[PATCH] D52306: [DAGCombine] Don't fold dependent loads across SELECT_CC.

[Bevin Hansson via Phabricator via llvm-commits]

ebevhan created this revision.
ebevhan added a reviewer: bogner.
Herald added a subscriber: llvm-commits.
ebevhan added a reviewer: niravd.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D52306

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp


Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -17866,6 +17866,11 @@
                                       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.
@@ -17875,10 +17880,7 @@
       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(),


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