[APInt] Fix isAllOnes and extractBits for zero width values.

isAllOnes() should return true for zero bit values because
there are no zeros in it.

Thanks to Jay Foad for pointing this out.

Differential Revision: https://reviews.llvm.org/D111241

llvm/include/llvm/ADT/APInt.h

@@ -343,14 +343,12 @@ public:
   /// \returns true if this APInt is non-positive.
   bool isNonPositive() const { return !isStrictlyPositive(); }
 
-  /// Determine if all bits are set.
+  /// Determine if all bits are set.  This is true for zero-width values.
   bool isAllOnes() const {
-    if (isSingleWord()) {
-      // Calculate the shift amount, handling the zero-bit wide case without UB.
-      unsigned ShiftAmt =
-          (APINT_BITS_PER_WORD - BitWidth) % APINT_BITS_PER_WORD;
-      return U.VAL == WORDTYPE_MAX >> ShiftAmt;
-    }
+    if (BitWidth == 0)
+      return true;
+    if (isSingleWord())
+      return U.VAL == WORDTYPE_MAX >> (APINT_BITS_PER_WORD - BitWidth);
     return countTrailingOnesSlowCase() == BitWidth;
   }
 

llvm/lib/Support/APInt.cpp

@@ -444,7 +444,6 @@ void APInt::insertBits(uint64_t subBits, unsigned bitPosition, unsigned numBits)
 }
 
 APInt APInt::extractBits(unsigned numBits, unsigned bitPosition) const {
-  assert(numBits > 0 && "Can't extract zero bits");
   assert(bitPosition < BitWidth && (numBits + bitPosition) <= BitWidth &&
          "Illegal bit extraction");
 

llvm/unittests/ADT/APIntTest.cpp

@@ -2948,6 +2948,7 @@ TEST(APIntTest, ZeroWidth) {
   // Methods like getLowBitsSet work with zero bits.
   EXPECT_EQ(0U, APInt::getLowBitsSet(0, 0).getBitWidth());
   EXPECT_EQ(0U, APInt::getSplat(0, ZW).getBitWidth());
+  EXPECT_EQ(0U, APInt(4, 10).extractBits(0, 2).getBitWidth());
 
   // Logical operators.
   ZW |= ZW2;
@@ -2990,6 +2991,7 @@ TEST(APIntTest, ZeroWidth) {
   EXPECT_EQ(0U, ZW.getLoBits(0).getBitWidth());
   EXPECT_EQ(0, ZW.zext(4));
   EXPECT_EQ(0U, APInt(4, 3).trunc(0).getBitWidth());
+  EXPECT_TRUE(ZW.isAllOnes());
 
   SmallString<42> STR;
   ZW.toStringUnsigned(STR);