johnplatts
commented
1 year ago Here is an example of Highway dynamic dispatch code updated to support multi-phase compilation (compiled more than once with different compiler options for the different compilation phases):
// Generates code for every target that this compiler can support.
#undef HWY_TARGET_INCLUDE
#define HWY_TARGET_INCLUDE "example.cpp" // this file
#include <hwy/foreach_target.h> // must come before highway.h
#include <hwy/highway.h>
HWY_BEFORE_NAMESPACE();
namespace project {
namespace HWY_NAMESPACE { // required: unique per target
// Can skip hn:: prefixes if already inside hwy::HWY_NAMESPACE.
namespace hn = hwy::HWY_NAMESPACE;
using T = float;
void MulAddLoop(const T* HWY_RESTRICT mul_array,
const T* HWY_RESTRICT add_array,
const size_t size, T* HWY_RESTRICT x_array);
#if HWY_IN_PER_TARGET_PHASE
void MulAddLoop(const T* HWY_RESTRICT mul_array,
const T* HWY_RESTRICT add_array,
const size_t size, T* HWY_RESTRICT x_array) {
const hn::ScalableTag<T> d;
for (size_t i = 0; i < size; i += hn::Lanes(d)) {
const auto mul = hn::Load(d, mul_array + i);
const auto add = hn::Load(d, add_array + i);
auto x = hn::Load(d, x_array + i);
x = hn::MulAdd(mul, x, add);
hn::Store(x, d, x_array + i);
}
}
#endif
} // namespace HWY_NAMESPACE
} // namespace project
HWY_AFTER_NAMESPACE();
// The table of pointers to the various implementations in HWY_NAMESPACE must
// be compiled only once (foreach_target #includes this file multiple times).
// HWY_ONCE is true for only one of these 'compilation passes'.
#if HWY_ONCE && HWY_IN_DYN_DISPATCH_PHASE
namespace project {
// This macro declares a static array used for dynamic dispatch.
HWY_EXPORT(MulAddLoop);
void CallMulAddLoop(const float* HWY_RESTRICT mul_array,
const float* HWY_RESTRICT add_array,
const size_t size, float* HWY_RESTRICT x_array) {
// This must reside outside of HWY_NAMESPACE because it references (calls the
// appropriate one from) the per-target implementations there.
// For static dispatch, use HWY_STATIC_DISPATCH.
return HWY_DYNAMIC_DISPATCH(MulAddLoop)(mul_array, add_array, size, x_array);
}
} // namespace project
#endif // HWY_ONCEHere is a link to the above example on Compiler Explorer that shows the above code compiled with different options for HWY_IN_PER_TARGET_PHASE/HWY_IN_DYN_DISPATCH_PHASE: https://gcc.godbolt.org/z/63xTfh1bj
jan-wassenberg
Pflugshaupt
There are some dynamic dispatch scenarios that require compiling the same C++ source files more than once (but with different C++ flags for some of the compilation phases), such as x86-64 with MSVC if AVX2/AVX3 targets are enabled, AArch64 if SVE/SVE2 targets are enabled, or PPC if PPC8/PPC9/PPC10 targets are enabled.
Here are the compilation phases for multi-phase compilation with MSVC on x86-64:
Here are the compilation phases for multi-phase compilation for AArch64 with SVE/SVE2 enabled:
Here are the compilation phases for multi-phase compilation for PPC64:
There are real-world use cases for multiple compilation dynamic dispatch, including improved performance on PPC9/PPC10/AArch64.