Simplify StringCharCodeAt in non-crankshaft codegen.

Simplify StringCharCodeAt in non-crankshaft codegen.

TEST=test/mjsunit/string-slices.js

Committed: http://code.google.com/p/v8/source/detail?r=9936

[Yang, 2011-11-09 10:14:58]

The existing implementation of StringCharCodeAt examines the string type. In the
case of cons or slice the underlying string is loaded and re-examined. In the
case of external string or unflattened cons, we jump to the runtime, where we
start out with the original string, and the unpacking of cons or slice has to be
redone in runtime. This of course also means that the original arguments (string
object and index) have to be kept for the case of runtime call.

However, we can also pass the already unpacked underlying external string to the
runtime directly, together with the adjusted index (if the original string was a
slice, by adding the slice-offset). This saves us from the trouble of saving the
original arguments, and also removes the need for a scratch register.

The crankshaft implementation already does this, even though unintentially up
till now.

Please take a look.

[Lasse Reichstein, 2011-11-09 12:48:11]

LGTM

http://codereview.chromium.org/8510005/diff/1/src/ia32/code-stubs-ia32.cc
File src/ia32/code-stubs-ia32.cc (right):

http://codereview.chromium.org/8510005/diff/1/src/ia32/code-stubs-ia32.cc#new...
src/ia32/code-stubs-ia32.cc:5110: // Put smi-tagged index into scratch register.
Remove the comment too.

http://codereview.chromium.org/8510005/diff/1/src/x64/code-stubs-x64.cc
File src/x64/code-stubs-x64.cc (right):

http://codereview.chromium.org/8510005/diff/1/src/x64/code-stubs-x64.cc#newco...
src/x64/code-stubs-x64.cc:4125: STATIC_ASSERT((kStringEncodingMask &
kTwoByteStringTag) == 0);
Could we convert index from smi to int32 here, instead of doing it in two places
below?

http://codereview.chromium.org/8510005/diff/1/test/mjsunit/string-slices.js
File test/mjsunit/string-slices.js (right):

http://codereview.chromium.org/8510005/diff/1/test/mjsunit/string-slices.js#n...
test/mjsunit/string-slices.js:28: // Flags: --string-slices
--expose-externalize-string --allow-natives-syntax
If string-slices is on by default now, we should remove the flag here.