[ConstantRange] Add saturating add/sub methods

[nikic]

Summary

Add support for uadd_sat and friends to ConstantRange, so we can handle uadd.sat and friends in LVI. The implementation is forwarding to the corresponding APInt methods with appropriate bounds.

One thing worth pointing out here is that the handling of wrapping ranges is not maximally accurate. A simple example is that adding 0 to a wrapped range will return a full range, rather than the original wrapping range. The tests also only check that the non-wrapping envelope is correct and minimal.

[lebedev.ri]

Looks good in general!

Are you confident the test passes?
If yes, i think there may be some much bigger problem with that whole test approach.

llvm/lib/IR/ConstantRange.cpp

1148–1150

Will need rebasing after D60946.

llvm/unittests/IR/ConstantRangeTest.cpp

1674–1675

Yes, i can see how that pattern is tedious to write :)

1692

Uhm, are you sure this is correct?
I'd expect this to be:

APInt Min = APInt::getSignedMaxValue(Bits);
APInt Max = APInt::getSigned*Min*Value(Bits);

[nikic]

Fix test.

[nikic]

Are you confident the test passes?

Well, that's embarrassing. The two signed tests were indeed failing...

[nikic]

Rebase over D60947.

[lebedev.ri]

In D60946#1473701, @nikic wrote:

Are you confident the test passes?

The two signed tests were indeed failing...

Aha, at least that didn't happen to expose some bigger problem then.
LG now.

Well, that's embarrassing.

llvm/include/llvm/IR/ConstantRange.h

public:
   /// Return a new range representing the possible values resulting from a
   /// logical right shift of a value in this range and a value in \p Other.
   ConstantRange lshr(const ConstantRange &Other) const;
 
   /// Return a new range representing the possible values resulting from a
   /// arithmetic right shift of a value in this range and a value in \p Other.
   ConstantRange ashr(const ConstantRange &Other) const;
 
+  /// Perform an unsigned saturating addition of two constant ranges.
+  ConstantRange uadd_sat(const ConstantRange &Other) const;
+
+  /// Perform a signed saturating addition of two constant ranges.
+  ConstantRange sadd_sat(const ConstantRange &Other) const;
+
+  /// Perform an unsigned saturating subtraction of two constant ranges.
+  ConstantRange usub_sat(const ConstantRange &Other) const;
+
+  /// Perform a signed saturating subtraction of two constant ranges.
+  ConstantRange ssub_sat(const ConstantRange &Other) const;
+
   /// Return a new range that is the logical not of the current set.
   ConstantRange inverse() const;
 
   /// Represents whether an operation on the given constant range is known to
   /// always or never overflow.
   enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows };
 
   /// Return whether unsigned add of the two ranges always/never overflows.

llvm/lib/IR/ConstantRange.cpp

if (getSignedMin().isNonNegative()) {
     max = PosMax;
   }
   if (min == max)
     return getFull();
 
   return ConstantRange(std::move(min), std::move(max));
 }
 
+ConstantRange ConstantRange::uadd_sat(const ConstantRange &Other) const {
+  if (isEmptySet() || Other.isEmptySet())
+    return getEmpty();
+
+  APInt NewL = getUnsignedMin().uadd_sat(Other.getUnsignedMin());
+  APInt NewU = getUnsignedMax().uadd_sat(Other.getUnsignedMax()) + 1;
+  if (NewU == NewL)
+    return getFull();
+  return ConstantRange(std::move(NewL), std::move(NewU));
+}
+
+ConstantRange ConstantRange::sadd_sat(const ConstantRange &Other) const {
+  if (isEmptySet() || Other.isEmptySet())
+    return getEmpty();
+
+  APInt NewL = getSignedMin().sadd_sat(Other.getSignedMin());
+  APInt NewU = getSignedMax().sadd_sat(Other.getSignedMax()) + 1;
+  if (NewU == NewL)
+    return getFull();
+  return ConstantRange(std::move(NewL), std::move(NewU));
+}
+
+ConstantRange ConstantRange::usub_sat(const ConstantRange &Other) const {
+  if (isEmptySet() || Other.isEmptySet())
+    return getEmpty();
+
+  APInt NewL = getUnsignedMin().usub_sat(Other.getUnsignedMax());
+  APInt NewU = getUnsignedMax().usub_sat(Other.getUnsignedMin()) + 1;
+  if (NewU == NewL)
+    return getFull();
+  return ConstantRange(std::move(NewL), std::move(NewU));
+}
+
+ConstantRange ConstantRange::ssub_sat(const ConstantRange &Other) const {
+  if (isEmptySet() || Other.isEmptySet())
+    return getEmpty();
+
+  APInt NewL = getSignedMin().ssub_sat(Other.getSignedMax());
+  APInt NewU = getSignedMax().ssub_sat(Other.getSignedMin()) + 1;
+  if (NewU == NewL)
+    return getFull();
+  return ConstantRange(std::move(NewL), std::move(NewU));
+}
+
 ConstantRange ConstantRange::inverse() const {
   if (isFullSet())
     return getEmpty();
   if (isEmptySet())
     return getFull();
   return ConstantRange(Upper, Lower);
 }
 

llvm/unittests/IR/ConstantRangeTest.cpp

EnumerateConstantRanges(Bits, [](const ConstantRange &CR) {
     assert((CR.isEmptySet() || !AllNegative || !AllNonNegative) &&
            "Only empty set can be both all negative and all non-negative");
 
     EXPECT_EQ(AllNegative, CR.isAllNegative());
     EXPECT_EQ(AllNonNegative, CR.isAllNonNegative());
   });
 }
 
+template<typename Fn1, typename Fn2>
+static void TestUnsignedBinOpExhaustive(Fn1 RangeFn, Fn2 IntFn) {
+  unsigned Bits = 4;
+  EnumerateTwoConstantRanges(Bits, [&](const ConstantRange &CR1,
+                                       const ConstantRange &CR2) {
+    ConstantRange CR = RangeFn(CR1, CR2);
+    if (CR1.isEmptySet() || CR2.isEmptySet()) {
+      EXPECT_TRUE(CR.isEmptySet());
+      return;
+    }
+
+    APInt Min = APInt::getMaxValue(Bits);
+    APInt Max = APInt::getMinValue(Bits);
+    ForeachNumInConstantRange(CR1, [&](const APInt &N1) {
+      ForeachNumInConstantRange(CR2, [&](const APInt &N2) {
+        APInt N = IntFn(N1, N2);
+        if (N.ult(Min))
+          Min = N;
+        if (N.ugt(Max))
+          Max = N;
+      });
+    });
+
+    ++Max;
+    ConstantRange Expected = Min == Max ? ConstantRange::getFull(Bits)
+                                        : ConstantRange(Min, Max);
+    EXPECT_EQ(Expected, CR);
+  });
+}
+
+template<typename Fn1, typename Fn2>
+static void TestSignedBinOpExhaustive(Fn1 RangeFn, Fn2 IntFn) {
+  unsigned Bits = 4;
+  EnumerateTwoConstantRanges(Bits, [&](const ConstantRange &CR1,
+                                       const ConstantRange &CR2) {
+    ConstantRange CR = RangeFn(CR1, CR2);
+    if (CR1.isEmptySet() || CR2.isEmptySet()) {
+      EXPECT_TRUE(CR.isEmptySet());
+      return;
+    }
+
+    APInt Min = APInt::getSignedMaxValue(Bits);
+    APInt Max = APInt::getSignedMaxValue(Bits);
+    ForeachNumInConstantRange(CR1, [&](const APInt &N1) {
+      ForeachNumInConstantRange(CR2, [&](const APInt &N2) {
+        APInt N = IntFn(N1, N2);
+        if (N.slt(Min))
+          Min = N;
+        if (N.sgt(Max))
+          Max = N;
+      });
+    });
+
+    ++Max;
+    ConstantRange Expected = Min == Max ? ConstantRange::getFull(Bits)
+                                        : ConstantRange(Min, Max);
+    EXPECT_EQ(Expected, CR);
+  });
+}
+
+TEST_F(ConstantRangeTest, UAddSat) {
+  TestUnsignedBinOpExhaustive(
+      [](const ConstantRange &CR1, const ConstantRange &CR2) {
+        return CR1.uadd_sat(CR2);
+      },
+      [](const APInt &N1, const APInt &N2) {
+        return N1.uadd_sat(N2);
+      });
+}
+
+TEST_F(ConstantRangeTest, USubSat) {
+  TestUnsignedBinOpExhaustive(
+      [](const ConstantRange &CR1, const ConstantRange &CR2) {
+        return CR1.usub_sat(CR2);
+      },
+      [](const APInt &N1, const APInt &N2) {
+        return N1.usub_sat(N2);
+      });
+}
+
+TEST_F(ConstantRangeTest, SAddSat) {
+  TestSignedBinOpExhaustive(
+      [](const ConstantRange &CR1, const ConstantRange &CR2) {
+        return CR1.sadd_sat(CR2);
+      },
+      [](const APInt &N1, const APInt &N2) {
+        return N1.sadd_sat(N2);
+      });
+}
+
+TEST_F(ConstantRangeTest, SSubSat) {
+  TestSignedBinOpExhaustive(
+      [](const ConstantRange &CR1, const ConstantRange &CR2) {
+        return CR1.ssub_sat(CR2);
+      },
+      [](const APInt &N1, const APInt &N2) {
+        return N1.ssub_sat(N2);
+      });
+}
+
 }  // anonymous namespace