Fix PR13969, a mini-phase-ordering issue with the new SROA pass.

Currently, we re-visit allocas when something changes about the way they
might be *split* to allow better scalarization to take place. However,
we weren't handling the case when the *promotion* is what would change
the behavior of SROA. When an address derived from an alloca is stored
into another alloca, we consider the first to have escaped. If the
second is ever promoted to an SSA value, we will suddenly be able to run
the SROA pass on the first alloca.

This patch adds explicit support for this form if iteration. When we
detect a store of a pointer derived from an alloca, we flag the
underlying alloca for reprocessing after promotion. The logic works hard
to only do this when there is definitely going to be promotion and it
might remove impediments to the analysis of the alloca.

Thanks to Nick for the great test case and Benjamin for some sanity
check review.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165223 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/SROA.cpp

@@ -1316,6 +1316,16 @@ class SROA : public FunctionPass {
   /// uses as dead. Only used to guard insertion into DeadInsts.
   SmallPtrSet<Instruction *, 4> DeadSplitInsts;
 
+  /// \brief Post-promotion worklist.
+  ///
+  /// Sometimes we discover an alloca which has a high probability of becoming
+  /// viable for SROA after a round of promotion takes place. In those cases,
+  /// the alloca is enqueued here for re-processing.
+  ///
+  /// Note that we have to be very careful to clear allocas out of this list in
+  /// the event they are deleted.
+  SetVector<AllocaInst *, SmallVector<AllocaInst *, 16> > PostPromotionWorklist;
+
   /// \brief A collection of alloca instructions we can directly promote.
   std::vector<AllocaInst *> PromotableAllocas;
 
@@ -2379,6 +2389,13 @@ private:
     if (IntPromotionTy)
       return rewriteIntegerStore(IRB, SI);
 
+    // Strip all inbounds GEPs and pointer casts to try to dig out any root
+    // alloca that should be re-examined after promoting this alloca.
+    if (SI.getValueOperand()->getType()->isPointerTy())
+      if (AllocaInst *AI = dyn_cast<AllocaInst>(SI.getValueOperand()
+                                                  ->stripInBoundsOffsets()))
+        Pass.PostPromotionWorklist.insert(AI);
+
     Value *NewPtr = getAdjustedAllocaPtr(IRB,
                                          SI.getPointerOperand()->getType());
     SI.setOperand(1, NewPtr);
@@ -3119,11 +3136,16 @@ bool SROA::rewriteAllocaPartition(AllocaInst &AI,
                << "[" << PI->BeginOffset << "," << PI->EndOffset << ") to: "
                << *NewAI << "\n");
 
+  // Track the high watermark of the post-promotion worklist. We will reset it
+  // to this point if the alloca is not in fact scheduled for promotion.
+  unsigned PPWOldSize = PostPromotionWorklist.size();
+
   AllocaPartitionRewriter Rewriter(*TD, P, PI, *this, AI, *NewAI,
                                    PI->BeginOffset, PI->EndOffset);
   DEBUG(dbgs() << "  rewriting ");
   DEBUG(P.print(dbgs(), PI, ""));
-  if (Rewriter.visitUsers(P.use_begin(PI), P.use_end(PI))) {
+  bool Promotable = Rewriter.visitUsers(P.use_begin(PI), P.use_end(PI));
+  if (Promotable) {
     DEBUG(dbgs() << "  and queuing for promotion\n");
     PromotableAllocas.push_back(NewAI);
   } else if (NewAI != &AI) {
@@ -3132,6 +3154,12 @@ bool SROA::rewriteAllocaPartition(AllocaInst &AI,
     // alloca which didn't actually change and didn't get promoted.
     Worklist.insert(NewAI);
   }
+
+  // Drop any post-promotion work items if promotion didn't happen.
+  if (!Promotable)
+    while (PostPromotionWorklist.size() > PPWOldSize)
+      PostPromotionWorklist.pop_back();
+
   return true;
 }
 
@@ -3165,13 +3193,6 @@ bool SROA::runOnAlloca(AllocaInst &AI) {
       TD->getTypeAllocSize(AI.getAllocatedType()) == 0)
     return false;
 
-  // First check if this is a non-aggregate type that we should simply promote.
-  if (!AI.getAllocatedType()->isAggregateType() && isAllocaPromotable(&AI)) {
-    DEBUG(dbgs() << "  Trivially scalar type, queuing for promotion...\n");
-    PromotableAllocas.push_back(&AI);
-    return false;
-  }
-
   bool Changed = false;
 
   // First, split any FCA loads and stores touching this alloca to promote
@@ -3339,23 +3360,29 @@ bool SROA::runOnFunction(Function &F) {
   // the list of promotable allocas.
   SmallPtrSet<AllocaInst *, 4> DeletedAllocas;
 
-  while (!Worklist.empty()) {
-    Changed |= runOnAlloca(*Worklist.pop_back_val());
-    deleteDeadInstructions(DeletedAllocas);
+  do {
+    while (!Worklist.empty()) {
+      Changed |= runOnAlloca(*Worklist.pop_back_val());
+      deleteDeadInstructions(DeletedAllocas);
 
-    // Remove the deleted allocas from various lists so that we don't try to
-    // continue processing them.
-    if (!DeletedAllocas.empty()) {
-      Worklist.remove_if(IsAllocaInSet(DeletedAllocas));
-      PromotableAllocas.erase(std::remove_if(PromotableAllocas.begin(),
-                                             PromotableAllocas.end(),
-                                             IsAllocaInSet(DeletedAllocas)),
-                              PromotableAllocas.end());
-      DeletedAllocas.clear();
+      // Remove the deleted allocas from various lists so that we don't try to
+      // continue processing them.
+      if (!DeletedAllocas.empty()) {
+        Worklist.remove_if(IsAllocaInSet(DeletedAllocas));
+        PostPromotionWorklist.remove_if(IsAllocaInSet(DeletedAllocas));
+        PromotableAllocas.erase(std::remove_if(PromotableAllocas.begin(),
+                                               PromotableAllocas.end(),
+                                               IsAllocaInSet(DeletedAllocas)),
+                                PromotableAllocas.end());
+        DeletedAllocas.clear();
+      }
     }
-  }
 
-  Changed |= promoteAllocas(F);
+    Changed |= promoteAllocas(F);
+
+    Worklist = PostPromotionWorklist;
+    PostPromotionWorklist.clear();
+  } while (!Worklist.empty());
 
   return Changed;
 }

test/Transforms/SROA/basictest.ll

@@ -926,3 +926,27 @@ bb3:
 bb4:
   unreachable
 }
+
+define double @PR13969(double %x) {
+; Check that we detect when promotion will un-escape an alloca and iterate to
+; re-try running SROA over that alloca. Without that, the two allocas that are
+; stored into a dead alloca don't get rewritten and promoted.
+; CHECK: @PR13969
+
+entry:
+  %a = alloca double
+  %b = alloca double*
+  %c = alloca double
+; CHECK-NOT: alloca
+
+  store double %x, double* %a
+  store double* %c, double** %b
+  store double* %a, double** %b
+  store double %x, double* %c
+  %ret = load double* %a
+; CHECK-NOT: store
+; CHECK-NOT: load
+
+  ret double %ret
+; CHECK: ret double %x
+}