Remove the redundant load on every context lookup.

Remove the redundant load on every context lookup.

There was an unnecessary load on every statically-resolved context lookup.
Remove it.

This revealed a hidden bug in const initializers inside 'with'.  They claim
to be statically resolved (having slot type CONTEXT) but they occur in a
spot where the runtime context chain and the static scope chain do not
agree.  This is fixed by special casing const initializers in the backend.

Committed: http://code.google.com/p/v8/source/detail?r=6635

[Kevin Millikin (Chromium), 2011-02-04 10:45:38]

This is only implemented on IA32.  It would be simple to port.  I left the
classic code generator unchanged.

Since we don't care about the performance of "const", it would arguably be
better to give const initializers inside "with" a slot type "LOOKUP" not
"CONTEXT".  (The issue is that "CONTEXT" claims they can be resolved statically
by following N links of the runtime context chain where N is computed from the
static scope chain.  "With" contexts are not accounted for in the scope chain,
so using it is pointless if safe.)

[fschneider, 2011-02-04 11:29:58]

LGTM.

http://codereview.chromium.org/6384020/diff/1/src/ia32/lithium-ia32.cc
File src/ia32/lithium-ia32.cc (right):

http://codereview.chromium.org/6384020/diff/1/src/ia32/lithium-ia32.cc#newcod...
src/ia32/lithium-ia32.cc:1697: value = UseRegisterAtStart(instr->value());
This is perfectly valid, but i think the AtStart will not have any benefit since
there are no temps or result registers here.

http://codereview.chromium.org/6384020/diff/1/src/ia32/macro-assembler-ia32.cc
File src/ia32/macro-assembler-ia32.cc (right):

http://codereview.chromium.org/6384020/diff/1/src/ia32/macro-assembler-ia32.c...
src/ia32/macro-assembler-ia32.cc:1537: cmp(dst, Operand(dst,
Context::SlotOffset(Context::FCONTEXT_INDEX)));
Shouldn't this be inside
if (FLAG_debug_code)?

[Kevin Millikin (Chromium), 2011-02-04 11:51:48]

http://codereview.chromium.org/6384020/diff/1/src/ia32/lithium-ia32.cc
File src/ia32/lithium-ia32.cc (right):

http://codereview.chromium.org/6384020/diff/1/src/ia32/lithium-ia32.cc#newcod...
src/ia32/lithium-ia32.cc:1697: value = UseRegisterAtStart(instr->value());
On 2011/02/04 11:29:58, fschneider wrote:
> This is perfectly valid, but i think the AtStart will not have any benefit
since
> there are no temps or result registers here.

I was trying to be consistent, but I guess UseRegister is a safer default so
I'll switch to that.

http://codereview.chromium.org/6384020/diff/1/src/ia32/macro-assembler-ia32.cc
File src/ia32/macro-assembler-ia32.cc (right):

http://codereview.chromium.org/6384020/diff/1/src/ia32/macro-assembler-ia32.c...
src/ia32/macro-assembler-ia32.cc:1537: cmp(dst, Operand(dst,
Context::SlotOffset(Context::FCONTEXT_INDEX)));
On 2011/02/04 11:29:58, fschneider wrote:
> Shouldn't this be inside
> if (FLAG_debug_code)?

Good catch.