[ConstantRange] Add urem support

Add urem support to ConstantRange, so we can handle in in LVI. This
is an approximate implementation that tries to capture the most useful
conditions: If the LHS is always strictly smaller than the RHS, then
the urem is a no-op and the result is the same as the LHS range.
Otherwise the lower bound is zero and the upper bound is
min(LHSMax, RHSMax - 1).

Differential Revision: https://reviews.llvm.org/D60952

llvm-svn: 359019

Author: nikic

llvm/include/llvm/IR/ConstantRange.h

@@ -356,6 +356,11 @@ class LLVM_NODISCARD ConstantRange {
   /// \p Other.
   ConstantRange udiv(const ConstantRange &Other) const;
 
+  /// Return a new range representing the possible values resulting
+  /// from an unsigned remainder operation of a value in this range and a
+  /// value in \p Other.
+  ConstantRange urem(const ConstantRange &Other) const;
+
   /// Return a new range representing the possible values resulting
   /// from a binary-and of a value in this range by a value in \p Other.
   ConstantRange binaryAnd(const ConstantRange &Other) const;

llvm/lib/IR/ConstantRange.cpp

@@ -794,6 +794,8 @@ ConstantRange ConstantRange::binaryOp(Instruction::BinaryOps BinOp,
     return multiply(Other);
   case Instruction::UDiv:
     return udiv(Other);
+  case Instruction::URem:
+    return urem(Other);
   case Instruction::Shl:
     return shl(Other);
   case Instruction::LShr:
@@ -991,6 +993,19 @@ ConstantRange::udiv(const ConstantRange &RHS) const {
   return getNonEmpty(std::move(Lower), std::move(Upper));
 }
 
+ConstantRange ConstantRange::urem(const ConstantRange &RHS) const {
+  if (isEmptySet() || RHS.isEmptySet() || RHS.getUnsignedMax().isNullValue())
+    return getEmpty();
+
+  // L % R for L < R is L.
+  if (getUnsignedMax().ult(RHS.getUnsignedMin()))
+    return *this;
+
+  // L % R is <= L and < R.
+  APInt Upper = APIntOps::umin(getUnsignedMax(), RHS.getUnsignedMax() - 1) + 1;
+  return getNonEmpty(APInt::getNullValue(getBitWidth()), std::move(Upper));
+}
+
 ConstantRange
 ConstantRange::binaryAnd(const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())

llvm/unittests/IR/ConstantRangeTest.cpp

@@ -59,20 +59,19 @@ static void ForeachNumInConstantRange(const ConstantRange &CR, Fn TestFn) {
 }
 
 template<typename Fn1, typename Fn2>
-static void TestUnsignedBinOpExhaustive(Fn1 RangeFn, Fn2 IntFn) {
+static void TestUnsignedBinOpExhaustive(
+    Fn1 RangeFn, Fn2 IntFn,
+    bool SkipZeroRHS = false, bool CorrectnessOnly = false) {
   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) {
+        if (SkipZeroRHS && N2 == 0)
+          return;
+
         APInt N = IntFn(N1, N2);
         if (N.ult(Min))
           Min = N;
@@ -81,7 +80,18 @@ static void TestUnsignedBinOpExhaustive(Fn1 RangeFn, Fn2 IntFn) {
       });
     });
 
-    EXPECT_EQ(ConstantRange::getNonEmpty(Min, Max + 1), CR);
+    ConstantRange CR = RangeFn(CR1, CR2);
+    if (Min.ugt(Max)) {
+      EXPECT_TRUE(CR.isEmptySet());
+      return;
+    }
+
+    ConstantRange Exact = ConstantRange::getNonEmpty(Min, Max + 1);
+    if (CorrectnessOnly) {
+      EXPECT_TRUE(CR.contains(Exact));
+    } else {
+      EXPECT_EQ(Exact, CR);
+    }
   });
 }
 
@@ -813,6 +823,42 @@ TEST_F(ConstantRangeTest, UDiv) {
   EXPECT_EQ(Wrap.udiv(Wrap), Full);
 }
 
+TEST_F(ConstantRangeTest, URem) {
+  EXPECT_EQ(Full.urem(Empty), Empty);
+  EXPECT_EQ(Empty.urem(Full), Empty);
+  // urem by zero is poison.
+  EXPECT_EQ(Full.urem(ConstantRange(APInt(16, 0))), Empty);
+  // urem by full range doesn't contain MaxValue.
+  EXPECT_EQ(Full.urem(Full), ConstantRange(APInt(16, 0), APInt(16, 0xffff)));
+  // urem is upper bounded by maximum RHS minus one.
+  EXPECT_EQ(Full.urem(ConstantRange(APInt(16, 0), APInt(16, 123))),
+            ConstantRange(APInt(16, 0), APInt(16, 122)));
+  // urem is upper bounded by maximum LHS.
+  EXPECT_EQ(ConstantRange(APInt(16, 0), APInt(16, 123)).urem(Full),
+            ConstantRange(APInt(16, 0), APInt(16, 123)));
+  // If the LHS is always lower than the RHS, the result is the LHS.
+  EXPECT_EQ(ConstantRange(APInt(16, 10), APInt(16, 20))
+                .urem(ConstantRange(APInt(16, 20), APInt(16, 30))),
+            ConstantRange(APInt(16, 10), APInt(16, 20)));
+  // It has to be strictly lower, otherwise the top value may wrap to zero.
+  EXPECT_EQ(ConstantRange(APInt(16, 10), APInt(16, 20))
+                .urem(ConstantRange(APInt(16, 19), APInt(16, 30))),
+            ConstantRange(APInt(16, 0), APInt(16, 20)));
+  // [12, 14] % 10 is [2, 4], but we conservatively compute [0, 9].
+  EXPECT_EQ(ConstantRange(APInt(16, 12), APInt(16, 15))
+                .urem(ConstantRange(APInt(16, 10))),
+            ConstantRange(APInt(16, 0), APInt(16, 10)));
+
+  TestUnsignedBinOpExhaustive(
+      [](const ConstantRange &CR1, const ConstantRange &CR2) {
+        return CR1.urem(CR2);
+      },
+      [](const APInt &N1, const APInt &N2) {
+        return N1.urem(N2);
+      },
+      /* SkipZeroRHS */ true, /* CorrectnessOnly */ true);
+}
+
 TEST_F(ConstantRangeTest, Shl) {
   ConstantRange Some2(APInt(16, 0xfff), APInt(16, 0x8000));
   ConstantRange WrapNullMax(APInt(16, 0x1), APInt(16, 0x0));