[Pipeliner] Add missing loop carried dependences

The pipeliner is not adding a dependence edge for a loop carried
dependence, and ends up scheduling a load from iteration n prior
to an aliased store in iteration n-1.

The code that adds the loop carried dependences in the pipeliner
doesn't check if the memory objects for loads and stores are
"identified" (i.e., distinct) objects. If they are not, then the
code that adds the dependences needs to be conservative. The
objects can be used to check dependences only when they are
distinct objects.

The code that checks for loop carried dependences has been updated
to classify loads and stores that are not identified as "unknown"
values. A store with an "unknown" value can potentially create
a loop carried dependence with any pending load.

Patch by Brendon Cahoon.

llvm-svn: 328547

llvm/lib/CodeGen/MachinePipeliner.cpp

@@ -1051,6 +1051,13 @@ static void getUnderlyingObjects(MachineInstr *MI,
   if (!MM->getValue())
     return;
   GetUnderlyingObjects(const_cast<Value *>(MM->getValue()), Objs, DL);
+  for (Value *V : Objs) {
+    if (!isIdentifiedObject(V)) {
+      Objs.clear();
+      return;
+    }
+    Objs.push_back(V);
+  }
 }
 
 /// Add a chain edge between a load and store if the store can be an
@@ -1059,6 +1066,8 @@ static void getUnderlyingObjects(MachineInstr *MI,
 /// but that code doesn't create loop carried dependences.
 void SwingSchedulerDAG::addLoopCarriedDependences(AliasAnalysis *AA) {
   MapVector<Value *, SmallVector<SUnit *, 4>> PendingLoads;
+  Value *UnknownValue =
+    UndefValue::get(Type::getVoidTy(MF.getFunction().getContext()));
   for (auto &SU : SUnits) {
     MachineInstr &MI = *SU.getInstr();
     if (isDependenceBarrier(MI, AA))
@@ -1066,6 +1075,8 @@ void SwingSchedulerDAG::addLoopCarriedDependences(AliasAnalysis *AA) {
     else if (MI.mayLoad()) {
       SmallVector<Value *, 4> Objs;
       getUnderlyingObjects(&MI, Objs, MF.getDataLayout());
+      if (Objs.empty())
+        Objs.push_back(UnknownValue);
       for (auto V : Objs) {
         SmallVector<SUnit *, 4> &SUs = PendingLoads[V];
         SUs.push_back(&SU);
@@ -1073,6 +1084,8 @@ void SwingSchedulerDAG::addLoopCarriedDependences(AliasAnalysis *AA) {
     } else if (MI.mayStore()) {
       SmallVector<Value *, 4> Objs;
       getUnderlyingObjects(&MI, Objs, MF.getDataLayout());
+      if (Objs.empty())
+        Objs.push_back(UnknownValue);
       for (auto V : Objs) {
         MapVector<Value *, SmallVector<SUnit *, 4>>::iterator I =
             PendingLoads.find(V);
@@ -1087,13 +1100,8 @@ void SwingSchedulerDAG::addLoopCarriedDependences(AliasAnalysis *AA) {
           // offset, then mark the dependence as loop carried potentially.
           unsigned BaseReg1, BaseReg2;
           int64_t Offset1, Offset2;
-          if (!TII->getMemOpBaseRegImmOfs(LdMI, BaseReg1, Offset1, TRI) ||
-              !TII->getMemOpBaseRegImmOfs(MI, BaseReg2, Offset2, TRI)) {
-            SDep Dep(Load, SDep::Barrier);
-            Dep.setLatency(1);
-            SU.addPred(Dep);
-            continue;            
-          }
+          if (TII->getMemOpBaseRegImmOfs(LdMI, BaseReg1, Offset1, TRI) &&
+              TII->getMemOpBaseRegImmOfs(MI, BaseReg2, Offset2, TRI)) {
             if (BaseReg1 == BaseReg2 && (int)Offset1 < (int)Offset2) {
               assert(TII->areMemAccessesTriviallyDisjoint(LdMI, MI, AA) &&
                      "What happened to the chain edge?");
@@ -1102,6 +1110,7 @@ void SwingSchedulerDAG::addLoopCarriedDependences(AliasAnalysis *AA) {
               SU.addPred(Dep);
               continue;
             }
+          }
           // Second, the more expensive check that uses alias analysis on the
           // base registers. If they alias, and the load offset is less than
           // the store offset, the mark the dependence as loop carried.

llvm/test/CodeGen/Hexagon/swp-loop-carried-unknown.ll

@@ -0,0 +1,54 @@
+; RUN: llc -march=hexagon < %s | FileCheck %s
+
+; Test that the pipeliner schedules a store before the load in which there is a
+; loop carried dependence. Previously, the loop carried dependence wasn't added
+; and the load from iteration n was scheduled prior to the store from iteration
+; n-1.
+
+; CHECK: loop0(.LBB0_[[LOOP:.]],
+; CHECK: .LBB0_[[LOOP]]:
+; CHECK: memh({{.*}}) =
+; CHECK: = memuh({{.*}})
+; CHECK: endloop0
+
+%s.0 = type { i16, i16 }
+
+; Function Attrs: nounwind
+define void @f0() local_unnamed_addr #0 {
+b0:
+  br label %b1
+
+b1:                                               ; preds = %b1, %b0
+  %v0 = phi i32 [ 0, %b0 ], [ %v22, %b1 ]
+  %v1 = load %s.0*, %s.0** undef, align 4
+  %v2 = getelementptr inbounds %s.0, %s.0* %v1, i32 0, i32 0
+  %v3 = load i16, i16* %v2, align 2
+  %v4 = add i16 0, %v3
+  %v5 = add i16 %v4, 0
+  %v6 = add i16 %v5, 0
+  %v7 = add i16 %v6, 0
+  %v8 = add i16 %v7, 0
+  %v9 = add i16 %v8, 0
+  %v10 = add i16 %v9, 0
+  %v11 = add i16 %v10, 0
+  %v12 = add i16 %v11, 0
+  %v13 = add i16 %v12, 0
+  %v14 = add i16 %v13, 0
+  %v15 = add i16 %v14, 0
+  %v16 = add i16 %v15, 0
+  %v17 = add i16 %v16, 0
+  %v18 = add i16 %v17, 0
+  %v19 = add i16 %v18, 0
+  %v20 = load %s.0*, %s.0** undef, align 4
+  store i16 %v19, i16* undef, align 2
+  %v21 = getelementptr inbounds %s.0, %s.0* %v20, i32 0, i32 1
+  store i16 0, i16* %v21, align 2
+  %v22 = add nuw nsw i32 %v0, 1
+  %v23 = icmp eq i32 %v22, 6
+  br i1 %v23, label %b2, label %b1
+
+b2:                                               ; preds = %b1
+  ret void
+}
+
+attributes #0 = { nounwind "target-cpu"="hexagonv60" "target-features"="+hvx-length64b,+hvxv60" }