[APInt] Add saturating truncation methods

Summary:
The signed one is needed for implementation of `ConstantRange::smul_sat()`,
unsigned is for completeness only.

Reviewers: nikic, RKSimon, spatel

Reviewed By: nikic

Subscribers: hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69993

3 files changed, 55 insertions, 0 deletions

diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
index 262363f..60a0db7 100644
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -1346,6 +1346,19 @@ public:
   /// that is greater than or equal to the current width.
   APInt trunc(unsigned width) const;
 
+  /// Truncate to new width with unsigned saturation.
+  ///
+  /// If the APInt, treated as unsigned integer, can be losslessly truncated to
+  /// the new bitwidth, then return truncated APInt. Else, return max value.
+  APInt truncUSat(unsigned width) const;
+
+  /// Truncate to new width with signed saturation.
+  ///
+  /// If this APInt, treated as signed integer, can be losslessly truncated to
+  /// the new bitwidth, then return truncated APInt. Else, return either
+  /// signed min value if the APInt was negative, or signed max value.
+  APInt truncSSat(unsigned width) const;
+
   /// Sign extend to a new width.
   ///
   /// This operation sign extends the APInt to a new width. If the high order
diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 431e2b7..9b9cd70 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -884,6 +884,31 @@ APInt APInt::trunc(unsigned width) const {
   return Result;
 }
 
+// Truncate to new width with unsigned saturation.
+APInt APInt::truncUSat(unsigned width) const {
+  assert(width < BitWidth && "Invalid APInt Truncate request");
+  assert(width && "Can't truncate to 0 bits");
+
+  // Can we just losslessly truncate it?
+  if (isIntN(width))
+    return trunc(width);
+  // If not, then just return the new limit.
+  return APInt::getMaxValue(width);
+}
+
+// Truncate to new width with signed saturation.
+APInt APInt::truncSSat(unsigned width) const {
+  assert(width < BitWidth && "Invalid APInt Truncate request");
+  assert(width && "Can't truncate to 0 bits");
+
+  // Can we just losslessly truncate it?
+  if (isSignedIntN(width))
+    return trunc(width);
+  // If not, then just return the new limits.
+  return isNegative() ? APInt::getSignedMinValue(width)
+                      : APInt::getSignedMaxValue(width);
+}
+
 // Sign extend to a new width.
 APInt APInt::sext(unsigned Width) const {
   assert(Width > BitWidth && "Invalid APInt SignExtend request");
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index fe4e742..030b000 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -1177,9 +1177,26 @@ TEST(APIntTest, fromString) {
 
 TEST(APIntTest, SaturatingMath) {
   APInt AP_10 = APInt(8, 10);
+  APInt AP_42 = APInt(8, 42);
   APInt AP_100 = APInt(8, 100);
   APInt AP_200 = APInt(8, 200);
 
+  EXPECT_EQ(APInt(7, 100), AP_100.truncUSat(7));
+  EXPECT_EQ(APInt(6, 63), AP_100.truncUSat(6));
+  EXPECT_EQ(APInt(5, 31), AP_100.truncUSat(5));
+
+  EXPECT_EQ(APInt(7, 127), AP_200.truncUSat(7));
+  EXPECT_EQ(APInt(6, 63), AP_200.truncUSat(6));
+  EXPECT_EQ(APInt(5, 31), AP_200.truncUSat(5));
+
+  EXPECT_EQ(APInt(7, 42), AP_42.truncSSat(7));
+  EXPECT_EQ(APInt(6, 31), AP_42.truncSSat(6));
+  EXPECT_EQ(APInt(5, 15), AP_42.truncSSat(5));
+
+  EXPECT_EQ(APInt(7, -56), AP_200.truncSSat(7));
+  EXPECT_EQ(APInt(6, -32), AP_200.truncSSat(6));
+  EXPECT_EQ(APInt(5, -16), AP_200.truncSSat(5));
+
   EXPECT_EQ(APInt(8, 200), AP_100.uadd_sat(AP_100));
   EXPECT_EQ(APInt(8, 255), AP_100.uadd_sat(AP_200));
   EXPECT_EQ(APInt(8, 255), APInt(8, 255).uadd_sat(APInt(8, 255)));