[APInt] Added APInt::clearBits() method (#137098)

Added APInt::clearBits(unsigned loBit, unsigned hiBit) that clears bits within a certain range.

Fixes #136550

---------

Co-authored-by: Simon Pilgrim <llvm-dev@redking.me.uk>

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 4e45416..0119cb2 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -336,6 +336,33 @@ void APInt::setBitsSlowCase(unsigned loBit, unsigned hiBit) {
     U.pVal[word] = WORDTYPE_MAX;
 }
 
+void APInt::clearBitsSlowCase(unsigned LoBit, unsigned HiBit) {
+  unsigned LoWord = whichWord(LoBit);
+  unsigned HiWord = whichWord(HiBit);
+
+  // Create an initial mask for the low word with ones below loBit.
+  uint64_t LoMask = ~(WORDTYPE_MAX << whichBit(LoBit));
+
+  // If HiBit is not aligned, we need a high mask.
+  unsigned HiShiftAmt = whichBit(HiBit);
+  if (HiShiftAmt != 0) {
+    // Create a high mask with ones above HiBit.
+    uint64_t HiMask = ~(WORDTYPE_MAX >> (APINT_BITS_PER_WORD - HiShiftAmt));
+    // If LoWord and HiWord are equal, then we combine the masks. Otherwise,
+    // set the bits in HiWord.
+    if (HiWord == LoWord)
+      LoMask &= HiMask;
+    else
+      U.pVal[HiWord] &= HiMask;
+  }
+  // Apply the mask to the low word.
+  U.pVal[LoWord] &= LoMask;
+
+  // Fill any words between LoWord and HiWord with all zeros.
+  for (unsigned Word = LoWord + 1; Word < HiWord; ++Word)
+    U.pVal[Word] = 0;
+}
+
 // Complement a bignum in-place.
 static void tcComplement(APInt::WordType *dst, unsigned parts) {
   for (unsigned i = 0; i < parts; i++)