Description
Here is the current Clang implementation of vec_pack_to_short_fp32:
llvm-project/clang/lib/Headers/altivec.h
Lines 7518 to 7527 in 5f5cf60
Here is the corrected implementation of vec_pack_to_short_fp32:
static __inline__ vector unsigned short __ATTRS_o_ai
vec_pack_to_short_fp32(vector float __a, vector float __b) {
vector unsigned int __resa = (vector unsigned int)__builtin_vsx_xvcvsphp(__a);
vector unsigned int __resb = (vector unsigned int)__builtin_vsx_xvcvsphp(__b);
return vec_pack(__resa, __resb);
}Here is a test program (which needs to be compiled with the -std=c11 -mcpu=power9 options) that can be used to check the results of the vec_pack_to_short_fp32 operation:
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdalign.h>
#pragma push_macro("vector")
#pragma push_macro("pixel")
#pragma push_macro("bool")
#undef vector
#undef pixel
#undef bool
#include <altivec.h>
#pragma pop_macro("vector")
#pragma pop_macro("pixel")
#pragma pop_macro("bool")
inline __attribute__((__always_inline__, __artificial__))
double Float16BitsToDouble(uint16_t f16_bits) {
double dbl_result;
__vector unsigned short f16_vect = vec_splats(f16_bits);
__asm__("xscvhpdp %x0,%x1"
: "=wa" (dbl_result)
: "wa" (f16_vect));
return dbl_result;
}
int main(int argc, char** argv) {
alignas(16) float input_vals[8];
alignas(16) uint16_t result_vals[8];
for(int i = 1; i < argc; ) {
input_vals[0] = strtof(argv[i++], NULL);
input_vals[1] = (i < argc) ? strtof(argv[i++], NULL) : 0.0f;
input_vals[2] = (i < argc) ? strtof(argv[i++], NULL) : 0.0f;
input_vals[3] = (i < argc) ? strtof(argv[i++], NULL) : 0.0f;
input_vals[4] = (i < argc) ? strtof(argv[i++], NULL) : 0.0f;
input_vals[5] = (i < argc) ? strtof(argv[i++], NULL) : 0.0f;
input_vals[6] = (i < argc) ? strtof(argv[i++], NULL) : 0.0f;
input_vals[7] = (i < argc) ? strtof(argv[i++], NULL) : 0.0f;
__vector float src_vect_a =
*((const __vector float*)input_vals);
__vector float src_vect_b =
*((const __vector float*)(input_vals + 4));
__vector unsigned short result_vect =
vec_pack_to_short_fp32(src_vect_a, src_vect_b);
*((__vector unsigned short*)result_vals) = result_vect;
for(int j = 0; j < 8; j++)
printf("Float32ToFloat16(%g) = %g\n",
input_vals[j], Float16BitsToDouble(result_vals[j]));
}
return 0;
}Here are the results of running the above test program when compiled with GCC 12:
$ ./vsx_vec_pack_to_short_fp32_test_021623_gcc 1.518 2.4447 3.3932 6.4842 -1.4912 -3.3938 -7.532 6.6662
Float32ToFloat16(1.518) = 1.51758
Float32ToFloat16(2.4447) = 2.44531
Float32ToFloat16(3.3932) = 3.39258
Float32ToFloat16(6.4842) = 6.48438
Float32ToFloat16(-1.4912) = -1.49121
Float32ToFloat16(-3.3938) = -3.39453
Float32ToFloat16(-7.532) = -7.53125
Float32ToFloat16(6.6662) = 6.66797Here are the results of running the above test program when compiled with Clang 15 or Clang 17:
$ ./vsx_vec_pack_to_short_fp32_test_021623_clang 1.518 2.4447 3.3932 6.4842 -1.4912 -3.3938 -7.532 6.6662
Float32ToFloat16(1.518) = 1.51758
Float32ToFloat16(2.4447) = 0
Float32ToFloat16(3.3932) = -1.49121
Float32ToFloat16(6.4842) = 0
Float32ToFloat16(-1.4912) = 3.39258
Float32ToFloat16(-3.3938) = 0
Float32ToFloat16(-7.532) = -7.53125
Float32ToFloat16(6.6662) = 0