TargetLowering: Add SimplifyDemandedBits() helper to TargetLoweringOpt

Summary:
The main purpose of this new helper is to enable simplifying operations that
have multiple uses.  SimplifyDemandedBits does not handle multiple uses
currently, and this new function makes it possible to optimize:

and v1, v0, 0xffffff
mul24 v2, v1, v1      ; Multiply ignoring high 8-bits.

To:

mul24 v2, v0, v0

Where before this would not be optimized, because v1 has multiple uses.

Reviewers: bogner, arsenm

Subscribers: nhaehnle, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D24964

llvm-svn: 284266

1 files changed, 55 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 0c04636424c..12282988129 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -418,6 +418,58 @@ bool TargetLowering::TargetLoweringOpt::ShrinkDemandedOp(SDValue Op,
   return false;
 }
 
+bool
+TargetLowering::TargetLoweringOpt::SimplifyDemandedBits(SDNode *User,
+                                                        unsigned OpIdx,
+                                                        const APInt &Demanded,
+                                                        DAGCombinerInfo &DCI) {
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  SDValue Op = User->getOperand(OpIdx);
+  APInt KnownZero, KnownOne;
+
+  if (!TLI.SimplifyDemandedBits(Op, Demanded, KnownZero, KnownOne,
+                                *this, 0, true))
+    return false;
+
+
+  // Old will not always be the same as Op.  For example:
+  //
+  // Demanded = 0xffffff
+  // Op = i64 truncate (i32 and x, 0xffffff)
+  // In this case simplify demand bits will want to replace the 'and' node
+  // with the value 'x', which will give us:
+  // Old = i32 and x, 0xffffff
+  // New = x
+  if (Old.hasOneUse()) {
+    // For the one use case, we just commit the change.
+    DCI.CommitTargetLoweringOpt(*this);
+    return true;
+  }
+
+  // If Old has more than one use then it must be Op, because the
+  // AssumeSingleUse flag is not propogated to recursive calls of
+  // SimplifyDemanded bits, so the only node with multiple use that
+  // it will attempt to combine will be opt.
+  assert(Old == Op);
+
+  SmallVector <SDValue, 4> NewOps;
+  for (unsigned i = 0, e = User->getNumOperands(); i != e; ++i) {
+    if (i == OpIdx) {
+      NewOps.push_back(New);
+      continue;
+    }
+    NewOps.push_back(User->getOperand(i));
+  }
+  DAG.UpdateNodeOperands(User, NewOps);
+  // Op has less users now, so we may be able to perform additional combines
+  // with it.
+  DCI.AddToWorklist(Op.getNode());
+  // User's operands have been updated, so we may be able to do new combines
+  // with it.
+  DCI.AddToWorklist(User);
+  return true;
+}
+
 /// Look at Op. At this point, we know that only the DemandedMask bits of the
 /// result of Op are ever used downstream. If we can use this information to
 /// simplify Op, create a new simplified DAG node and return true, returning the
@@ -430,7 +482,8 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
                                           APInt &KnownZero,
                                           APInt &KnownOne,
                                           TargetLoweringOpt &TLO,
-                                          unsigned Depth) const {
+                                          unsigned Depth,
+                                          bool AssumeSingleUse) const {
   unsigned BitWidth = DemandedMask.getBitWidth();
   assert(Op.getScalarValueSizeInBits() == BitWidth &&
          "Mask size mismatches value type size!");
@@ -442,7 +495,7 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
   KnownZero = KnownOne = APInt(BitWidth, 0);
 
   // Other users may use these bits.
-  if (!Op.getNode()->hasOneUse()) {
+  if (!Op.getNode()->hasOneUse() && !AssumeSingleUse) {
     if (Depth != 0) {
       // If not at the root, Just compute the KnownZero/KnownOne bits to
       // simplify things downstream.