[fp32tofp16] fix subnormal round to normal error

Author: DongBaiYue

paddle/phi/common/float16.h

@@ -123,24 +123,37 @@ struct PADDLE_ALIGN(2) float16 {
     // http://stackoverflow.com/questions/1659440/32-bit-to-16-bit-floating-point-conversion
     Bits v, s;
     v.f = val;
+    // Extract sign bit and clear from value
     uint32_t sign = v.si & sigN;
     v.si ^= sign;
-    sign >>= shiftSign;  // logical shift
+    sign >>= shiftSign;
+
+    // Handle subnormals: normalize using multiplication
+    const uint32_t subnormal_mask = -(minN > v.si);
     s.si = mulN;
-    s.si = s.f * v.f;  // correct subnormals
-    v.si ^= (s.si ^ v.si) & -(minN > v.si);
+    s.si = s.f * v.f;  // Extract the fraction of the subnormal number through
+                       // multiplication and conversion from float to int
+    v.si ^= (s.si ^ v.si) & subnormal_mask;
+
+    // Handle special values: infinity and NaN
     v.si ^= (infN ^ v.si) & -((infN > v.si) & (v.si > maxN));
     v.si ^= (nanN ^ v.si) & -((nanN > v.si) & (v.si > infN));
-    // Rounding conditions (round to nearest, ties to even).
+
+    // Rounding: round to nearest, ties to even
     // https://en.wikipedia.org/wiki/Rounding#Rounding_half_to_even
-    if (v.ui < infN) {  // Skip special values (infinity and NaN)
-      // Lowest significant bit of the retained part
-      const uint32_t lsb = (v.ui >> shift) & 0x1;
-      v.ui = (v.ui + 0xFFF + lsb);  // rounding up
-    }
+    const uint32_t lsb =
+        (v.ui >> shift) & 0x1;               // Least significant retained bit
+    v.ui += (0xFFF + lsb) & -(v.ui < infN);  // Round with overflow protection
+
     v.ui >>= shift;  // logical shift
+
+    // Exponent adjustment for overflow (max values)
     v.si ^= ((v.si - maxD) ^ v.si) & -(v.si > maxC);
-    v.si ^= ((v.si - minD) ^ v.si) & -(v.si > subC);
+    // Exponent adjustment for normal numbers
+    const uint32_t normal_mask = ~subnormal_mask;
+    v.si ^= ((v.si - minD) ^ v.si) & normal_mask;
+
+    // Combine sign and value bits
     x = v.ui | sign;
 
 #endif