[ConstantRange] Compute precise shl range for single elements

For the common case where the shift amount is constant (a single
element range) we can easily compute a precise range (up to
unsigned envelope), so do that.

Author: nikic

llvm/lib/IR/ConstantRange.cpp

@@ -1346,24 +1346,33 @@ ConstantRange::shl(const ConstantRange &Other) const {
   if (isEmptySet() || Other.isEmptySet())
     return getEmpty();
 
-  APInt max = getUnsignedMax();
-  APInt Other_umax = Other.getUnsignedMax();
+  APInt Min = getUnsignedMin();
+  APInt Max = getUnsignedMax();
+  if (const APInt *RHS = Other.getSingleElement()) {
+    unsigned BW = getBitWidth();
+    if (RHS->uge(BW))
+      return getEmpty();
 
-  // If we are shifting by maximum amount of
-  // zero return return the original range.
-  if (Other_umax.isZero())
-    return *this;
-  // there's overflow!
-  if (Other_umax.ugt(max.countLeadingZeros()))
+    unsigned EqualLeadingBits = (Min ^ Max).countLeadingZeros();
+    if (RHS->ule(EqualLeadingBits))
+      return getNonEmpty(Min << *RHS, (Max << *RHS) + 1);
+
+    return getNonEmpty(APInt::getZero(BW),
+                       APInt::getBitsSetFrom(BW, RHS->getZExtValue()) + 1);
+  }
+
+  APInt OtherMax = Other.getUnsignedMax();
+
+  // There's overflow!
+  if (OtherMax.ugt(Max.countLeadingZeros()))
     return getFull();
 
   // FIXME: implement the other tricky cases
 
-  APInt min = getUnsignedMin();
-  min <<= Other.getUnsignedMin();
-  max <<= Other_umax;
+  Min <<= Other.getUnsignedMin();
+  Max <<= OtherMax;
 
-  return ConstantRange(std::move(min), std::move(max) + 1);
+  return ConstantRange::getNonEmpty(std::move(Min), std::move(Max) + 1);
 }
 
 ConstantRange

llvm/unittests/IR/ConstantRangeTest.cpp

@@ -1400,7 +1400,8 @@ TEST_F(ConstantRangeTest, Shl) {
   ConstantRange WrapNullMax(APInt(16, 0x1), APInt(16, 0x0));
   EXPECT_EQ(Full.shl(Full), Full);
   EXPECT_EQ(Full.shl(Empty), Empty);
-  EXPECT_EQ(Full.shl(One), Full);    // TODO: [0, (-1 << 0xa) + 1)
+  EXPECT_EQ(Full.shl(One), ConstantRange(APInt(16, 0),
+                                         APInt(16, 0xfc00) + 1));
   EXPECT_EQ(Full.shl(Some), Full);   // TODO: [0, (-1 << 0xa) + 1)
   EXPECT_EQ(Full.shl(Wrap), Full);
   EXPECT_EQ(Empty.shl(Empty), Empty);
@@ -1418,6 +1419,21 @@ TEST_F(ConstantRangeTest, Shl) {
       Some2.shl(ConstantRange(APInt(16, 0x1))),
       ConstantRange(APInt(16, 0xfff << 0x1), APInt(16, 0x7fff << 0x1) + 1));
   EXPECT_EQ(One.shl(WrapNullMax), Full);
+
+  TestBinaryOpExhaustive(
+      [](const ConstantRange &CR1, const ConstantRange &CR2) {
+        return CR1.shl(CR2);
+      },
+      [](const APInt &N1, const APInt &N2) -> Optional<APInt> {
+        if (N2.uge(N2.getBitWidth()))
+          return None;
+        return N1.shl(N2);
+      },
+      PreferSmallestUnsigned,
+      [](const ConstantRange &, const ConstantRange &CR2) {
+        // We currently only produce precise results for single element RHS.
+        return CR2.isSingleElement();
+      });
 }
 
 TEST_F(ConstantRangeTest, Lshr) {