[ConstantRange] Add abs() support

Add support for abs() to ConstantRange. This will allow to handle
SPF_ABS select flavor in LVI and will also come in handy as a
primitive for the srem implementation.

The implementation is slightly tricky, because a) abs of signed min
is signed min and b) sign-wrapped ranges may have an abs() that is
smaller than a full range, so we need to explicitly handle them.

Differential Revision: https://reviews.llvm.org/D61084

llvm-svn: 359321

include/llvm/IR/ConstantRange.h

@@ -397,6 +397,10 @@ public:
   /// Return a new range that is the logical not of the current set.
   ConstantRange inverse() const;
 
+  /// Calculate absolute value range. If the original range contains signed
+  /// min, then the resulting range will also contain signed min.
+  ConstantRange abs() const;
+
   /// Represents whether an operation on the given constant range is known to
   /// always or never overflow.
   enum class OverflowResult { AlwaysOverflows, MayOverflow, NeverOverflows };

lib/IR/ConstantRange.cpp

@@ -1156,6 +1156,37 @@ ConstantRange ConstantRange::inverse() const {
   return ConstantRange(Upper, Lower);
 }
 
+ConstantRange ConstantRange::abs() const {
+  if (isEmptySet())
+    return getEmpty();
+
+  if (isSignWrappedSet()) {
+    APInt Lo;
+    // Check whether the range crosses zero.
+    if (Upper.isStrictlyPositive() || !Lower.isStrictlyPositive())
+      Lo = APInt::getNullValue(getBitWidth());
+    else
+      Lo = APIntOps::umin(Lower, -Upper + 1);
+
+    // SignedMin is included in the result range.
+    return ConstantRange(Lo, APInt::getSignedMinValue(getBitWidth()) + 1);
+  }
+
+  APInt SMin = getSignedMin(), SMax = getSignedMax();
+
+  // All non-negative.
+  if (SMin.isNonNegative())
+    return *this;
+
+  // All negative.
+  if (SMax.isNegative())
+    return ConstantRange(-SMax, -SMin + 1);
+
+  // Range crosses zero.
+  return ConstantRange(APInt::getNullValue(getBitWidth()),
+                       APIntOps::umax(-SMin, SMax) + 1);
+}
+
 ConstantRange::OverflowResult ConstantRange::unsignedAddMayOverflow(
     const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())

unittests/IR/ConstantRangeTest.cpp

@@ -1797,4 +1797,30 @@ TEST_F(ConstantRangeTest, SSubSat) {
       });
 }
 
+TEST_F(ConstantRangeTest, Abs) {
+  unsigned Bits = 4;
+  EnumerateConstantRanges(Bits, [&](const ConstantRange &CR) {
+    // We're working with unsigned integers here, because it makes the signed
+    // min case non-wrapping.
+    APInt Min = APInt::getMaxValue(Bits);
+    APInt Max = APInt::getMinValue(Bits);
+    ForeachNumInConstantRange(CR, [&](const APInt &N) {
+      APInt AbsN = N.abs();
+      if (AbsN.ult(Min))
+        Min = AbsN;
+      if (AbsN.ugt(Max))
+        Max = AbsN;
+    });
+
+    ConstantRange AbsCR = CR.abs();
+    if (Min.ugt(Max)) {
+      EXPECT_TRUE(AbsCR.isEmptySet());
+      return;
+    }
+
+    ConstantRange Exact = ConstantRange::getNonEmpty(Min, Max + 1);
+    EXPECT_EQ(Exact, AbsCR);
+  });
+}
+
 }  // anonymous namespace