From c2fd2cd7a149673b7021f2489cccdb89c9eae192 Mon Sep 17 00:00:00 2001
From: Roman Lebedev <lebedev.ri@gmail.com>
Date: Tue, 22 Sep 2020 15:34:45 +0300
Subject: [PATCH] [NFC][CVP] processUDivOrURem(): refactor to use
 ConstantRange::getActiveBits()

As an exhaustive test shows, this logic is fully identical to the old
implementation, with exception of the case where both of the operands
had empty ranges:

```
TEST_F(ConstantRangeTest, CVP_UDiv) {
  unsigned Bits = 4;
  EnumerateConstantRanges(Bits, [&](const ConstantRange &CR0) {
    if(CR0.isEmptySet())
      return;
    EnumerateConstantRanges(Bits, [&](const ConstantRange &CR1) {
      if(CR0.isEmptySet())
        return;

      unsigned MaxActiveBits = 0;
      for (const ConstantRange &CR : {CR0, CR1})
        MaxActiveBits = std::max(MaxActiveBits, CR.getActiveBits());

      ConstantRange OperandRange(Bits, /*isFullSet=*/false);
      for (const ConstantRange &CR : {CR0, CR1})
        OperandRange = OperandRange.unionWith(CR);
      unsigned NewWidth = OperandRange.getUnsignedMax().getActiveBits();

      EXPECT_EQ(MaxActiveBits, NewWidth) << CR0 << " " << CR1;
    });
  });
}
```
---
 .../Scalar/CorrelatedValuePropagation.cpp        | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index 3d51a9f2c6a..50b7d9e229d 100644
--- a/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -633,18 +633,20 @@ static bool processUDivOrURem(BinaryOperator *Instr, LazyValueInfo *LVI) {
 
   // Find the smallest power of two bitwidth that's sufficient to hold Instr's
   // operands.
-  auto OrigWidth = Instr->getType()->getIntegerBitWidth();
-  ConstantRange OperandRange(OrigWidth, /*isFullSet=*/false);
+
+  // What is the smallest bit width that can accomodate the entire value ranges
+  // of both of the operands?
+  unsigned MaxActiveBits = 0;
   for (Value *Operand : Instr->operands()) {
-    OperandRange = OperandRange.unionWith(
-        LVI->getConstantRange(Operand, Instr->getParent()));
+    ConstantRange CR = LVI->getConstantRange(Operand, Instr->getParent());
+    MaxActiveBits = std::max(CR.getActiveBits(), MaxActiveBits);
   }
   // Don't shrink below 8 bits wide.
-  unsigned NewWidth = std::max<unsigned>(
-      PowerOf2Ceil(OperandRange.getUnsignedMax().getActiveBits()), 8);
+  unsigned NewWidth = std::max<unsigned>(PowerOf2Ceil(MaxActiveBits), 8);
+
   // NewWidth might be greater than OrigWidth if OrigWidth is not a power of
   // two.
-  if (NewWidth >= OrigWidth)
+  if (NewWidth >= Instr->getType()->getIntegerBitWidth())
     return false;
 
   ++NumUDivs;