[APInt] Add APIntOps::absdiff to compute the absolute difference of 2 unsigned values

[RKSimon]

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

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

[llvmbot]

@llvm/pr-subscribers-llvm-adt

Author: Simon Pilgrim (RKSimon)

Changes

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

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

---

Full diff: https://github.com/llvm/llvm-project/pull/82255.diff

2 Files Affected:

• (modified) llvm/include/llvm/ADT/APInt.h (+5)
• (modified) llvm/unittests/ADT/APIntTest.cpp (+34)

diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
index 6f2f25548cc84b..ef7e1878f303ba 100644
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/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 umax(A, B) - umin(A, B);
+}
+
 /// Compute GCD of two unsigned APInt values.
 ///
 /// This function returns the greatest common divisor of the two APInt values
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index 3b909f8f7d14e2..2fe59f05ca753a 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/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;