[DAGCombiner] disable truncation of binops by default

As discussed in the post-commit thread of r347917, this
transform is fighting with an existing transform causing
an infinite loop or out-of-memory, so this is effectively 
reverting r347917 and its follow-up r348195 while we
investigate the bug.

llvm-svn: 348604

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index d0f7f150001..c1cfaa5abd2 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -112,6 +112,12 @@ static cl::opt<bool>
   MaySplitLoadIndex("combiner-split-load-index", cl::Hidden, cl::init(true),
                     cl::desc("DAG combiner may split indexing from loads"));
 
+// This is a temporary debug flag to disable a combine that is known to
+// conflict with another combine.
+static cl::opt<bool>
+NarrowTruncatedBinops("narrow-truncated-binops", cl::Hidden, cl::init(false),
+                      cl::desc("Move truncates ahead of binops"));
+
 namespace {
 
   class DAGCombiner {
@@ -9804,7 +9810,7 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
   case ISD::AND:
   case ISD::OR:
   case ISD::XOR:
-    if (!LegalOperations && N0.hasOneUse() &&
+    if (NarrowTruncatedBinops && !LegalOperations && N0.hasOneUse() &&
         (isConstantOrConstantVector(N0.getOperand(0)) ||
          isConstantOrConstantVector(N0.getOperand(1)))) {
       // TODO: We already restricted this to pre-legalization, but for vectors