[Assumptions] Make collecting ephemeral values not quadratic in the

number of assume intrinsics.

The classical way to have a cache-friendly vector style container when
we need queue semantics for BFS instead of stack semantics for DFS is to
use an ever-growing vector and an index. Erasing from the front requires
O(size) work, and unless we expect the worklist to grow *very* large,
its probably cheaper to just grow and race down the list.

But that makes it more bad that we're putting the assume intrinsics in
this at all. We end up looking at the (by definition empty) use list to
see if they're ephemeral (when we've already put them in that set), etc.

Instead, directly populate the worklist with the operands when we mark
the assume intrinsics as ephemeral. Also, test the visited set *before*
putting things into the worklist so we don't accumulate the same value
in the list 100s of times.

It would be nice to use a set-vector for this but I think its useful to
test the set earlier to avoid repeatedly querying whether the same
instruction is safe to speculate.

Hopefully with these changes the number of values pushed onto the
worklist is smaller, and we avoid quadratic work by letting it grow as
necessary.

Differential Revision: https://reviews.llvm.org/D23396

llvm-svn: 279099

llvm/lib/Analysis/CodeMetrics.cpp

@@ -27,22 +27,35 @@
 
 using namespace llvm;
 
-static void completeEphemeralValues(SmallVector<const Value *, 16> &WorkSet,
-                                    SmallPtrSetImpl<const Value*> &EphValues) {
-  SmallPtrSet<const Value *, 32> Visited;
-
-  // Make sure that all of the items in WorkSet are in our EphValues set.
-  EphValues.insert(WorkSet.begin(), WorkSet.end());
+static void
+appendSpeculatableOperands(const Value *V,
+                           SmallPtrSetImpl<const Value *> &Visited,
+                           SmallVectorImpl<const Value *> &Worklist) {
+  const User *U = dyn_cast<User>(V);
+  if (!U)
+    return;
+
+  for (const Value *Operand : U->operands())
+    if (Visited.insert(Operand).second)
+      if (isSafeToSpeculativelyExecute(Operand))
+        Worklist.push_back(Operand);
+}
 
+static void completeEphemeralValues(SmallPtrSetImpl<const Value *> &Visited,
+                                    SmallVectorImpl<const Value *> &Worklist,
+                                    SmallPtrSetImpl<const Value *> &EphValues) {
   // Note: We don't speculate PHIs here, so we'll miss instruction chains kept
   // alive only by ephemeral values.
 
-  while (!WorkSet.empty()) {
-    const Value *V = WorkSet.front();
-    WorkSet.erase(WorkSet.begin());
+  // Walk the worklist using an index but without caching the size so we can
+  // append more entries as we process the worklist. This forms a queue without
+  // quadratic behavior by just leaving processed nodes at the head of the
+  // worklist forever.
+  for (int i = 0; i < (int)Worklist.size(); ++i) {
+    const Value *V = Worklist[i];
 
-    if (!Visited.insert(V).second)
-      continue;
+    assert(Visited.count(V) &&
+           "Failed to add a worklist entry to our visited set!");
 
     // If all uses of this value are ephemeral, then so is this value.
     if (!all_of(V->users(), [&](const User *U) { return EphValues.count(U); }))
@@ -51,52 +64,54 @@ static void completeEphemeralValues(SmallVector<const Value *, 16> &WorkSet,
     EphValues.insert(V);
     DEBUG(dbgs() << "Ephemeral Value: " << *V << "\n");
 
-    if (const User *U = dyn_cast<User>(V))
-      for (const Value *J : U->operands()) {
-        if (isSafeToSpeculativelyExecute(J))
-          WorkSet.push_back(J);
-      }
+    // Append any more operands to consider.
+    appendSpeculatableOperands(V, Visited, Worklist);
   }
 }
 
 // Find all ephemeral values.
 void CodeMetrics::collectEphemeralValues(
     const Loop *L, AssumptionCache *AC,
     SmallPtrSetImpl<const Value *> &EphValues) {
-  SmallVector<const Value *, 16> WorkSet;
+  SmallPtrSet<const Value *, 32> Visited;
+  SmallVector<const Value *, 16> Worklist;
 
   for (auto &AssumeVH : AC->assumptions()) {
     if (!AssumeVH)
       continue;
     Instruction *I = cast<Instruction>(AssumeVH);
 
-    // Filter out call sites outside of the loop so we don't to a function's
+    // Filter out call sites outside of the loop so we don't do a function's
     // worth of work for each of its loops (and, in the common case, ephemeral
     // values in the loop are likely due to @llvm.assume calls in the loop).
     if (!L->contains(I->getParent()))
       continue;
 
-    WorkSet.push_back(I);
+    if (EphValues.insert(I).second)
+      appendSpeculatableOperands(I, Visited, Worklist);
   }
 
-  completeEphemeralValues(WorkSet, EphValues);
+  completeEphemeralValues(Visited, Worklist, EphValues);
 }
 
 void CodeMetrics::collectEphemeralValues(
     const Function *F, AssumptionCache *AC,
     SmallPtrSetImpl<const Value *> &EphValues) {
-  SmallVector<const Value *, 16> WorkSet;
+  SmallPtrSet<const Value *, 32> Visited;
+  SmallVector<const Value *, 16> Worklist;
 
   for (auto &AssumeVH : AC->assumptions()) {
     if (!AssumeVH)
       continue;
     Instruction *I = cast<Instruction>(AssumeVH);
     assert(I->getParent()->getParent() == F &&
            "Found assumption for the wrong function!");
-    WorkSet.push_back(I);
+
+    if (EphValues.insert(I).second)
+      appendSpeculatableOperands(I, Visited, Worklist);
   }
 
-  completeEphemeralValues(WorkSet, EphValues);
+  completeEphemeralValues(Visited, Worklist, EphValues);
 }
 
 /// Fill in the current structure with information gleaned from the specified