use intrinsics correctly

Author: VSadov

src/coreclr/inc/gcinfotypes.h

@@ -47,28 +47,31 @@ __forceinline size_t SAFE_SHIFT_RIGHT(size_t x, size_t count)
 
 inline UINT32 CeilOfLog2(size_t x)
 {
-    // we want lzcnt, but bsr is ok too
+    // it is ok to use bsr or clz unconditionally
     _ASSERTE(x > 0);
 
     x = (x << 1) - 1;
 
 #ifdef TARGET_64BIT
 #ifdef _MSC_VER
-    DWORD lzcountCeil;
-    _BitScanReverse64(&lzcountCeil, (unsigned long)x);
+    DWORD result;
+    _BitScanReverse64(&result, (unsigned long)x);
+    return (UINT32)result;
 #else // _MSC_VER
-    UINT32 lzcountCeil = (UINT32)__builtin_clzl((unsigned long)x);
+    // LZCNT returns index starting from MSB, whereas BSR gives the index from LSB.
+    // 63 ^ BSR here is equivalent to 63 - BSR since the BSR result is always between 0 and 63.
+    // This saves an instruction, as subtraction from constant requires either MOV/SUB or NEG/ADD.
+    return (UINT32)63 ^ (UINT32)__builtin_clzl((unsigned long)x);
 #endif // _MSC_VER
 #else // TARGET_64BIT
 #ifdef _MSC_VER
-    DWORD lzcountCeil;
-    _BitScanReverse(&lzcountCeil, (unsigned long)x);
+    DWORD result;
+    _BitScanReverse(&result, (unsigned int)x);
+    return (UINT32)result;
 #else // _MSC_VER
-    UINT32 lzcountCeil = (UINT32)__builtin_clz((unsigned int)x);
+    return (UINT32)31 ^ (UINT32)__builtin_clz((unsigned int)x);
 #endif // _MSC_VER
 #endif
-
-    return BITS_PER_SIZE_T - lzcountCeil;
 }
 
 enum GcSlotFlags