[PATCH] D15330: Fix cycle in selection DAG introduced by extractelement legalization

[Robert Lougher via llvm-commits]

rob.lougher updated this revision to Diff 42281.
rob.lougher added a comment.

Updated patch to use hasPredecessorHelper.


http://reviews.llvm.org/D15330

Files:
  lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
  test/CodeGen/X86/extractelement-legalization-cycle.ll

Index: test/CodeGen/X86/extractelement-legalization-cycle.ll
===================================================================
--- test/CodeGen/X86/extractelement-legalization-cycle.ll
+++ test/CodeGen/X86/extractelement-legalization-cycle.ll
@@ -0,0 +1,21 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s
+
+; When the extractelement is converted to a load the store can be re-used.
+; This will, however, introduce a cycle into the selection DAG (the load
+; of the extractelement index is dependent on the store, and so after the
+; conversion it becomes dependent on the new load, which is dependent on
+; the index).  Make sure we skip the store, and conservatively instead
+; use a store to the stack.
+
+define float @foo(i32* %i, <4 x float>* %v) {
+; CHECK-LABEL: foo:
+; CHECK:    movaps %xmm0, -[[OFFSET:[0-9]+]](%rsp)
+; CHECK:    movss -[[OFFSET]](%rsp,{{.*}}), %xmm0 {{.*}}
+; CHECK-NEXT:    retq
+  %1 = load <4 x float>, <4 x float>* %v, align 16
+  %mul = fmul <4 x float> %1, %1
+  store <4 x float> %mul, <4 x float>* %v, align 16
+  %2 = load i32, i32* %i, align 4
+  %vecext = extractelement <4 x float> %mul, i32 %2
+  ret float %vecext
+}
Index: lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -1463,6 +1463,11 @@
   // 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 @@
       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;


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