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

src/ia32/virtual-frame-ia32.h

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
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@@ skipping 97 matching lines @@
   // Spill all values from the frame to memory.
   void SpillAll();
 
   // Spill all occurrences of a specific register from the frame.
   void Spill(Register reg);
 
   // Spill all occurrences of an arbitrary register if possible.  Return the
   // register spilled or no_reg if it was not possible to free any register
   // (ie, they all have frame-external references).
   Register SpillAnyRegister();
 

[William Hesse, 2009/05/15 07:45:51]

How about:
// Remove copies and constants from the top mergeable_elements of the frame,
// so that a frame of the same height that differs only on
these elements can be merged to it.
// Use JumpTarget::kAllElements if the entire frame should be made mergeable.

[Kevin Millikin (Chromium), 2009/05/15 08:03:35]

Seems misleading.  The arbitrary frame could differ anywhere (could have memory
where this one has register, or they could have different registers).  I'd
rather stick to the accurate wording even if it's more verbose.

I'll change the "not allowed" so it's clear it's a postcondition and not a
precondition.

+  // 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 not allowed anywhere in the 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.
   void PrepareMergeTo(VirtualFrame* expected);
 
   // Make this virtual frame have a state identical to an expected virtual
   // frame.  As a side effect, code may be emitted to make this frame match
   // the expected one.
   void MergeTo(VirtualFrame* expected);
 
@@ skipping 355 matching lines @@
   Result RawCallCodeObject(Handle<Code> code, RelocInfo::Mode rmode);
 
   bool Equals(VirtualFrame* other);
 
   friend class JumpTarget;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_IA32_VIRTUAL_FRAME_IA32_H_