[APInt] Introduce clearLowBits()

[lebedev.ri]

Summary

Equivalent to x & -2^K.

[craig.topper]

LGTM

[RKSimon]

LGTM - cheers

[lebedev.ri]

Thank you for the review.

llvm/include/llvm/ADT/APInt.h

void clearBit(unsigned BitPosition) {
     assert(BitPosition < BitWidth && "BitPosition out of range");
     WordType Mask = ~maskBit(BitPosition);
     if (isSingleWord())
       U.VAL &= Mask;
     else
       U.pVal[whichWord(BitPosition)] &= Mask;
   }
 
+  /// Set bottom loBits bits to 0.
+  void clearLowBits(unsigned loBits) {
+    assert(loBits <= BitWidth && "More bits than bitwidth");
+    APInt Keep = getHighBitsSet(BitWidth, BitWidth - loBits);
+    *this &= Keep;
+  }
+
   /// Set the sign bit to 0.
   void clearSignBit() {
     clearBit(BitWidth - 1);
   }
 
   /// Toggle every bit to its opposite value.
   void flipAllBits() {
     if (isSingleWord()) {

llvm/unittests/ADT/APIntTest.cpp

TEST(APIntTest, getHiBits) {
   APInt i32(32, 0xfa);
   i32.setHighBits(2);
   EXPECT_EQ(0xc, i32.getHiBits(4));
   APInt i128(128, 0xfa);
   i128.setHighBits(2);
   EXPECT_EQ(0xc, i128.getHiBits(4));
 }
 
+TEST(APIntTest, clearLowBits) {
+  APInt i64hi32 = APInt::getAllOnesValue(64);
+  i64hi32.clearLowBits(32);
+  EXPECT_EQ(32u, i64hi32.countLeadingOnes());
+  EXPECT_EQ(0u, i64hi32.countLeadingZeros());
+  EXPECT_EQ(64u, i64hi32.getActiveBits());
+  EXPECT_EQ(32u, i64hi32.countTrailingZeros());
+  EXPECT_EQ(0u, i64hi32.countTrailingOnes());
+  EXPECT_EQ(32u, i64hi32.countPopulation());
+
+  APInt i128hi64 = APInt::getAllOnesValue(128);
+  i128hi64.clearLowBits(64);
+  EXPECT_EQ(64u, i128hi64.countLeadingOnes());
+  EXPECT_EQ(0u, i128hi64.countLeadingZeros());
+  EXPECT_EQ(128u, i128hi64.getActiveBits());
+  EXPECT_EQ(64u, i128hi64.countTrailingZeros());
+  EXPECT_EQ(0u, i128hi64.countTrailingOnes());
+  EXPECT_EQ(64u, i128hi64.countPopulation());
+
+  APInt i128hi24 = APInt::getAllOnesValue(128);
+  i128hi24.clearLowBits(104);
+  EXPECT_EQ(24u, i128hi24.countLeadingOnes());
+  EXPECT_EQ(0u, i128hi24.countLeadingZeros());
+  EXPECT_EQ(128u, i128hi24.getActiveBits());
+  EXPECT_EQ(104u, i128hi24.countTrailingZeros());
+  EXPECT_EQ(0u, i128hi24.countTrailingOnes());
+  EXPECT_EQ(24u, i128hi24.countPopulation());
+
+  APInt i128hi104 = APInt::getAllOnesValue(128);
+  i128hi104.clearLowBits(24);
+  EXPECT_EQ(104u, i128hi104.countLeadingOnes());
+  EXPECT_EQ(0u, i128hi104.countLeadingZeros());
+  EXPECT_EQ(128u, i128hi104.getActiveBits());
+  EXPECT_EQ(24u, i128hi104.countTrailingZeros());
+  EXPECT_EQ(0u, i128hi104.countTrailingOnes());
+  EXPECT_EQ(104u, i128hi104.countPopulation());
+
+  APInt i128hi0 = APInt::getAllOnesValue(128);
+  i128hi0.clearLowBits(128);
+  EXPECT_EQ(0u, i128hi0.countLeadingOnes());
+  EXPECT_EQ(128u, i128hi0.countLeadingZeros());
+  EXPECT_EQ(0u, i128hi0.getActiveBits());
+  EXPECT_EQ(128u, i128hi0.countTrailingZeros());
+  EXPECT_EQ(0u, i128hi0.countTrailingOnes());
+  EXPECT_EQ(0u, i128hi0.countPopulation());
+
+  APInt i80hi1 = APInt::getAllOnesValue(80);
+  i80hi1.clearLowBits(79);
+  EXPECT_EQ(1u, i80hi1.countLeadingOnes());
+  EXPECT_EQ(0u, i80hi1.countLeadingZeros());
+  EXPECT_EQ(80u, i80hi1.getActiveBits());
+  EXPECT_EQ(79u, i80hi1.countTrailingZeros());
+  EXPECT_EQ(0u, i80hi1.countTrailingOnes());
+  EXPECT_EQ(1u, i80hi1.countPopulation());
+
+  APInt i32hi16 = APInt::getAllOnesValue(32);
+  i32hi16.clearLowBits(16);
+  EXPECT_EQ(16u, i32hi16.countLeadingOnes());
+  EXPECT_EQ(0u, i32hi16.countLeadingZeros());
+  EXPECT_EQ(32u, i32hi16.getActiveBits());
+  EXPECT_EQ(16u, i32hi16.countTrailingZeros());
+  EXPECT_EQ(0u, i32hi16.countTrailingOnes());
+  EXPECT_EQ(16u, i32hi16.countPopulation());
+}
+
 TEST(APIntTest, GCD) {
   using APIntOps::GreatestCommonDivisor;
 
   for (unsigned Bits : {1, 2, 32, 63, 64, 65}) {
     // Test some corner cases near zero.
     APInt Zero(Bits, 0), One(Bits, 1);
     EXPECT_EQ(GreatestCommonDivisor(Zero, Zero), Zero);
     EXPECT_EQ(GreatestCommonDivisor(Zero, One), One);