversion
> rejecting vector types. Instead of letting them pass there unchanged
> I chose to instead skip it when parsing conditionals instead as a
> variant with less possible fallout. The part missing was promotion
> of a scalar second or third operand to vector, I copied the logic
> from binary operator handling for this.
>
> I've moved the testcases I could easily spot over to c-c++-common.
>
> The C++ frontend implements vector ? '1' : '0' with promotion of
> both scalar to a vector but without applying integer promotions
> first (r0-124280-g07298ffd6f7c2d). I don't think this is
> what we document or backed by OpenCL or C++ valarray.
I think we do document that:
If both b and c are scalars and the type of true?b:c has the same size
as the element type of a, then b and c are converted to a vector type
whose elements have this type and with the same number of elements as a.
(in https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html )
and I added
In contrast to scalar operations in C and C++, operands of integer vector
operations do not undergo integer promotions.
when spelling out the behavior of vector shifts.
Clang also supports 'vector ? scalar : scalar'.
> I've left
> this out for now. I think we need better coverage in the testsuite
> for this and others, for example the patch below now accepts
> vector(int) ? vector(short) : 0 but the C++ frontend rejects this
> (I think OpenCL is fine with this). OpenCL says the conditional
> op is equivalent to select(exp3,exp2,exp1) which is defined as
> result[i] = if MSB of c[i] is set ? b[i] : a[i], with no restriction
> on the conditional.
I think we require that sizes match because that's natural considering
how it is lowered (bitwise blending of comparison mask with op2/op3).
Note that in Clang ext_vector_type has OpenCL semantics for c?a:b
(taking the MSB of 'c') while vector_size has GNU semantics ('c' is
compared with zero).
Thanks.
Alexander
