Hi GCC Team, Aarch64 Maintainers,
The rules in Vector Function Application Binary Interface Specification for
OpenMP
(https://sourceware.org/glibc/wiki/libmvec?action=AttachFile&do=view&target=VectorABI.txt)
is used in x86 for generating the simd clones of a function.
Is there a similar one defined for Aarch64?
If not, would like to start a discussion on the same for Aarch64. To kick
start the same, a draft proposal for Aarch64 (on the same lines as x86 ABI) is
included below. The only change from x86 ABI is in the function name mangling.
Here the letter 'b' is used for indicating the ASIMD isa.
Please review and comment.
Thanks and Regards,
Ashwin Sekhar T K
CUT HERE --
Aarch64 Vector Function Application Binary Interface Specification for OpenMP
1. Vector Function ABI Overview
Aarch64 Vector Function ABI provides ABI for the vector functions generated by
compiler supporting SIMD constructs of OpenMP 4.0 [1] in Aarch64. This is
based on the x86 Vector Function Application Binary Interface Specification for
OpenMP [2].
2. Vector Function ABI
Vector Function ABI defines a set of rules that the caller and the callee
functions must obey.
These rules consist of:
* Calling convention
* Vector length (the number of concurrent scalar invocations to be processed
per invocation of the vector function)
* Mapping from element data types to vector data types
* Ordering of vector arguments
* Vector function masking
* Vector function name mangling
* Compiler generated variants of vector function
2.1. Calling Convention
The vector functions should use calling convention described in Procedure Call
Standard for the ARM 64-bit Architecture (AArch64) [3].
2.2. Vector Length
Every vector variant of a SIMD-enabled function has a vector length (VLEN). If
OpenMP clause "simdlen" is used, the VLEN is the value of the argument of that
clause. The VLEN value must be power of 2. In other case the notion of the
function`s "characteristic data type" (CDT) is used to compute the vector
length.
CDT is defined in the following order:
a) For non-void function, the CDT is the return type.
b) If the function has any non-uniform, non-linear parameters, then the CDT
is the type of the first such parameter.
c) If the CDT determined by a) or b) above is struct, union, or class type
which is pass-by-value (except for the type that maps to the built-in
complex data type), the characteristic data type is int.
d) If none of the above three cases is applicable, the CDT is int.
VLEN = sizeof(vector_register) / sizeof(CDT),
For example, if ISA is ASIMD, sizeof(vector_register) = 16, as the vector
registers are 128 bit. And if the CDT of the function is "int", sizeof(CDT) = 4.
So, VLEN = 4.
2.3. Element Data Type to Vector Data Type Mapping
The vector data types for parameters are selected depending on ISA, vector
length, data type of original parameter, and parameter specification.
For uniform and linear parameters (detailed description could be found in [1]),
the original data type is preserved.
For vector parameters, vector data types are selected by the compiler. The
mapping from element data type to vector data type is described as below.
* The bit size of vector data type of parameter is computed as:
size_of_vector_data_type = VLEN * sizeof(original_parameter_data_type) * 8
For instance, for ASIMD version of vector function with parameter data type
"int": If VLEN = 4, size_of_vector_data_type = 4 * 4 * 8 = 128 (bits), which
means one argument of type __m128 to be passed.
* If the size_of_vector_data_type is greater than the width of the vector
register, multiple vector registers are selected and the parameter will be
passed in multiple vector registers.
For instance, for ASIMD version of vector function with parameter data type
"int":
If VLEN = 8, size_of_vector_data_type = 8 * 4 * 8 = 256 (bits), so the
vector data type is __m256, which means 2 arguments of type __m128 are to
be passed.
2.4. Ordering of Vector Arguments
* When a parameter in the original data type results in one argument in the
vector function, the ordering rule is a simple one to one match with the
original argument order.
For example, when the original argument list is (int a, float b,
