[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

1 files changed, 25 insertions, 0 deletions

diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 431e2b70df0..9b9cd70078b 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");