Common: add count trailing zeros Ctz

Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>

Author: f3sch

GPU/Common/GPUCommonMath.h

@@ -108,6 +108,7 @@ class GPUCommonMath
   GPUd() constexpr static float QuietNaN() { return GPUCA_CHOICE(std::numeric_limits<float>::quiet_NaN(), __builtin_nanf(""), nan(0u)); }
 #endif
   GPUd() constexpr static uint32_t Clz(uint32_t val);
+  GPUd() constexpr static uint32_t Ctz(uint32_t val);
   GPUd() constexpr static uint32_t Popcount(uint32_t val);
 
   GPUd() static void memcpy(void* dst, const void* src, size_t size);
@@ -332,6 +333,20 @@ GPUdi() constexpr uint32_t GPUCommonMath::Clz(uint32_t x)
 #endif
 }
 
+GPUdi() constexpr uint32_t GPUCommonMath::Ctz(uint32_t x)
+{
+#if (defined(__GNUC__) || defined(__clang__) || defined(__CUDACC__) || defined(__HIPCC__))
+  return x == 0 ? 32 : GPUCA_CHOICE(__builtin_ctz(x), __ffs(x) - 1, __builtin_ctz(x));
+#else
+  for (uint32_t i = 0; i < 32; ++i) {
+    if (x & (1u << i)) {
+      return i;
+    }
+  }
+  return 32;
+#endif
+}
+
 GPUdi() constexpr uint32_t GPUCommonMath::Popcount(uint32_t x)
 {
 #if (defined(__GNUC__) || defined(__clang__) || defined(__CUDACC__) || defined(__HIPCC__)) && !defined(__OPENCL__) // TODO: remove OPENCL when reported SPIR-V bug is fixed