fix(dnn): specialize pow to make it consistent

GitOrigin-RevId: cff3bbbadd28297e6704e19d27b1c1882bd88a66

dnn/src/common/elemwise/kern_defs.cuh

@@ -26,6 +26,8 @@
 #include <algorithm>
 using std::max;
 using std::min;
+
+#define rsqrtf(x) (1.f / sqrt(x))
 #endif
 
 #ifndef MEGDNN_ELEMWISE_MODE_ENABLE
@@ -93,6 +95,30 @@ __device__ __host__ inline float gelu_grad(float x, float dy) {
     return dy * (normcdf_v + x * phi);
 }
 
+__device__ __host__ inline bool feq(float a, float b) {
+    return fabsf(a - b) < 1e-6;
+}
+
+__device__ __host__ inline float dispatch_powf(float x, float y) {
+#define CALL_IF(_v, _stmt) \
+    do {                   \
+        if (feq(y, _v)) {  \
+            return _stmt;  \
+        }                  \
+    } while (0)
+
+    CALL_IF(2.f, x * x);
+    CALL_IF(0.5f, sqrtf(x));
+    CALL_IF(-0.5f, rsqrtf(x));
+    CALL_IF(0.f, 1.f);
+    CALL_IF(1.f, x);
+    CALL_IF(3.f, x * x * x);
+    CALL_IF(-1.f, 1.f / x);
+    CALL_IF(-2.f, 1.f / (x * x));
+#undef CALL_IF
+    return powf(x, y);
+}
+
 #include "src/common/elemwise/each_mode.inl"
 
 template <megcorePlatform_t plat, uint32_t mode, typename dtype>