Initial implementation of sparse conditional constant propagation.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 339b3eea6f85e2365dde8406e7c9c19ccc707740..d875a953471dfe60ffc2f598fdacb54a29759097 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -9,6 +9,7 @@
 #include "vm/flow_graph_allocator.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/flow_graph_compiler.h"
+#include "vm/flow_graph_optimizer.h"
 #include "vm/locations.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
@@ -22,6 +23,18 @@ namespace dart {
 DECLARE_FLAG(bool, enable_type_checks);
 
 
+Definition::Definition()
+    : temp_index_(-1),
+      ssa_temp_index_(-1),
+      propagated_type_(AbstractType::Handle()),
+      propagated_cid_(kIllegalCid),
+      input_use_list_(NULL),
+      env_use_list_(NULL),
+      use_kind_(kValue),  // Phis and parameters rely on this default.
+      constant_value_(Object::ZoneHandle(ConstantPropagator::Unknown())) {
+}
+
+
 intptr_t Definition::Hashcode() const {
   intptr_t result = tag();
   for (intptr_t i = 0; i < InputCount(); ++i) {
@@ -422,6 +435,54 @@ intptr_t JoinEntryInstr::IndexOfPredecessor(BlockEntryInstr* pred) const {
 }
 
 
+void JoinEntryInstr::EliminateUnreachablePhiInputs() {
+  if (phis_ == NULL || phis_->is_empty()) return;
+
+  // Loop over the predecessors, reorganize phi inputs.
+  // TODO(kmillikin): Replace phis that have a single remaining input with
+  // the input.  This requires being a bit careful about use lists.
+  intptr_t input_count = predecessors_.length();
+  for (intptr_t new_idx = 0; new_idx < input_count; ++new_idx) {
+    BlockEntryInstr* pred = predecessors_[new_idx];
+    // Linear search for the old predecessor index.  We can't directly
+    // compare block entries, because unreachable code elimination has
+    // replaced some targets with joins.
+    intptr_t old_idx = 0;
+    for (; old_idx < stale_predecessors_.length(); ++old_idx) {
+      if (stale_predecessors_[old_idx]->next() == pred->next()) break;
+    }
+    ASSERT(old_idx < stale_predecessors_.length());
+    // If the index has changed, adjust all phi inputs.
+    if (old_idx != new_idx) {
+      ASSERT(new_idx < old_idx);
+      // Swap each phi's inputs so the input at new_idx is correct.
+      // Preserve the previous value in case it is from a reachable
+      // predecessor.
+      for (intptr_t phi_idx = 0; phi_idx < phis_->length(); ++phi_idx) {
+        PhiInstr* phi = (*phis_)[phi_idx];
+        if (phi == NULL) continue;
+        Value* temp = phi->InputAt(new_idx);
+        phi->SetInputAt(new_idx, phi->InputAt(old_idx));
+        phi->SetInputAt(old_idx, temp);
+      }
+      // The old input at new_idx is now found at old_idx.  It may be a
+      // reachable predecessor so swap the old predecessors too.
+      BlockEntryInstr* temp = stale_predecessors_[new_idx];
+      stale_predecessors_[new_idx] = stale_predecessors_[old_idx];
+      stale_predecessors_[old_idx] = temp;
+    }
+  }
+  // Now truncate each phi if necessary.
+  if (input_count < stale_predecessors_.length()) {
+    for (intptr_t phi_idx = 0; phi_idx < phis_->length(); ++phi_idx) {
+      PhiInstr* phi = (*phis_)[phi_idx];
+      if (phi == NULL) continue;
+      phi->inputs_.TruncateTo(input_count);
+    }
+  }
+}
+
+
 // ==== Recording assigned variables.
 void Definition::RecordAssignedVars(BitVector* assigned_vars,
                                     intptr_t fixed_parameter_count) {
@@ -531,6 +592,17 @@ intptr_t ParameterInstr::GetPropagatedCid() {
 
 
 // ==== Postorder graph traversal.
+static bool IsMarked(BlockEntryInstr* block,
+                     GrowableArray<BlockEntryInstr*>* preorder) {
+  // Detect that a block has been visited as part of the current
+  // DiscoverBlocks (we can call DiscoverBlocks multiple times).  The block
+  // will be 'marked' by (1) having a preorder number in the range of the
+  // preorder array and (2) being in the preorder array at that index.
+  intptr_t i = block->preorder_number();
+  return (i >= 0) && (i < preorder->length()) && ((*preorder)[i] == block);
+}
+
+
 void GraphEntryInstr::DiscoverBlocks(
     BlockEntryInstr* current_block,
     GrowableArray<BlockEntryInstr*>* preorder,
@@ -540,7 +612,7 @@ void GraphEntryInstr::DiscoverBlocks(
     intptr_t variable_count,
     intptr_t fixed_parameter_count) {
   // We only visit this block once, first of all blocks.
-  ASSERT(preorder_number() == -1);
+  ASSERT(!IsMarked(this, preorder));
   ASSERT(current_block == NULL);
   ASSERT(preorder->is_empty());
   ASSERT(postorder->is_empty());
@@ -588,16 +660,23 @@ void BlockEntryInstr::DiscoverBlocks(
   // is non-null and preorder array is non-empty.
   ASSERT(current_block != NULL);
   ASSERT(!preorder->is_empty());
+  // Blocks with a single predecessor cannot have been reached before.
+  ASSERT(!IsTargetEntry() || !IsMarked(this, preorder));
+
+  // 1. If the block has already been reached, add current_block as a
+  // basic-block predecessor and we are done.
+  if (IsMarked(this, preorder)) {
+    AddPredecessor(current_block);
+    return;
+  }
 
-  // 1. Record control-flow-graph basic-block predecessors.
+  // 2. Otherwise, clear the predecessors which might have been computed on
+  // some earlier call to DiscoverBlocks and record this predecessor.  For
+  // joins save the original predecessors, if any, so we can garbage collect
+  // phi inputs from unreachable predecessors without recomputing SSA.
+  ClearPredecessors();
   AddPredecessor(current_block);
 
-  // 2. If the block has already been reached by the traversal, we are
-  // done.  Blocks with a single predecessor cannot have been reached
-  // before.
-  ASSERT(!IsTargetEntry() || (preorder_number() == -1));
-  if (preorder_number() >= 0) return;
-
   // 3. The current block is the spanning-tree parent.
   parent->Add(current_block->preorder_number());