[BypassSlowDivision] Handle division by constant numerators better.

Summary:
We don't do BypassSlowDivision when the denominator is a constant, but
we do do it when the numerator is a constant.

This patch makes two related changes to BypassSlowDivision when the
numerator is a constant:

 * If the numerator is too large to fit into the bypass width, don't
   bypass slow division (because we'll never run the smaller-width
   code).

 * If we bypass slow division where the numerator is a constant, don't
   OR together the numerator and denominator when determining whether
   both operands fit within the bypass width.  We need to check only the
   denominator.

Reviewers: tra

Subscribers: llvm-commits, jholewinski

Differential Revision: https://reviews.llvm.org/D26699

llvm-svn: 287062

1 files changed, 16 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
index d74d5299db5..bc2cef26edc 100644
--- a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
+++ b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
@@ -89,6 +89,11 @@ static bool insertFastDiv(Instruction *I, IntegerType *BypassType,
     return false;
   }
 
+  // If the numerator is a constant, bail if it doesn't fit into BypassType.
+  if (ConstantInt *ConstDividend = dyn_cast<ConstantInt>(Dividend))
+    if (ConstDividend->getValue().getActiveBits() > BypassType->getBitWidth())
+      return false;
+
   // Basic Block is split before divide
   BasicBlock *MainBB = &*I->getParent();
   BasicBlock *SuccessorBB = MainBB->splitBasicBlock(I);
@@ -150,7 +155,17 @@ static bool insertFastDiv(Instruction *I, IntegerType *BypassType,
   // Combine operands into a single value with OR for value testing below
   MainBB->getInstList().back().eraseFromParent();
   IRBuilder<> MainBuilder(MainBB, MainBB->end());
-  Value *OrV = MainBuilder.CreateOr(Dividend, Divisor);
+
+  // We should have bailed out above if the divisor is a constant, but the
+  // dividend may still be a constant.  Set OrV to our non-constant operands
+  // OR'ed together.
+  assert(!isa<ConstantInt>(Divisor));
+
+  Value *OrV;
+  if (!isa<ConstantInt>(Dividend))
+    OrV = MainBuilder.CreateOr(Dividend, Divisor);
+  else
+    OrV = Divisor;
 
   // BitMask is inverted to check if the operands are
   // larger than the bypass type