[APInt] Add the truncOrSelf resizing operator to APInt

[kmclaughlin]

Summary

Truncates the APInt if the bit width is greater than the width specified,
otherwise do nothing

[lebedev.ri]

Can you perhaps update some code to use this?

[RKSimon]

LGTM with a couple of minors - cheers

llvm/include/llvm/ADT/APInt.h

1398

Maybe move this down to be next to zextOrSelf/sextOrSelf?

llvm/lib/Support/APInt.cpp

952

Maybe move this down to be next to zextOrSelf/sextOrSelf?

llvm/include/llvm/ADT/APInt.h

#endif
 
   /// Zero extend to a new width.
   ///
   /// This operation zero extends the APInt to a new width. The high order bits
   /// are filled with 0 bits.  It is an error to specify a width that is less
   /// than or equal to the current width.
   APInt zext(unsigned width) const;
 
+  /// Truncate to width
+  ///
+  /// Make this APInt have the bit width given by \p width. The value is
+  /// truncated or left alone to make it that width.
+  APInt truncOrSelf(unsigned width) const;
+
   /// Sign extend or truncate to width
   ///
   /// Make this APInt have the bit width given by \p width. The value is sign
   /// extended, truncated, or left alone to make it that width.
   APInt sextOrTrunc(unsigned width) const;
 
   /// Zero extend or truncate to width
   ///

llvm/lib/Support/APInt.cpp

APInt APInt::zext(unsigned width) const {
 
   // Zero remaining words.
   std::memset(Result.U.pVal + getNumWords(), 0,
               (Result.getNumWords() - getNumWords()) * APINT_WORD_SIZE);
 
   return Result;
 }
 
+APInt APInt::truncOrSelf(unsigned width) const {
+  if (BitWidth > width)
+    return trunc(width);
+  return *this;
+}
+
 APInt APInt::zextOrTrunc(unsigned width) const {
   if (BitWidth < width)
     return zext(width);
   if (BitWidth > width)
     return trunc(width);
   return *this;
 }
 

llvm/unittests/ADT/APIntTest.cpp

TEST(APIntTest, sext) {
   EXPECT_EQ(32U, i32_min.countPopulation());
 
   APInt i32_neg1(APInt(32, ~uint64_t(0)).sext(63));
   EXPECT_EQ(63U, i32_neg1.countLeadingOnes());
   EXPECT_EQ(0U, i32_neg1.countTrailingZeros());
   EXPECT_EQ(63U, i32_neg1.countPopulation());
 }
 
+TEST(APIntTest, truncOrSelf) {
+  APInt val(32, 0xFFFFFFFF);
+  EXPECT_EQ(0xFFFF, val.truncOrSelf(16));
+  EXPECT_EQ(0xFFFFFFFF, val.truncOrSelf(32));
+  EXPECT_EQ(0xFFFFFFFF, val.truncOrSelf(64));
+}
+
 TEST(APIntTest, multiply) {
   APInt i64(64, 1234);
 
   EXPECT_EQ(7006652, i64 * 5678);
   EXPECT_EQ(7006652, 5678 * i64);
 
   APInt i128 = APInt::getOneBitSet(128, 64);
   APInt i128_1234(128, 1234);