Fix DAGCombiner to avoid going into an infinite loop when it

encounters (and:i64 (shl:i64 (load:i64), 1), 0xffffffff).
This fixes rdar://8606584.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118143 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -3667,6 +3667,20 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
   // fold (zext (truncate x)) -> (and x, mask)
   if (N0.getOpcode() == ISD::TRUNCATE &&
       (!LegalOperations || TLI.isOperationLegal(ISD::AND, VT))) {
+
+    // fold (zext (truncate (load x))) -> (zext (smaller load x))
+    // fold (zext (truncate (srl (load x), c))) -> (zext (smaller load (x+c/n)))
+    SDValue NarrowLoad = ReduceLoadWidth(N0.getNode());
+    if (NarrowLoad.getNode()) {
+      SDNode* oye = N0.getNode()->getOperand(0).getNode();
+      if (NarrowLoad.getNode() != N0.getNode()) {
+        CombineTo(N0.getNode(), NarrowLoad);
+        // CombineTo deleted the truncate, if needed, but not what's under it.
+        AddToWorkList(oye);
+      }
+      return SDValue(N, 0);   // Return N so it doesn't get rechecked!
+    }
+
     SDValue Op = N0.getOperand(0);
     if (Op.getValueType().bitsLT(VT)) {
       Op = DAG.getNode(ISD::ANY_EXTEND, N->getDebugLoc(), VT, Op);
@@ -4102,6 +4116,17 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
     }
   }
 
+  // If the load is shifted left (and the result isn't shifted back right),
+  // we can fold the truncate through the shift.
+  unsigned ShLeftAmt = 0;
+  if (ShAmt == 0 && N0.getOpcode() == ISD::SHL && N0.hasOneUse() &&
+      TLI.isNarrowingProfitable(N0.getValueType(), VT)) {
+    if (ConstantSDNode *N01 = dyn_cast<ConstantSDNode>(N0.getOperand(1))) {
+      ShLeftAmt = N01->getZExtValue();
+      N0 = N0.getOperand(0);
+    }
+  }
+
   // Do not generate loads of non-round integer types since these can
   // be expensive (and would be wrong if the type is not byte sized).
   if (isa<LoadSDNode>(N0) && N0.hasOneUse() && ExtVT.isRound() &&
@@ -4140,8 +4165,18 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
     DAG.ReplaceAllUsesOfValueWith(N0.getValue(1), Load.getValue(1),
                                   &DeadNodes);
 
+    // Shift the result left, if we've swallowed a left shift.
+    SDValue Result = Load;
+    if (ShLeftAmt != 0) {
+      EVT ShImmTy = getShiftAmountTy();
+      if (!isUIntN(ShImmTy.getSizeInBits(), ShLeftAmt))
+        ShImmTy = VT;
+      Result = DAG.getNode(ISD::SHL, N0.getDebugLoc(), VT,
+                           Result, DAG.getConstant(ShLeftAmt, ShImmTy));
+    }
+
     // Return the new loaded value.
-    return Load;
+    return Result;
   }
 
   return SDValue();

test/CodeGen/X86/narrow-shl-load.ll

@@ -0,0 +1,32 @@
+; RUN: llc -march=x86-64 < %s
+
+; DAGCombiner should fold this code in finite time.
+
+; rdar://8606584
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+target triple = "x86_64-pc-linux-gnu"
+
+define void @D() nounwind readnone {
+bb.nph:
+  br label %while.cond
+
+while.cond:                                       ; preds = %while.cond, %bb.nph
+  %tmp6 = load i32* undef, align 4
+  %and = or i64 undef, undef
+  %conv11 = zext i32 undef to i64
+  %conv14 = zext i32 %tmp6 to i64
+  %shl15 = shl i64 %conv14, 1
+  %shl15.masked = and i64 %shl15, 4294967294
+  %and17 = or i64 %shl15.masked, %conv11
+  %add = add i64 %and17, 1
+  %xor = xor i64 %add, %and
+  %tmp20 = load i64* undef, align 8
+  %add21 = add i64 %xor, %tmp20
+  %conv22 = trunc i64 %add21 to i32
+  store i32 %conv22, i32* undef, align 4
+  br i1 false, label %while.end, label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  ret void
+}