I've been trying to get some std::atan2 code to auto vectorize. I've been able to get it to the point where GCC doesn't complain about asin, but it doesn't seem to be able to handle atan2 (nor atan for that matter).
Here is a link to the godbolt version. here is the source code:
#include <cmath>
#include <cstddef>
#include <array>
constexpr std::size_t array_length = 16;
void calc_uv_coordinates(
const std::array<float,array_length>& pos_x,
const std::array<float,array_length>& pos_y,
const std::array<float,array_length>& pos_z,
std::array<float,array_length>& tex_u,
std::array<float,array_length>& tex_v) noexcept{
#pragma clang loop vectorize(enable)
for(std::size_t i = 0; i < array_length; ++i){
tex_u[i] = std::atan2(pos_z[i], pos_x[i]) / (2 * M_PI);
tex_v[i] = std::asin(pos_y[i]) / M_PI;
}
}
Here are the compiler arguments:
clang 9.0: -c -Ofast -ffast-math -Rpass-analysis=loop-vectorize
gcc 9.2: -c -Ofast -ffast-math -fopt-info-vec-missed -fopt-info-vec
Clang says:
In file included from <source>:2:
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/cmath:145:12: remark: loop not vectorized: call instruction cannot be vectorized [-Rpass-analysis]
{ return __builtin_atan2f(__y, __x); }
^
<source>:15:5: warning: loop not vectorized: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering [-Wpass-failed=transform-warning]
for(std::size_t i = 0; i < array_length; ++i){
^
1 warning generated.
ASM generation compiler returned: 0
clang-9: warning: -Wl,-rpath,/opt/compiler-explorer/clang-9.0.0/lib: 'linker' input unused [-Wunused-command-line-argument]
clang-9: warning: -Wl,-rpath,/opt/compiler-explorer/clang-9.0.0/lib32: 'linker' input unused [-Wunused-command-line-argument]
clang-9: warning: -Wl,-rpath,/opt/compiler-explorer/clang-9.0.0/lib64: 'linker' input unused [-Wunused-command-line-argument]
In file included from <source>:2:
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/cmath:145:12: remark: loop not vectorized: call instruction cannot be vectorized [-Rpass-analysis]
{ return __builtin_atan2f(__y, __x); }
^
<source>:15:5: warning: loop not vectorized: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering [-Wpass-failed=transform-warning]
for(std::size_t i = 0; i < array_length; ++i){
^
1 warning generated.
Execution build compiler returned: 0
Program returned: 1
Error: no suitable ./output.s executable found
GCC says:
<source>:15:30: missed: couldn't vectorize loop
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/cmath:145:28: missed: not vectorized: relevant stmt not supported: _15 = __builtin_atan2f (_2, _1);
ASM generation compiler returned: 0
<source>:15:30: missed: couldn't vectorize loop
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/cmath:145:28: missed: not vectorized: relevant stmt not supported: _15 = __builtin_atan2f (_2, _1);
Execution build compiler returned: 0
Program returned: 1
Error: no suitable ./output.s executable found
I'm not sure I understand what relevant stmt not supported: _15 = __builtin_atan2f (_2, _1); is saying, I believe it is saying that GCC has not put in the manual effort to auto vectorize the built in. It appears that GCC only auto vectorizes trig with built-in assembly instructions, but I don't understand why that forces atan2f off the table for vectorization, or why that makes it any different than an arbitrary arithmetic function.
Other libraries have SIMD atan and atan2, so it isn't like this function is impossible to vectorize. In principle this should just be each part of atan2 being vectorized along all elements.