Change the default implementation of DataEquals for Hydrogen instructions.

Change the default implementation of DataEquals for Hydrogen instructions.

The former default was true. The new default is false and the default
implementation is UNREACHABLE so it asserts in debug builds.  The function
is overridden in all concrete instruction classes that might have the flag
kUseGVN set.

Committed: http://code.google.com/p/v8/source/detail?r=6493

[Kevin Millikin (Chromium), 2011-01-26 13:49:42]

I override only in concrete classes, even if kUseGVN is set in a non-concrete
instruction base class (setting kUseGVN in such a way is error prone too, so we
should consider getting rid of it).

We could also consider when verifying instructions:

if (instr->CheckFlag(kUseGVN)) { instr->DataEquals(instr); }

to verify that we don't have the UNREACHABLE() implementation.

[fschneider, 2011-01-26 14:13:41]

LGTM.

http://codereview.chromium.org/6255013/diff/1/src/hydrogen-instructions.h
File src/hydrogen-instructions.h (right):

http://codereview.chromium.org/6255013/diff/1/src/hydrogen-instructions.h#new...
src/hydrogen-instructions.h:2905: virtual bool DataEquals(HValue* other) const {
Since stores do not participate in GVN we don't need DataEquals here.

[fschneider, 2011-01-26 15:06:08]

LGTM as well.