[SYCL][ESIMD] Implement compile-time evaluated math util. (#2133)

Can't have real recursion, even if constexpr - SYCL device compiler does
not allow that according to the spec.

Signed-off-by: Konstantin S Bobrovsky <[email protected]>

Author: kbobrovs

sycl/include/CL/sycl/intel/esimd/detail/esimd_util.hpp

@@ -17,10 +17,42 @@ namespace __esimd {
 /// Constant in number of bytes.
 enum { BYTE = 1, WORD = 2, DWORD = 4, QWORD = 8, OWORD = 16, GRF = 32 };
 
-/// Compute the next power of 2 at compile time.
-static ESIMD_INLINE constexpr unsigned int getNextPowerOf2(unsigned int n,
-                                                           unsigned int k = 1) {
-  return (k >= n) ? k : getNextPowerOf2(n, k * 2);
+/// Compute next power of 2 of a constexpr with guaranteed compile-time
+/// evaluation.
+template <unsigned int N, unsigned int K, bool K_gt_eq_N> struct NextPowerOf2;
+
+template <unsigned int N, unsigned int K> struct NextPowerOf2<N, K, true> {
+  static constexpr unsigned int get() { return K; }
+};
+
+template <unsigned int N, unsigned int K> struct NextPowerOf2<N, K, false> {
+  static constexpr unsigned int get() {
+    return NextPowerOf2<N, K * 2, K * 2 >= N>::get();
+  }
+};
+
+template <unsigned int N> unsigned int getNextPowerOf2() {
+  return NextPowerOf2<N, 1, (1 >= N)>::get();
+}
+
+template <> unsigned int getNextPowerOf2<0>() { return 0; }
+
+/// Compute binary logarithm of a constexpr with guaranteed compile-time
+/// evaluation.
+template <unsigned int N, bool N_gt_1> struct Log2;
+
+template <unsigned int N> struct Log2<N, false> {
+  static constexpr unsigned int get() { return 0; }
+};
+
+template <unsigned int N> struct Log2<N, true> {
+  static constexpr unsigned int get() {
+    return 1 + Log2<(N >> 1), ((N >> 1) > 1)>::get();
+  }
+};
+
+template <unsigned int N> constexpr unsigned int log2() {
+  return Log2<N, (N > 1)>::get();
 }
 
 /// Check if a given 32 bit positive integer is a power of 2 at compile time.
@@ -33,10 +65,6 @@ static ESIMD_INLINE constexpr bool isPowerOf2(unsigned int n,
   return (n & (n - 1)) == 0 && n <= limit;
 }
 
-static ESIMD_INLINE constexpr unsigned log2(unsigned n) {
-  return (n > 1) ? 1 + log2(n >> 1) : 0;
-}
-
 } // namespace __esimd
 
 __SYCL_INLINE_NAMESPACE(cl) {

sycl/include/CL/sycl/intel/esimd/esimd_math.hpp

@@ -1884,7 +1884,7 @@ T1 esimd_reduce(simd<T1, SZ> v) {
   if constexpr (isPowerOf2) {
     return esimd_reduce_single<T0, T1, SZ, OpType>(v);
   } else {
-    constexpr unsigned N1 = 1u << __esimd::log2(SZ);
+    constexpr unsigned N1 = 1u << __esimd::log2<SZ>();
     constexpr unsigned N2 = SZ - N1;
 
     simd<T1, N1> v1 = v.template select<N1, 1>(0);

sycl/include/CL/sycl/intel/esimd/esimd_memory.hpp

@@ -567,7 +567,7 @@ media_block_load(AccessorTy acc, unsigned x, unsigned y) {
   static_assert(plane <= 3u, "valid plane index is in range [0, 3]");
 #if defined(__SYCL_DEVICE_ONLY__) && defined(__SYCL_EXPLICIT_SIMD__)
   constexpr unsigned int RoundedWidth =
-      Width < 4 ? 4 : __esimd::getNextPowerOf2(Width);
+      Width < 4 ? 4 : __esimd::getNextPowerOf2<Width>();
 
   if constexpr (Width < RoundedWidth) {
     constexpr unsigned int n1 = RoundedWidth / sizeof(T);
@@ -617,7 +617,7 @@ media_block_store(AccessorTy acc, unsigned x, unsigned y, simd<T, m * n> vals) {
   static_assert(plane <= 3u, "valid plane index is in range [0, 3]");
 #if defined(__SYCL_DEVICE_ONLY__) && defined(__SYCL_EXPLICIT_SIMD__)
   constexpr unsigned int RoundedWidth =
-      Width < 4 ? 4 : __esimd::getNextPowerOf2(Width);
+      Width < 4 ? 4 : __esimd::getNextPowerOf2<Width>();
   constexpr unsigned int n1 = RoundedWidth / sizeof(T);
 
   if constexpr (Width < RoundedWidth) {