1 files changed, 30 insertions, 10 deletions

diff --git a/polly/lib/Transform/ForwardOpTree.cpp b/polly/lib/Transform/ForwardOpTree.cpp
index 81ace39b8de..852fc188542 100644
--- a/polly/lib/Transform/ForwardOpTree.cpp
+++ b/polly/lib/Transform/ForwardOpTree.cpp
@@ -140,13 +140,6 @@ private:
   ForwardingDecision canForwardTree(ScopStmt *TargetStmt, Value *UseVal,
                                     ScopStmt *UseStmt, Loop *UseLoop,
                                     bool DoIt) {
-
-    // PHis are not yet supported.
-    if (isa<PHINode>(UseVal)) {
-      DEBUG(dbgs() << "    Cannot forward PHI: " << *UseVal << "\n");
-      return FD_CannotForward;
-    }
-
     VirtualUse VUse = VirtualUse::create(UseStmt, UseLoop, UseVal, true);
     switch (VUse.getKind()) {
     case VirtualUse::Constant:
@@ -157,10 +150,31 @@ private:
         return FD_DidForward;
       return FD_CanForwardLeaf;
 
-    case VirtualUse::Synthesizable:
-      // Not supported yet.
-      DEBUG(dbgs() << "    Cannot forward synthesizable: " << *UseVal << "\n");
+    case VirtualUse::Synthesizable: {
+      // ScopExpander will take care for of generating the code at the new
+      // location.
+      if (DoIt)
+        return FD_DidForward;
+
+      // Check if the value is synthesizable at the new location as well. This
+      // might be possible when leaving a loop for which ScalarEvolution is
+      // unable to derive the exit value for.
+      // TODO: If there is a LCSSA PHI at the loop exit, use that one.
+      // If the SCEV contains a SCEVAddRecExpr, we currently depend on that we
+      // do not forward past its loop header. This would require us to use a
+      // previous loop induction variable instead the current one. We currently
+      // do not allow forwarding PHI nodes, thus this should never occur (the
+      // only exception where no phi is necessary being an unreachable loop
+      // without edge from the outside).
+      VirtualUse TargetUse = VirtualUse::create(
+          S, TargetStmt, TargetStmt->getSurroundingLoop(), UseVal, true);
+      if (TargetUse.getKind() == VirtualUse::Synthesizable)
+        return FD_CanForwardLeaf;
+
+      DEBUG(dbgs() << "    Synthesizable would not be synthesizable anymore: "
+                   << *UseVal << "\n");
       return FD_CannotForward;
+    }
 
     case VirtualUse::ReadOnly:
       // Note that we cannot return FD_CanForwardTree here. With a operand tree
@@ -185,6 +199,12 @@ private:
     case VirtualUse::Inter:
       auto Inst = cast<Instruction>(UseVal);
 
+      // PHIs, unless synthesizable, are not yet supported.
+      if (isa<PHINode>(Inst)) {
+        DEBUG(dbgs() << "    Cannot forward PHI: " << *UseVal << "\n");
+        return FD_CannotForward;
+      }
+
       // Compatible instructions must satisfy the following conditions:
       // 1. Idempotent (instruction will be copied, not moved; although its
       //    original instance might be removed by simplification)