SIMD: Portable Masks, C++20 (#4607)

## Summary

Ensure we can use any producing type's `simd_mask`, e.g., `double`, when
setting (in)valid particles, by manipulating our `uint64` in `idcpu`.
This currently requires a conversion/cast in C++. Make this conversion
portable, so far it only worked on GCC/Linux.

This helper needs C++20. Thus, build AMReX conditionally always with
C++20 or newer when SIMD is requested (non-default). Eventually, we will
require C++26 once this becomes standard, and C++29 will have even more
updates.

## Additional background

Portability to macOS/Windows:
- conda-forge/impactx-feedstock#54
- mattkretz/vir-simd#45

## Checklist

The proposed changes:
- [x] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Author: ax3l

Src/Base/AMReX_SIMD.H

@@ -8,6 +8,9 @@
 #ifdef AMREX_USE_SIMD
 // TODO make SIMD provider configurable: VIR (C++17 TS2) or C++26 (later)
 #   include <vir/simd.h>  // includes SIMD TS2 header <experimental/simd>
+#   if __cplusplus >= 202002L
+#       include <vir/simd_cvt.h>
+#   endif
 #endif
 
 #include <cstdint>
@@ -21,6 +24,9 @@ namespace amrex::simd
     namespace stdx {
 #ifdef AMREX_USE_SIMD
         using namespace vir::stdx;
+#   if __cplusplus >= 202002L
+        using vir::cvt;
+#   endif
 #else
         // fallback implementations for functions that are commonly used in portable code paths
 

Src/Particle/AMReX_Particle.H

@@ -106,8 +106,9 @@ namespace amrex {
 #ifdef AMREX_USE_SIMD
             // SIMD
             static_assert(!std::is_same_v<T_SIMD, uint64_t>, "make_invalid: forgot template on wrapper?");
-            // .__cvt() because of: https://github.com/VcDevel/std-simd/issues/41
-            simd::stdx::where(mask.__cvt(), idcpu) &= ~(T_SIMD{1} << 63);
+            // cvt / .__cvt() because of: https://github.com/VcDevel/std-simd/issues/41#issuecomment-3185164952
+            // see also https://github.com/mattkretz/vir-simd/issues/45
+            simd::stdx::where(simd::stdx::cvt(mask), idcpu) &= ~(T_SIMD{1} << 63);
 #else
             AMREX_ALWAYS_ASSERT_WITH_MESSAGE(false, "make_invalid: AMReX_SIMD was not enabled!");
 #endif
@@ -132,8 +133,9 @@ namespace amrex {
 #ifdef AMREX_USE_SIMD
             // SIMD
             static_assert(!std::is_same_v<T_SIMD, uint64_t>, "make_valid: forgot template on wrapper?");
-            // .__cvt() because of: https://github.com/VcDevel/std-simd/issues/41
-            simd::stdx::where(mask.__cvt(), idcpu) |= T_SIMD{1} << 63;
+            // cvt / .__cvt() because of: https://github.com/VcDevel/std-simd/issues/41#issuecomment-3185164952
+            // see also https://github.com/mattkretz/vir-simd/issues/45
+            simd::stdx::where(simd::stdx::cvt(mask), idcpu) |= T_SIMD{1} << 63;
 #else
             AMREX_ALWAYS_ASSERT_WITH_MESSAGE(false, "make_valid: AMReX_SIMD was not enabled!");
 #endif

Tools/CMake/AMReX_Config.cmake

@@ -47,6 +47,12 @@ function (configure_amrex AMREX_TARGET)
    # minimum: C++17
    target_compile_features(${AMREX_TARGET} PUBLIC cxx_std_17)
 
+   # vir::cvt
+   # https://github.com/mattkretz/vir-simd/issues/45
+   if (AMReX_SIMD)
+       target_compile_features(${AMREX_TARGET} PUBLIC cxx_std_20)
+   endif()
+
    if (AMReX_CUDA)
       set_target_properties(${AMREX_TARGET} PROPERTIES CUDA_EXTENSIONS OFF)
       # minimum: C++17