Bypass an expensive computation of a basic block's entry frame for a

common case of a single forward edge (a fall-through).  This includes
exits from deferred code.

The fall-through frame is used, after removing constants and copies
above the high-water mark.
Review URL: http://codereview.chromium.org/113400

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1960 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ia32/jump-target-ia32.cc

@@ -195,12 +195,12 @@ void JumpTarget::DoBind(int mergable_elements) {
     return;
   }
 
-  if (direction_ == FORWARD_ONLY) {
-    // A simple case: no forward jumps and no possible backward jumps.
   if (!is_linked()) {
+    ASSERT(cgen_->has_valid_frame());
+    if (direction_ == FORWARD_ONLY) {
+      // Fast case: no forward jumps and no possible backward jumps.
       // The stack pointer can be floating above the top of the
       // virtual frame before the bind.  Afterward, it should not.
-      ASSERT(cgen_->has_valid_frame());
       VirtualFrame* frame = cgen_->frame();
       int difference =
           frame->stack_pointer_ - (frame->elements_.length() - 1);
@@ -209,15 +209,25 @@ void JumpTarget::DoBind(int mergable_elements) {
         __ add(Operand(esp), Immediate(difference * kPointerSize));
       }
 
+    } else {
+      ASSERT(direction_ == BIDIRECTIONAL);
+      // Fast case: no forward jumps, possible backward ones.  Remove
+      // constants and copies above the watermark on the fall-through
+      // frame and use it as the entry frame.
+      cgen_->frame()->MakeMergable(mergable_elements);
+      entry_frame_ = new VirtualFrame(cgen_->frame());
+      __ bind(&entry_label_);
+    }
     is_bound_ = true;
     return;
   }
 
-    // Another simple case: no fall through, a single forward jump,
-    // and no possible backward jumps.
-    if (!cgen_->has_valid_frame() && reaching_frames_.length() == 1) {
-      // Pick up the only reaching frame, take ownership of it, and
-      // use it for the block about to be emitted.
+  if (direction_ == FORWARD_ONLY &&
+      !cgen_->has_valid_frame() &&
+      reaching_frames_.length() == 1) {
+    // Fast case: no fall-through, a single forward jump, and no
+    // possible backward jumps.  Pick up the only reaching frame, take
+    // ownership of it, and use it for the block about to be emitted.
     VirtualFrame* frame = reaching_frames_[0];
     RegisterFile reserved = RegisterAllocator::Reserved();
     cgen_->SetFrame(frame, &reserved);
@@ -237,7 +247,6 @@ void JumpTarget::DoBind(int mergable_elements) {
     is_bound_ = true;
     return;
   }
-  }
 
   // If there is a current frame, record it as the fall-through.  It
   // is owned by the reaching frames for now.
@@ -250,9 +259,7 @@ void JumpTarget::DoBind(int mergable_elements) {
   }
 
   // Compute the frame to use for entry to the block.
-  if (entry_frame_ == NULL) {
   ComputeEntryFrame(mergable_elements);
-  }
 
   // Some moves required to merge to an expected frame require purely
   // frame state changes, and do not require any code generation.

src/ia32/virtual-frame-ia32.cc

@@ -180,6 +180,80 @@ void VirtualFrame::SyncRange(int begin, int end) {
 }
 
 
+void VirtualFrame::MakeMergable(int mergable_elements) {
+  if (mergable_elements == JumpTarget::kAllElements) {
+    mergable_elements = elements_.length();
+  }
+  ASSERT(mergable_elements <= elements_.length());
+
+  int start_index = elements_.length() - mergable_elements;
+
+  // The is_copied flags on entry frame elements are expected to be
+  // exact.  Set them for the elements below the water mark.
+  for (int i = 0; i < start_index; i++) {
+    elements_[i].clear_copied();
+    if (elements_[i].is_copy()) {
+      elements_[elements_[i].index()].set_copied();
+    }
+  }
+
+  for (int i = start_index; i < elements_.length(); i++) {
+    FrameElement element = elements_[i];
+
+    if (element.is_constant() || element.is_copy()) {
+      if (element.is_synced()) {
+        // Just spill.
+        elements_[i] = FrameElement::MemoryElement();
+      } else {
+        // Allocate to a register.
+        FrameElement backing_element;  // Invalid if not a copy.
+        if (element.is_copy()) {
+          backing_element = elements_[element.index()];
+        }
+        Result fresh = cgen_->allocator()->Allocate();
+        ASSERT(fresh.is_valid());
+        elements_[i] =
+            FrameElement::RegisterElement(fresh.reg(),
+                                          FrameElement::NOT_SYNCED);
+        Use(fresh.reg(), i);
+
+        // Emit a move.
+        if (element.is_constant()) {
+          if (cgen_->IsUnsafeSmi(element.handle())) {
+            cgen_->LoadUnsafeSmi(fresh.reg(), element.handle());
+          } else {
+            __ Set(fresh.reg(), Immediate(element.handle()));
+          }
+        } else {
+          ASSERT(element.is_copy());
+          // Copies are only backed by register or memory locations.
+          if (backing_element.is_register()) {
+            // The backing store may have been spilled by allocating,
+            // but that's OK.  If it was, the value is right where we
+            // want it.
+            if (!fresh.reg().is(backing_element.reg())) {
+              __ mov(fresh.reg(), backing_element.reg());
+            }
+          } else {
+            ASSERT(backing_element.is_memory());
+            __ mov(fresh.reg(), Operand(ebp, fp_relative(element.index())));
+          }
+        }
+      }
+      // No need to set the copied flag---there are no copies of
+      // copies or constants so the original was not copied.
+      elements_[i].set_static_type(element.static_type());
+    } else {
+      // Clear the copy flag of non-constant, non-copy elements above
+      // the high water mark.  They cannot be copied because copes are
+      // always higher than their backing store and copies are not
+      // allowed above the water mark.
+      elements_[i].clear_copied();
+    }
+  }
+}
+
+
 void VirtualFrame::MergeTo(VirtualFrame* expected) {
   Comment cmnt(masm_, "[ Merge frame");
   // We should always be merging the code generator's current frame to an

src/ia32/virtual-frame-ia32.h

@@ -116,6 +116,13 @@ class VirtualFrame : public ZoneObject {
   // (ie, they all have frame-external references).
   Register SpillAnyRegister();
 
+  // Make this frame so that an arbitrary frame of the same height can
+  // be merged to it.  Copies and constants are removed from the
+  // topmost mergable_elements elements of the frame.  A
+  // mergable_elements of JumpTarget::kAllElements indicates constants
+  // and copies are should be removed from the entire frame.
+  void MakeMergable(int mergable_elements);
+
   // Prepare this virtual frame for merging to an expected frame by
   // performing some state changes that do not require generating
   // code.  It is guaranteed that no code will be generated.