[llvm] [ADT] Bitset: add shift operators, word accessors, and etc (PR #193400)

[Jiachen Yuan via llvm-commits]

================
@@ -194,6 +194,95 @@ template <unsigned NumBits> class Bitset {
     }
     return false;
   }
+
+  constexpr Bitset &operator<<=(unsigned N) {
+    if (N == 0)
+      return *this;
+    if (N >= NumBits) {
+      return *this = Bitset();
+    }
+    const unsigned WordShift = N / BitwordBits;
+    const unsigned BitShift = N % BitwordBits;
+    if (BitShift == 0) {
+      for (unsigned I = NumWords; I-- > WordShift;)
+        Bits[I] = Bits[I - WordShift];
+    } else {
+      const unsigned CarryShift = BitwordBits - BitShift;
+      for (unsigned I = NumWords - 1; I > WordShift; --I) {
+        Bits[I] = (Bits[I - WordShift] << BitShift) |
+                  (Bits[I - WordShift - 1] >> CarryShift);
+      }
+      Bits[WordShift] = Bits[0] << BitShift;
+    }
+    for (unsigned I = 0; I < WordShift; ++I)
+      Bits[I] = 0;
+    maskLastWord();
+    return *this;
+  }
+
+  constexpr Bitset operator<<(unsigned N) const {
+    Bitset Result(*this);
+    Result <<= N;
+    return Result;
+  }
+
+  constexpr Bitset &operator>>=(unsigned N) {
+    if (N == 0)
+      return *this;
+    if (N >= NumBits) {
+      return *this = Bitset();
+    }
+    const unsigned WordShift = N / BitwordBits;
+    const unsigned BitShift = N % BitwordBits;
+    if (BitShift == 0) {
+      for (unsigned I = 0; I < NumWords - WordShift; ++I)
+        Bits[I] = Bits[I + WordShift];
+    } else {
+      const unsigned CarryShift = BitwordBits - BitShift;
+      for (unsigned I = 0; I < NumWords - WordShift - 1; ++I) {
+        Bits[I] = (Bits[I + WordShift] >> BitShift) |
+                  (Bits[I + WordShift + 1] << CarryShift);
+      }
+      Bits[NumWords - WordShift - 1] = Bits[NumWords - 1] >> BitShift;
+    }
+    for (unsigned I = NumWords - WordShift; I < NumWords; ++I)
+      Bits[I] = 0;
+    maskLastWord();
+    return *this;
+  }
+
+  constexpr Bitset operator>>(unsigned N) const {
+    Bitset Result(*this);
+    Result >>= N;
+    return Result;
+  }
+
+  /// Return the I-th 64-bit word of the bitset, from least significant to most.
----------------
JiachenYuan wrote:

Sure, I have added some more explanations about the case where last word with trailing bits are zero-extended.

https://github.com/llvm/llvm-project/pull/193400