Split PrunedLiveness::findBoundariesInBlock into fast paths

This allows dynamic dispatch into the fast paths. It's also easier to
understand the fast paths when written as separate loops.

This way, we can used MultiDefPrunedLiveness to compute ownership
liveness, but still benefit from the fact that the common case (owned
non-phi value) is much simpler and faster to compute.

This approach is necessary because we use stack-based data structures
within the liveness result, so we need to statically instantiate
something that can handle all cases.

Author: atrick

lib/SIL/Utils/PrunedLiveness.cpp

@@ -442,32 +442,66 @@ template class PrunedLiveRange<MultiDefPrunedLiveness>;
 //                             SSAPrunedLiveness
 //===----------------------------------------------------------------------===//
 
-void SSAPrunedLiveness::findBoundariesInBlock(
-    SILBasicBlock *block, bool isLiveOut,
-    PrunedLivenessBoundary &boundary) const {
-  assert(isInitialized());
-
-  // For SSA, a live-out block cannot have a boundary.
-  if (isLiveOut)
-    return;
-
-  bool isDefBlockState = isDefBlock(block);
+/// Given live-within (non-live-out) \p block, find the last user.
+void findBoundaryInNonDefBlock(SILBasicBlock *block,
+                               PrunedLivenessBoundary &boundary,
+                               const PrunedLiveness &liveness) {
   for (SILInstruction &inst : llvm::reverse(*block)) {
-    if (isDefBlockState && isDef(&inst)) {
+    if (liveness.isInterestingUser(&inst)) {
+      boundary.lastUsers.push_back(&inst);
+      return;
+    }
+  }
+  llvm_unreachable("live-within block must contain an interesting use");
+}
+
+/// Given a live-within \p block that contains an SSA definition, and knowledge
+/// that all live uses are dominated by that single definition, find either the
+/// last user or a dead def.
+///
+/// A live range with a single definition cannot have any uses above that
+/// definition in the same block. This even holds for unreachable self-loops.
+void findBoundaryInSSADefBlock(SILNode *ssaDef,
+                               PrunedLivenessBoundary &boundary,
+                               const PrunedLiveness &liveness) {
+  // defInst is null for argument defs.
+  SILInstruction *defInst = dyn_cast<SILInstruction>(ssaDef);
+  for (SILInstruction &inst : llvm::reverse(*ssaDef->getParentBlock())) {
+    if (&inst == defInst) {
       boundary.deadDefs.push_back(cast<SILNode>(&inst));
       return;
     }
-    if (isInterestingUser(&inst)) {
+    if (liveness.isInterestingUser(&inst)) {
       boundary.lastUsers.push_back(&inst);
       return;
     }
   }
-  auto *deadArg = dyn_cast<SILArgument>(def);
-  assert(deadArg && deadArg->getParent() == block
+  auto *deadArg = dyn_cast<SILArgument>(ssaDef);
+  assert(deadArg
          && "findBoundariesInBlock must be called on a live block");
   boundary.deadDefs.push_back(deadArg);
 }
 
+void SSAPrunedLiveness::findBoundariesInBlock(
+    SILBasicBlock *block, bool isLiveOut,
+    PrunedLivenessBoundary &boundary) const {
+  assert(isInitialized());
+
+  // For SSA, a live-out block cannot have a boundary.
+  if (isLiveOut)
+    return;
+
+  // Handle live-within block
+  if (!isDefBlock(block)) {
+    findBoundaryInNonDefBlock(block, boundary, *this);
+    return;
+  }
+  // Find either the last user or a dead def
+  auto *defInst = def->getDefiningInstruction();
+  SILNode *defNode = defInst ? cast<SILNode>(defInst) : cast<SILArgument>(def);
+  findBoundaryInSSADefBlock(defNode, boundary, *this);
+}
+
 //===----------------------------------------------------------------------===//
 //                           MultiDefPrunedLiveness
 //===----------------------------------------------------------------------===//
@@ -476,25 +510,45 @@ void MultiDefPrunedLiveness::findBoundariesInBlock(
     SILBasicBlock *block, bool isLiveOut,
     PrunedLivenessBoundary &boundary) const {
   assert(isInitialized());
-  unsigned prevCount = boundary.deadDefs.size() + boundary.lastUsers.size();
 
+  if (!isDefBlock(block)) {
+    // A live-out block with no defs cannot have a boundary.
+    if (!isLiveOut) {
+      findBoundaryInNonDefBlock(block, boundary, *this);
+    }
+    return;
+  }
+  // Handle def blocks...
+  //
+  // First, check for an SSA live range
+  if (++defs.begin() == defs.end()) {
+    // For SSA, a live-out block cannot have a boundary.
+    if (!isLiveOut) {
+      findBoundaryInSSADefBlock(*defs.begin(), boundary, *this);
+    }
+    return;
+  }
+  // Handle a live-out or live-within block with potentially multiple defs
+  unsigned prevCount = boundary.deadDefs.size() + boundary.lastUsers.size();
   bool isLive = isLiveOut;
-  bool isDefBlockState = isDefBlock(block);
   for (auto &inst : llvm::reverse(*block)) {
     // Check if the instruction is a def before checking whether it is a
     // use. The same instruction can be both a dead def and boundary use.
-    if (isDefBlockState && isDef(&inst)) {
+    if (isDef(&inst)) {
       if (!isLive) {
         boundary.deadDefs.push_back(cast<SILNode>(&inst));
       }
       isLive = false;
     }
+    // Note: the same instruction could potentially be both a dead def and last
+    // user. The liveness boundary supports this, although it won't happen in
+    // any context where we care about inserting code on the boundary.
     if (!isLive && isInterestingUser(&inst)) {
       boundary.lastUsers.push_back(&inst);
       isLive = true;
     }
   }
-  if (!isLive && isDefBlockState) {
+  if (!isLive) {
     for (SILArgument *deadArg : block->getArguments()) {
       if (defs.contains(deadArg)) {
         boundary.deadDefs.push_back(deadArg);