[KnownBits] Add blsi and blsmsk

[foad]

Upload the right patch

[foad]

@goldstein.w.n is doing this in D142519 now.

llvm/include/llvm/Support/KnownBits.h

public:
   KnownBits byteSwap() {
     return KnownBits(Zero.byteSwap(), One.byteSwap());
   }
 
   KnownBits reverseBits() {
     return KnownBits(Zero.reverseBits(), One.reverseBits());
   }
 
+  /// Compute known bits for X & -X. The name comes from the X86 BMI instruction
+  /// BLSI.
+  KnownBits blsi() const;
+
+  /// Compute known bits for X ^ (X - 1). The name comes from the X86 BMI
+  /// instruction BLSMSK.
+  KnownBits blsmsk() const;
+
   bool operator==(const KnownBits &Other) const {
     return Zero == Other.Zero && One == Other.One;
   }
 
   bool operator!=(const KnownBits &Other) const { return !(*this == Other); }
 
   void print(raw_ostream &OS) const;
   void dump() const;

llvm/lib/Support/KnownBits.cpp

KnownBits &KnownBits::operator^=(const KnownBits &RHS) {
   // Result bit is 0 if both operand bits are 0 or both are 1.
   APInt Z = (Zero & RHS.Zero) | (One & RHS.One);
   // Result bit is 1 if one operand bit is 0 and the other is 1.
   One = (Zero & RHS.One) | (One & RHS.Zero);
   Zero = std::move(Z);
   return *this;
 }
 
+KnownBits KnownBits::blsi() const {
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(Zero, APInt(BitWidth, 0));
+  unsigned Max = countMaxTrailingZeros();
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  if (Max == Min && Max < BitWidth)
+    Known.One.setBit(Max);
+  return Known;
+}
+
+KnownBits KnownBits::blsmsk() const {
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(BitWidth);
+  unsigned Max = countMaxTrailingZeros();
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  Known.One.setLowBits(std::min(Min + 1, BitWidth));
+  return Known;
+}
+
 void KnownBits::print(raw_ostream &OS) const {
   OS << "{Zero=" << Zero << ", One=" << One << "}";
 }
 void KnownBits::dump() const {
   print(dbgs());
   dbgs() << "\n";
 }

llvm/unittests/Support/KnownBitsTest.cpp

 
 TEST(KnownBitsTest, UnaryExhaustive) {
   unsigned Bits = 4;
   ForeachKnownBits(Bits, [&](const KnownBits &Known) {
     KnownBits KnownAbs(Bits);
     KnownAbs.Zero.setAllBits();
     KnownAbs.One.setAllBits();
     KnownBits KnownAbsPoison(KnownAbs);
+    KnownBits KnownBlsi(Bits);
+    KnownBlsi.Zero.setAllBits();
+    KnownBlsi.One.setAllBits();
+    KnownBits KnownBlsmsk(Bits);
+    KnownBlsmsk.Zero.setAllBits();
+    KnownBlsmsk.One.setAllBits();
 
     ForeachNumInKnownBits(Known, [&](const APInt &N) {
       APInt Res = N.abs();
       KnownAbs.One &= Res;
       KnownAbs.Zero &= ~Res;
 
       if (!N.isMinSignedValue()) {
         KnownAbsPoison.One &= Res;
         KnownAbsPoison.Zero &= ~Res;
       }
+
+      Res = N & -N;
+      KnownBlsi.One &= Res;
+      KnownBlsi.Zero &= ~Res;
+
+      Res = N ^ (N - 1);
+      KnownBlsmsk.One &= Res;
+      KnownBlsmsk.Zero &= ~Res;
     });
 
     // abs() is conservatively correct, but not guaranteed to be precise.
     KnownBits ComputedAbs = Known.abs();
     EXPECT_TRUE(ComputedAbs.Zero.isSubsetOf(KnownAbs.Zero));
     EXPECT_TRUE(ComputedAbs.One.isSubsetOf(KnownAbs.One));
 
     KnownBits ComputedAbsPoison = Known.abs(true);
     EXPECT_TRUE(ComputedAbsPoison.Zero.isSubsetOf(KnownAbsPoison.Zero));
     EXPECT_TRUE(ComputedAbsPoison.One.isSubsetOf(KnownAbsPoison.One));
+
+    KnownBits ComputedBlsi = Known.blsi();
+    EXPECT_EQ(KnownBlsi, ComputedBlsi);
+
+    KnownBits ComputedBlsmsk = Known.blsmsk();
+    EXPECT_EQ(KnownBlsmsk, ComputedBlsmsk);
   });
 }
 
 TEST(KnownBitsTest, ICmpExhaustive) {
   unsigned Bits = 4;
   ForeachKnownBits(Bits, [&](const KnownBits &Known1) {
     ForeachKnownBits(Bits, [&](const KnownBits &Known2) {
       bool AllEQ = true, NoneEQ = true;