Fix PR 8916 (qv for analysis), at least the immediate problem.

There's an inherent tension in DAGCombine between assuming
that things will be put in canonical form, and the Depth
mechanism that disables transformations when recursion gets
too deep.  It would not surprise me if there's a lot of little
bugs like this one waiting to be discovered.  The mechanism
seems fragile and I'd suggest looking at it from a design viewpoint.

llvm-svn: 123191

2 files changed, 17 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 396ebc15c6d..f9e0992ea14 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1176,8 +1176,9 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
     // the RHS.
     if (ConstantSDNode *RHSC = dyn_cast<ConstantSDNode>(Op.getOperand(1))) {
       APInt LHSZero, LHSOne;
+      // Do not increment Depth here; that can cause an infinite loop.
       TLO.DAG.ComputeMaskedBits(Op.getOperand(0), NewMask,
-                                LHSZero, LHSOne, Depth+1);
+                                LHSZero, LHSOne, Depth);
       // If the LHS already has zeros where RHSC does, this and is dead.
       if ((LHSZero & NewMask) == (~RHSC->getAPIntValue() & NewMask))
         return TLO.CombineTo(Op, Op.getOperand(0));
diff --git a/llvm/test/CodeGen/X86/2011-01-10-DagCombineHang.ll b/llvm/test/CodeGen/X86/2011-01-10-DagCombineHang.ll
new file mode 100644
index 00000000000..bf438b82edf
--- /dev/null
+++ b/llvm/test/CodeGen/X86/2011-01-10-DagCombineHang.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s -mtriple=x86_64-apple-darwin10
+; This formerly got DagCombine into a loop, PR 8916.
+
+define i32 @foo(i64 %x, i64 %y, i64 %z, i32 %a, i32 %b) {
+entry:
+  %"alloca point" = bitcast i32 0 to i32          ; <i32> [#uses=0]
+  %t1 = shl i64 %x, 15
+  %t2 = and i64 %t1, 4294934528
+  %t3 = or i64 %t2, %y
+  %t4 = xor i64 %z, %t3
+  %t5 = trunc i64 %t4 to i32
+  %t6 = add i32 %a, %t5
+  %t7 = add i32 %t6, %b
+  ret i32 %t7
+}