[compiler] Introduce a simple store-store elimination, disabled by default.

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index c23f08f6fdaa3241e11bfb76bb1697bf859d407d..d03fd79a392452411a83283261e47d33de159c1e 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -446,6 +446,7 @@ DEFINE_BOOL(trace_turbo_scheduler, false, "trace TurboFan's scheduler")
 DEFINE_BOOL(trace_turbo_reduction, false, "trace TurboFan's various reducers")
 DEFINE_BOOL(trace_turbo_jt, false, "trace TurboFan's jump threading")
 DEFINE_BOOL(trace_turbo_ceq, false, "trace TurboFan's control equivalence")
+DEFINE_BOOL(trace_turbo_ssel, false, "trace TurboFan's store-store elimination")

[Jarin, 2016/06/21 13:28:33]

Can't you just reuse the trace_store_elimination flag?

Also, you should introduce turbo_store_elimination flag that turns off the store
elimination. This is useful if we need to turn it of because of a bug or
performance regression.

[bgeron, 2016/06/21 17:48:59]

Sounds a good idea. I feel we ought to move it from the Crankshaft section to a
more general section, but where would that be?
Done. Note that we now have both FLAG_store_elimination and
FLAG_turbo_store_elimination, but I guess it's better that way because the first
one is off by default.

[bgeron, 2016/06/22 18:00:58]

Done.

 DEFINE_BOOL(turbo_asm, true, "enable TurboFan for asm.js code")
 DEFINE_BOOL(turbo_asm_deoptimization, false,
             "enable deoptimization in TurboFan for asm.js code")