[llvm] r342980 - [DAGCombine] Don't fold dependent loads across SELECT_CC.

[Nirav Dave via llvm-commits]

Author: niravd
Date: Tue Sep 25 07:43:05 2018
New Revision: 342980

URL: http://llvm.org/viewvc/llvm-project?rev=342980&view=rev
Log:
[DAGCombine] Don't fold dependent loads across SELECT_CC.

DAGCombine will try to fold two loads that feed a SELECT or SELECT_CC
after the select, resulting in a select of an address and a single
load after.

If either of the loads depend on the other, this is not legal as it
could introduce cycles. However, it only checked this if the opcode
was a SELECT, and not for a SELECT_CC.

Unfortunately, the only reproducer I have for this is for our
downstream target. I've tried getting it to trigger on an upstream one
but haven't been successful.

Patch thanks to Bevin Hansson.

Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=342980&r1=342979&r2=342980&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Tue Sep 25 07:43:05 2018
@@ -17873,6 +17873,10 @@ bool DAGCombiner::SimplifySelectOps(SDNo
                                       LLD->getBasePtr().getValueType()))
       return false;
 
+    // The loads must not depend on one another.
+    if (LLD->isPredecessorOf(RLD) || RLD->isPredecessorOf(LLD))
+      return false;
+
     // Check that the select condition doesn't reach either load.  If so,
     // folding this will induce a cycle into the DAG.  If not, this is safe to
     // xform, so create a select of the addresses.
@@ -17882,10 +17886,7 @@ bool DAGCombiner::SimplifySelectOps(SDNo
       if ((LLD->hasAnyUseOfValue(1) && LLD->isPredecessorOf(CondNode)) ||
           (RLD->hasAnyUseOfValue(1) && RLD->isPredecessorOf(CondNode)))
         return false;
-      // The loads must not depend on one another.
-      if (LLD->isPredecessorOf(RLD) ||
-          RLD->isPredecessorOf(LLD))
-        return false;
+
       Addr = DAG.getSelect(SDLoc(TheSelect),
                            LLD->getBasePtr().getValueType(),
                            TheSelect->getOperand(0), LLD->getBasePtr(),