[APInt] Add APIntOps::absdiff to compute the absolute difference of 2 unsigned values (llvm#82255)

Equivalent to "umax(A, B) - umin(A, B)"

First step towards adding knownbits support for absdiff patterns for llvm#81765

Author: RKSimon

llvm/include/llvm/ADT/APInt.h

@@ -2187,6 +2187,11 @@ inline const APInt &umax(const APInt &A, const APInt &B) {
   return A.ugt(B) ? A : B;
 }
 
+/// Determine the absolute difference of two APInts considered to be unsigned.
+inline const APInt absdiff(const APInt &A, const APInt &B) {
+  return A.uge(B) ? (A - B) : (B - A);
+}
+
 /// Compute GCD of two unsigned APInt values.
 ///
 /// This function returns the greatest common divisor of the two APInt values

llvm/unittests/ADT/APIntTest.cpp

@@ -2497,6 +2497,40 @@ TEST(APIntTest, clearLowBits) {
   EXPECT_EQ(16u, i32hi16.popcount());
 }
 
+TEST(APIntTest, AbsDiff) {
+  using APIntOps::absdiff;
+
+  APInt MaxU1(1, 1, false);
+  APInt MinU1(1, 0, false);
+  EXPECT_EQ(1u, absdiff(MaxU1, MinU1).getZExtValue());
+  EXPECT_EQ(1u, absdiff(MinU1, MaxU1).getZExtValue());
+
+  APInt MaxU4(4, 15, false);
+  APInt MinU4(4, 0, false);
+  EXPECT_EQ(15u, absdiff(MaxU4, MinU4).getZExtValue());
+  EXPECT_EQ(15u, absdiff(MinU4, MaxU4).getZExtValue());
+
+  APInt MaxS8(8, 127, true);
+  APInt MinS8(8, -128, true);
+  EXPECT_EQ(1u, absdiff(MaxS8, MinS8).getZExtValue());
+  EXPECT_EQ(1u, absdiff(MinS8, MaxS8).getZExtValue());
+
+  APInt MaxU16(16, 65535, false);
+  APInt MinU16(16, 0, false);
+  EXPECT_EQ(65535u, absdiff(MaxU16, MinU16).getZExtValue());
+  EXPECT_EQ(65535u, absdiff(MinU16, MaxU16).getZExtValue());
+
+  APInt MaxS16(16, 32767, true);
+  APInt MinS16(16, -32768, true);
+  APInt ZeroS16(16, 0, true);
+  EXPECT_EQ(1u, absdiff(MaxS16, MinS16).getZExtValue());
+  EXPECT_EQ(1u, absdiff(MinS16, MaxS16).getZExtValue());
+  EXPECT_EQ(32768u, absdiff(ZeroS16, MinS16));
+  EXPECT_EQ(32768u, absdiff(MinS16, ZeroS16));
+  EXPECT_EQ(32767u, absdiff(ZeroS16, MaxS16));
+  EXPECT_EQ(32767u, absdiff(MaxS16, ZeroS16));
+}
+
 TEST(APIntTest, GCD) {
   using APIntOps::GreatestCommonDivisor;