[APInt] Support zero-width extract in extractBitsAsZExtValue()

[nikic]

Summary

D111241 added support for extractBits() with zero width. Extend this to extractBitsAsZExtValue() as well for consistency (in which case it will always return zero).

[lattner]

cool

[foad]

Thanks!

llvm/lib/Support/APInt.cpp

for (unsigned word = 0; word < NumDstWords; ++word) {
     DestPtr[word] = (w0 >> loBit) | (w1 << (APINT_BITS_PER_WORD - loBit));
   }
 
   return Result.clearUnusedBits();
 }
 
 uint64_t APInt::extractBitsAsZExtValue(unsigned numBits,
                                        unsigned bitPosition) const {
-  assert(numBits > 0 && "Can't extract zero bits");
   assert(bitPosition < BitWidth && (numBits + bitPosition) <= BitWidth &&
          "Illegal bit extraction");
   assert(numBits <= 64 && "Illegal bit extraction");
 
   uint64_t maskBits = maskTrailingOnes<uint64_t>(numBits);
   if (isSingleWord())
     return (U.VAL >> bitPosition) & maskBits;
 

llvm/unittests/ADT/APIntTest.cpp

TEST(APIntTest, ZeroWidth) {
   // Assignment
   ZW = ZW2;
   EXPECT_EQ(0U, ZW.getBitWidth());
 
   // 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());
+  EXPECT_EQ(0U, APInt(4, 10).extractBitsAsZExtValue(0, 2));
 
   // Logical operators.
   ZW |= ZW2;
   ZW &= ZW2;
   ZW ^= ZW2;
   ZW |= 42; // These ignore high bits of the literal.
   ZW &= 42;
   ZW ^= 42;