Experimental: three small codegen changes....

Experimental: three small codegen changes.

1. Do not let the esi register (reserved for the context) be available
   for register allocation.  This was unsafe as it stood.  We noe
   generate better code, because we do not spill it at a call (it's
   callee saved) and do not restore it after a call.

2. Move the code for making frames mergable from the common CFG edge
   leading to a branch and onto the labeled (non-fall-through) basic
   block.  This needs improvement: it should be triggered by a test
   that VirtualFrame::MakeMergable will actually generate code
   (sometimes it updates internal state and generates no code, in
   which case we now get an annoying branch to a jump).

3. Push the virtual frame farther.  It is enabled for blocks and some
   code paths through assignments.  It can now reach the entry of loop
   statements in the two loop benchmarks.

[iposva, 2008-12-08 06:32:32]

Kevin,

I need some more time to take another look at this. Ideally by Monday night.

-Ivan

http://codereview.chromium.org/13234/diff/1/4
File src/jump-target-ia32.cc (right):

http://codereview.chromium.org/13234/diff/1/4#newcode119
Line 119: __ bind(&original_fall_through);
How often do you expect this double jump to happen?

[Kevin Millikin (Chromium), 2008-12-08 08:26:28]

http://codereview.chromium.org/13234/diff/1/4
File src/jump-target-ia32.cc (right):

http://codereview.chromium.org/13234/diff/1/4#newcode119
Line 119: __ bind(&original_fall_through);
On 2008/12/08 06:32:32, iposva wrote:
> How often do you expect this double jump to happen?

Only when the frame contains constants that are synched to memory and nothing
else that's problematic---then it can become mergable by just forgetting about
the constants.

I don't expect it to be the common case, but it happens right now for the stack
check on function entry where the locals are constants (the undefined value). 
For total correctness with the debugger, the stack check should probably be
special-cased and avoid the double jump but I'd still like to avoid it in other
cases too.