Simplify JumpTarget::ComputeEntryFrame. Eliminate a separate pass

over the frame elements to find registers used (before allocating
undetermined elements) and another pass to set the frame's register
indices (after allocating all elements).
Review URL: http://codereview.chromium.org/114018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1906 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/arm/register-allocator-arm.cc

@@ -66,6 +66,14 @@ void RegisterAllocator::UnuseReserved(RegisterFile* register_file) {
 }
 
 
+bool RegisterAllocator::IsReserved(int reg_code) {
+  return (reg_code == sp.code())
+      || (reg_code == fp.code())
+      || (reg_code == cp.code())
+      || (reg_code == pc.code());
+}
+
+
 void RegisterAllocator::Initialize() {
   Reset();
   // The following registers are live on function entry, saved in the

src/ia32/register-allocator-ia32.cc

@@ -97,6 +97,12 @@ void RegisterAllocator::UnuseReserved(RegisterFile* register_file) {
 }
 
 
+bool RegisterAllocator::IsReserved(int reg_code) {
+  // Test below relies on the order of register codes.
+  return reg_code >= esp.code() && reg_code <= esi.code();
+}
+
+
 void RegisterAllocator::Initialize() {
   Reset();
   // The following register is live on function entry, saved in the

src/jump-target.cc

@@ -189,28 +189,37 @@ void JumpTarget::ComputeEntryFrame(int mergable_elements) {
     }
   }
 
-  // Compute the registers already reserved by values in the frame.
-  // Count the reserved registers to avoid using them.
-  RegisterFile frame_registers = RegisterAllocator::Reserved();
-  for (int i = 0; i < length; i++) {
-    FrameElement* element = elements[i];
-    if (element != NULL && element->is_register()) {
-      frame_registers.Use(element->reg());
+  // Build the new frame.  A freshly allocated frame has memory elements
+  // for the parameters and some platform-dependent elements (e.g.,
+  // return address).  Replace those first.
+  entry_frame_ = new VirtualFrame(cgen_);
+  int index = 0;
+  for (; index < entry_frame_->elements_.length(); index++) {
+    // If the element is determined, set it now and count registers.
+    // Undetermined elements are initially recorded as if in memory.
+    if (elements[index] != NULL) {
+      entry_frame_->elements_[index] = *elements[index];
+      if (elements[index]->is_register()) {
+        entry_frame_->register_locations_[elements[index]->reg().code()] =
+            index;
       }
     }
-
-  // Build the new frame.  The frame already has memory elements for
-  // the parameters (including the receiver) and the return address.
-  // We will fill it up with memory elements.
-  entry_frame_ = new VirtualFrame(cgen_);
-  while (entry_frame_->elements_.length() < length) {
+  }
+  // Then fill in the rest of the frame with new elements.
+  for (; index < length; index++) {
+    if (elements[index] == NULL) {
       entry_frame_->elements_.Add(FrameElement::MemoryElement());
+    } else {
+      entry_frame_->elements_.Add(*elements[index]);
+      if (elements[index]->is_register()) {
+        entry_frame_->register_locations_[elements[index]->reg().code()] =
+            index;
+      }
+    }
   }
 
-
-  // Copy the already-determined frame elements to the entry frame,
-  // and allocate any still-undetermined frame elements to registers
-  // or memory, from the top down.
+  // Allocate any still-undetermined frame elements to registers or
+  // memory, from the top down.
   for (int i = length - 1; i >= 0; i--) {
     if (elements[i] == NULL) {
       // If the value is synced on all frames, put it in memory.  This
@@ -234,7 +243,7 @@ void JumpTarget::ComputeEntryFrame(int mergable_elements) {
       for (int j = 0; j < reaching_frames_.length(); j++) {
         FrameElement element = reaching_frames_[j]->elements_[i];
         if (element.is_register() &&
-            !frame_registers.is_used(element.reg())) {
+            !entry_frame_->is_used(element.reg())) {
           candidate_registers.Use(element.reg());
           if (candidate_registers.count(element.reg()) > max_count) {
             max_count = candidate_registers.count(element.reg());
@@ -245,7 +254,7 @@ void JumpTarget::ComputeEntryFrame(int mergable_elements) {
       // If there was no preferred choice consider any free register.
       if (best_reg_code == no_reg.code_) {
         for (int j = 0; j < kNumRegisters; j++) {
-          if (!frame_registers.is_used(j)) {
+          if (!entry_frame_->is_used(j) && !RegisterAllocator::IsReserved(j)) {
             best_reg_code = j;
             break;
           }
@@ -256,27 +265,21 @@ void JumpTarget::ComputeEntryFrame(int mergable_elements) {
       // (the element is already recorded as in memory)
       if (best_reg_code != no_reg.code_) {
         Register reg = { best_reg_code };
-        frame_registers.Use(reg);
         entry_frame_->elements_[i] =
             FrameElement::RegisterElement(reg,
                                           FrameElement::NOT_SYNCED);
+        entry_frame_->register_locations_[best_reg_code] = i;
       }
-    } else {
-      // The element is already determined.
-      entry_frame_->elements_[i] = *elements[i];
     }
   }
 
   // Set the copied flags in the frame to be exact.  This assumes that
   // the backing store of copies is always lower in the frame.
-  // Set the register locations to their index in the frame.
   for (int i = 0; i < length; i++) {
     FrameElement* current = &entry_frame_->elements_[i];
     current->clear_copied();
     if (current->is_copy()) {
       entry_frame_->elements_[current->index()].set_copied();
-    } else if (current->is_register()) {
-      entry_frame_->register_locations_[current->reg().code()] = i;
     }
 
     if (direction_ == BIDIRECTIONAL && i >= high_water_mark) {

src/register-allocator.h

@@ -275,6 +275,11 @@ class RegisterAllocator BASE_EMBEDDED {
   // Unuse all the reserved registers in a register file.
   static void UnuseReserved(RegisterFile* register_file);
 
+  // True if the register is reserved by the code generator, false if it
+  // can be freely used by the allocator.
+  static bool IsReserved(int reg_code);
+  static bool IsReserved(Register reg) { return IsReserved(reg); }
+
   // Predicates and accessors for the registers' reference counts.
   bool is_used(int reg_code) const { return registers_.is_used(reg_code); }
   bool is_used(Register reg) const { return registers_.is_used(reg.code()); }