[ConstantRange] Teach multiply to be cleverer about signed ranges.

Multiplication is not dependent on signedness, so just treating
all input ranges as unsigned is not incorrect. However it will cause
overly pessimistic ranges (such as full-set) when used with signed
negative values.

Teach multiply to try to interpret its inputs as both signed and
unsigned, and then to take the most specific (smallest population)
as its result.

llvm-svn: 231483

llvm/include/llvm/IR/ConstantRange.h

@@ -208,8 +208,8 @@ public:
   ConstantRange sub(const ConstantRange &Other) const;
 
   /// Return a new range representing the possible values resulting
-  /// from a multiplication of a value in this range and a value in \p Other.
-  /// TODO: This isn't fully implemented yet.
+  /// from a multiplication of a value in this range and a value in \p Other,
+  /// treating both this and \p Other as unsigned ranges.
   ConstantRange multiply(const ConstantRange &Other) const;
 
   /// Return a new range representing the possible values resulting

llvm/lib/IR/ConstantRange.cpp

@@ -587,6 +587,13 @@ ConstantRange::multiply(const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())
     return ConstantRange(getBitWidth(), /*isFullSet=*/false);
 
+  // Multiplication is signedness-independent. However different ranges can be
+  // obtained depending on how the input ranges are treated. These different
+  // ranges are all conservatively correct, but one might be better than the
+  // other. We calculate two ranges; one treating the inputs as unsigned
+  // and the other signed, then return the smallest of these ranges.
+
+  // Unsigned range first.
   APInt this_min = getUnsignedMin().zext(getBitWidth() * 2);
   APInt this_max = getUnsignedMax().zext(getBitWidth() * 2);
   APInt Other_min = Other.getUnsignedMin().zext(getBitWidth() * 2);
@@ -594,7 +601,26 @@ ConstantRange::multiply(const ConstantRange &Other) const {
 
   ConstantRange Result_zext = ConstantRange(this_min * Other_min,
                                             this_max * Other_max + 1);
-  return Result_zext.truncate(getBitWidth());
+  ConstantRange UR = Result_zext.truncate(getBitWidth());
+
+  // Now the signed range. Because we could be dealing with negative numbers
+  // here, the lower bound is the smallest of the cartesian product of the
+  // lower and upper ranges; for example:
+  //   [-1,4) * [-2,3) = min(-1*-2, -1*2, 3*-2, 3*2) = -6.
+  // Similarly for the upper bound, swapping min for max.
+
+  this_min = getSignedMin().sext(getBitWidth() * 2);
+  this_max = getSignedMax().sext(getBitWidth() * 2);
+  Other_min = Other.getSignedMin().sext(getBitWidth() * 2);
+  Other_max = Other.getSignedMax().sext(getBitWidth() * 2);
+  
+  auto L = {this_min * Other_min, this_min * Other_max,
+            this_max * Other_min, this_max * Other_max};
+  auto Compare = [](const APInt &A, const APInt &B) { return A.slt(B); };
+  ConstantRange Result_sext(std::min(L, Compare), std::max(L, Compare) + 1);
+  ConstantRange SR = Result_sext.truncate(getBitWidth());
+
+  return UR.getSetSize().ult(SR.getSetSize()) ? UR : SR;
 }
 
 ConstantRange

llvm/unittests/IR/ConstantRangeTest.cpp

@@ -400,6 +400,13 @@ TEST_F(ConstantRangeTest, Multiply) {
   EXPECT_EQ(ConstantRange(APInt(4, 1), APInt(4, 6)).multiply(
                 ConstantRange(APInt(4, 6), APInt(4, 2))),
             ConstantRange(4, /*isFullSet=*/true));
+
+  EXPECT_EQ(ConstantRange(APInt(8, 254), APInt(8, 0)).multiply(
+              ConstantRange(APInt(8, 252), APInt(8, 4))),
+            ConstantRange(APInt(8, 250), APInt(8, 9)));
+  EXPECT_EQ(ConstantRange(APInt(8, 254), APInt(8, 255)).multiply(
+              ConstantRange(APInt(8, 2), APInt(8, 4))),
+            ConstantRange(APInt(8, 250), APInt(8, 253)));
 }
 
 TEST_F(ConstantRangeTest, UMax) {