Add amrex::Math::rsqrt (#4777)

This PR adds a function to compute the inverse square root of a single-
or double-precision value. On GPUs this should be faster than doing the
inverse and square root separately.

Author: AlexanderSinn

Src/Base/AMReX_Math.H

@@ -325,6 +325,34 @@ T comp_ellint_2 (T k)
     return Kcomp*sum_val;
 }
 
+//! Return inverse square root of x
+AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+double rsqrt (double x)
+{
+    double r;
+#if defined(AMREX_USE_SYCL)
+    AMREX_IF_ON_DEVICE(( r = sycl::rsqrt(x); ))
+#else
+    AMREX_IF_ON_DEVICE(( r = ::rsqrt(x); ))
+#endif
+    AMREX_IF_ON_HOST(( r = 1. / std::sqrt(x); ))
+    return r;
+}
+
+//! Return inverse square root of x
+AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+float rsqrt (float x)
+{
+    float r;
+#if defined(AMREX_USE_SYCL)
+    AMREX_IF_ON_DEVICE(( r = sycl::rsqrt(x); ))
+#else
+    AMREX_IF_ON_DEVICE(( r = ::rsqrtf(x); ))
+#endif
+    AMREX_IF_ON_HOST(( r = 1.F / std::sqrt(x); ))
+    return r;
+}
+
 /***************************************************************************************************
  * Copyright (c) 2017 - 2024 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: BSD-3-Clause