[APInt} Add saturating multiply ops

[lebedev.ri]

Summary

There are *_ov() functions already, so at least for consistency it may be good to also have saturating variants.
These may or may not be needed for ConstantRange's mulWithNoWrap()

[nikic]

LGTM

llvm/include/llvm/ADT/APInt.h

#endif
   APInt sshl_ov(const APInt &Amt, bool &Overflow) const;
   APInt ushl_ov(const APInt &Amt, bool &Overflow) const;
 
   // Operations that saturate
   APInt sadd_sat(const APInt &RHS) const;
   APInt uadd_sat(const APInt &RHS) const;
   APInt ssub_sat(const APInt &RHS) const;
   APInt usub_sat(const APInt &RHS) const;
+  APInt smul_sat(const APInt &RHS) const;
+  APInt umul_sat(const APInt &RHS) const;
 
   /// Array-indexing support.
   ///
   /// \returns the bit value at bitPosition
   bool operator[](unsigned bitPosition) const {
     assert(bitPosition < getBitWidth() && "Bit position out of bounds!");
     return (maskBit(bitPosition) & getWord(bitPosition)) != 0;
   }

llvm/lib/Support/APInt.cpp

APInt APInt::usub_sat(const APInt &RHS) const {
   bool Overflow;
   APInt Res = usub_ov(RHS, Overflow);
   if (!Overflow)
     return Res;
 
   return APInt(BitWidth, 0);
 }
 
+APInt APInt::smul_sat(const APInt &RHS) const {
+  bool Overflow;
+  APInt Res = smul_ov(RHS, Overflow);
+  if (!Overflow)
+    return Res;
+
+  // The result is negative if one and only one of inputs is negative.
+  bool ResIsNegative = isNegative() ^ RHS.isNegative();
+
+  return ResIsNegative ? APInt::getSignedMinValue(BitWidth)
+                       : APInt::getSignedMaxValue(BitWidth);
+}
+
+APInt APInt::umul_sat(const APInt &RHS) const {
+  bool Overflow;
+  APInt Res = umul_ov(RHS, Overflow);
+  if (!Overflow)
+    return Res;
+
+  return APInt::getMaxValue(BitWidth);
+}
 
 void APInt::fromString(unsigned numbits, StringRef str, uint8_t radix) {
   // Check our assumptions here
   assert(!str.empty() && "Invalid string length");
   assert((radix == 10 || radix == 8 || radix == 16 || radix == 2 ||
           radix == 36) &&
          "Radix should be 2, 8, 10, 16, or 36!");
 

llvm/unittests/ADT/APIntTest.cpp

TEST(APIntTest, SaturatingMath) {
   EXPECT_EQ(APInt(8, 90), AP_100.usub_sat(AP_10));
   EXPECT_EQ(APInt(8, 0), AP_100.usub_sat(AP_200));
   EXPECT_EQ(APInt(8, 0), APInt(8, 0).usub_sat(APInt(8, 255)));
 
   EXPECT_EQ(APInt(8, -90), AP_10.ssub_sat(AP_100));
   EXPECT_EQ(APInt(8, 127), AP_100.ssub_sat(-AP_100));
   EXPECT_EQ(APInt(8, -128), (-AP_100).ssub_sat(AP_100));
   EXPECT_EQ(APInt(8, -128), APInt(8, -128).ssub_sat(APInt(8, 127)));
+
+  EXPECT_EQ(APInt(8, 250), APInt(8, 50).umul_sat(APInt(8, 5)));
+  EXPECT_EQ(APInt(8, 255), APInt(8, 50).umul_sat(APInt(8, 6)));
+  EXPECT_EQ(APInt(8, 255), APInt(8, -128).umul_sat(APInt(8, 3)));
+  EXPECT_EQ(APInt(8, 255), APInt(8, 3).umul_sat(APInt(8, -128)));
+  EXPECT_EQ(APInt(8, 255), APInt(8, -128).umul_sat(APInt(8, -128)));
+
+  EXPECT_EQ(APInt(8, 125), APInt(8, 25).smul_sat(APInt(8, 5)));
+  EXPECT_EQ(APInt(8, 127), APInt(8, 25).smul_sat(APInt(8, 6)));
+  EXPECT_EQ(APInt(8, 127), APInt(8, 127).smul_sat(APInt(8, 127)));
+  EXPECT_EQ(APInt(8, -125), APInt(8, -25).smul_sat(APInt(8, 5)));
+  EXPECT_EQ(APInt(8, -125), APInt(8, 25).smul_sat(APInt(8, -5)));
+  EXPECT_EQ(APInt(8, 125), APInt(8, -25).smul_sat(APInt(8, -5)));
+  EXPECT_EQ(APInt(8, 125), APInt(8, 25).smul_sat(APInt(8, 5)));
+  EXPECT_EQ(APInt(8, -128), APInt(8, -25).smul_sat(APInt(8, 6)));
+  EXPECT_EQ(APInt(8, -128), APInt(8, 25).smul_sat(APInt(8, -6)));
+  EXPECT_EQ(APInt(8, 127), APInt(8, -25).smul_sat(APInt(8, -6)));
+  EXPECT_EQ(APInt(8, 127), APInt(8, 25).smul_sat(APInt(8, 6)));
 }
 
 TEST(APIntTest, FromArray) {
   EXPECT_EQ(APInt(32, uint64_t(1)), APInt(32, ArrayRef<uint64_t>(1)));
 }
 
 TEST(APIntTest, StringBitsNeeded2) {
   EXPECT_EQ(1U, APInt::getBitsNeeded(  "0", 2));