APInt: Slightly simplify countLeadingZerosSlowCase()

[MatzeB]

Summary

We always clear the unused bits in the most signifant word so there is
no need to mask them out in countLeadingZerosSlowCase().

This is a drive-by simplification while I was readining APInt code...

[MatzeB]

ping

[bkramer]

lg, with one minor comment.

lib/Support/APInt.cpp

692 ↗

(On Diff #46095)

Typo: signifant 🐘

llvm/trunk/lib/Support/APInt.cpp

 
 /// This function returns the low "numBits" bits of this APInt.
 APInt APInt::getLoBits(unsigned numBits) const {
   return APIntOps::lshr(APIntOps::shl(*this, BitWidth - numBits),
                         BitWidth - numBits);
 }
 
 unsigned APInt::countLeadingZerosSlowCase() const {
-  // Treat the most significand word differently because it might have
-  // meaningless bits set beyond the precision.
-  unsigned BitsInMSW = BitWidth % APINT_BITS_PER_WORD;
-  integerPart MSWMask;
-  if (BitsInMSW) MSWMask = (integerPart(1) << BitsInMSW) - 1;
-  else {
-    MSWMask = ~integerPart(0);
-    BitsInMSW = APINT_BITS_PER_WORD;
-  }
-
-  unsigned i = getNumWords();
-  integerPart MSW = pVal[i-1] & MSWMask;
-  if (MSW)
-    return llvm::countLeadingZeros(MSW) - (APINT_BITS_PER_WORD - BitsInMSW);
-
-  unsigned Count = BitsInMSW;
-  for (--i; i > 0u; --i) {
-    if (pVal[i-1] == 0)
+  unsigned Count = 0;
+  for (int i = getNumWords()-1; i >= 0; --i) {
+    integerPart V = pVal[i];
+    if (V == 0)
       Count += APINT_BITS_PER_WORD;
     else {
-      Count += llvm::countLeadingZeros(pVal[i-1]);
+      Count += llvm::countLeadingZeros(V);
       break;
     }
   }
+  // Adjust for unused bits in the most significant word (they are zero).
+  unsigned Mod = BitWidth % APINT_BITS_PER_WORD;
+  Count -= Mod > 0 ? APINT_BITS_PER_WORD - Mod : 0;
   return Count;
 }
 
 unsigned APInt::countLeadingOnes() const {
   if (isSingleWord())
     return llvm::countLeadingOnes(VAL << (APINT_BITS_PER_WORD - BitWidth));
 
   unsigned highWordBits = BitWidth % APINT_BITS_PER_WORD;