Fix cycle in selection DAG introduced by extractelement legalization

During selection DAG legalization, extractelement is replaced with a load
instruction.  To do this, a temporary store to the stack is used unless an
existing store is found that can be re-used.
    
If re-using a store, the chain going out of the store must be replaced by
the one going out of the new load (this ensures that any stores that must
take place after the store happens after the load, else the value might
be overwritten before it is loaded).
    
The problem is, if the extractelement index is dependent on the store
replacing the chain will introduce a cycle in the selection DAG (the load
uses the index, and by replacing the chain we will make the index dependent
on the load).
    
To fix this, if the index is dependent on the store, the store is skipped.
This is conservative as we may end up creating an unnecessary extra store
to the stack.  However, the situation is not expected to occur very often.

Differential Revision: http://reviews.llvm.org/D15330

llvm-svn: 255114

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 3393e17b8e0..f46767f6c4a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -1463,6 +1463,11 @@ SDValue SelectionDAGLegalize::ExpandExtractFromVectorThroughStack(SDValue Op) {
   // series of EXTRACT_VECTOR_ELT nodes are generated, one for each element in
   // the vector. If all are expanded here, we don't want one store per vector
   // element.
+
+  // Caches for hasPredecessorHelper
+  SmallPtrSet<const SDNode *, 32> Visited;
+  SmallVector<const SDNode *, 16> Worklist;
+
   SDValue StackPtr, Ch;
   for (SDNode::use_iterator UI = Vec.getNode()->use_begin(),
        UE = Vec.getNode()->use_end(); UI != UE; ++UI) {
@@ -1477,6 +1482,12 @@ SDValue SelectionDAGLegalize::ExpandExtractFromVectorThroughStack(SDValue Op) {
       if (!ST->getChain().reachesChainWithoutSideEffects(DAG.getEntryNode()))
         continue;
 
+      // If the index is dependent on the store we will introduce a cycle when
+      // creating the load (the load uses the index, and by replacing the chain
+      // we will make the index dependent on the load).
+      if (Idx.getNode()->hasPredecessorHelper(ST, Visited, Worklist))
+        continue;
+
       StackPtr = ST->getBasePtr();
       Ch = SDValue(ST, 0);
       break;