X64: Optimize access to external references.

X64: Optimize access to external references.

Access to an ExternalReference in non-serializable code will try to use
an offset relative to the root-array register.
Since the root-array is in the Heap object, and the Heap object is in
the Isolate object, there's a good chance that any external data field
is within a 32-bit offset of the root array register.
It falls back on the original behavior if the serializer is enabled,
if the root register isn't initialized or if the offset is not representable
as a 32-bit value.

Committed: http://code.google.com/p/v8/source/detail?r=7315

[William Hesse, 2011-03-22 14:05:08]

LGTM.

Shouldn't you get rid of 
movq(Register, ExternalReference), and eliminate all uses of it, or make it
protected?  Your new functions should be able to replace all uses of it, right?

http://codereview.chromium.org/6716018/diff/1/src/x64/deoptimizer-x64.cc
File src/x64/deoptimizer-x64.cc (left):

http://codereview.chromium.org/6716018/diff/1/src/x64/deoptimizer-x64.cc#oldc...
src/x64/deoptimizer-x64.cc:760: ExternalReference roots_address =
ExternalReference::roots_address();
Could this be necessary for serialization reasons?

http://codereview.chromium.org/6716018/diff/1/src/x64/regexp-macro-assembler-...
File src/x64/regexp-macro-assembler-x64.cc (right):

http://codereview.chromium.org/6716018/diff/1/src/x64/regexp-macro-assembler-...
src/x64/regexp-macro-assembler-x64.cc:67: * kRootRegister) are not initialized
during RegExp code.
"are not initialized" is confusing.  Do you mean "free for general purpose use
during RegExp code"?

[Lasse Reichstein, 2011-03-22 14:12:21]

I can't get rid of movq(Register, ExternalReference) yet,  since I didn't change
codegen-x64.cc (soon to be dead anyway) and I probably left a few uses in
debug-only code as well.

However, I do think it's a good idea for a future patch.

http://codereview.chromium.org/6716018/diff/1/src/x64/deoptimizer-x64.cc
File src/x64/deoptimizer-x64.cc (left):

http://codereview.chromium.org/6716018/diff/1/src/x64/deoptimizer-x64.cc#oldc...
src/x64/deoptimizer-x64.cc:760: ExternalReference roots_address =
ExternalReference::roots_address();
I don't see how. It's a local variable that isn't used anywhere.
I think it's just a leftover from when InitializeRootRegister was introduced.

http://codereview.chromium.org/6716018/diff/1/src/x64/regexp-macro-assembler-...
File src/x64/regexp-macro-assembler-x64.cc (right):

http://codereview.chromium.org/6716018/diff/1/src/x64/regexp-macro-assembler-...
src/x64/regexp-macro-assembler-x64.cc:67: * kRootRegister) are not initialized
during RegExp code.
Except that they are not free either (since we don't save them and they are
callee-save).
Rewording.