[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.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328547 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 23 insertions, 14 deletions

diff --git a/lib/CodeGen/MachinePipeliner.cpp b/lib/CodeGen/MachinePipeliner.cpp
index a3c600300b4..8015eda1605 100644
--- a/lib/CodeGen/MachinePipeliner.cpp
+++ b/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);
@@ -1087,20 +1100,16 @@ 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 (BaseReg1 == BaseReg2 && (int)Offset1 < (int)Offset2) {
-            assert(TII->areMemAccessesTriviallyDisjoint(LdMI, MI, AA) &&
-                   "What happened to the chain edge?");
-            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?");
+              SDep Dep(Load, SDep::Barrier);
+              Dep.setLatency(1);
+              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