Avoid bailing out to runtime for short substrings.

Avoid bailing out to runtime for short substrings.

This significantly improves the speed for creating short substrings (less than 13 characters) from slices, flat cons strings and external strings.

TEST=string-external-cached.js, string-slices.js

Committed: http://code.google.com/p/v8/source/detail?r=10221

[Yang, 2011-12-08 14:27:14]

PTAL.

[rossberg, 2011-12-08 15:36:32]

lgtm

http://codereview.chromium.org/8889012/diff/1/src/ia32/code-stubs-ia32.cc
File src/ia32/code-stubs-ia32.cc (right):

http://codereview.chromium.org/8889012/diff/1/src/ia32/code-stubs-ia32.cc#new...
src/ia32/code-stubs-ia32.cc:6102: __ cmp(ecx, 2);
Nit: could avoid the SmiUntag here and SmiTag later and just compare to
Smi::FromInt(2) directly here, and load Smi::FromInt(2) in line 6131, and cmp to
Smi::FromInt(SlicedString::kMinLength) in line 6182.

http://codereview.chromium.org/8889012/diff/1/src/ia32/code-stubs-ia32.cc#new...
src/ia32/code-stubs-ia32.cc:6151: // Cons string.  Check whether it is flat,
then fetch first part.
Nit: Maybe add comment about the flat cons invariant.