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

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 428 matching lines @@
 DEFINE_BOOL(trace_turbo, false, "trace generated TurboFan IR")
 DEFINE_BOOL(trace_turbo_graph, false, "trace generated TurboFan graphs")
 DEFINE_IMPLICATION(trace_turbo_graph, trace_turbo)
 DEFINE_STRING(trace_turbo_cfg_file, NULL,
               "trace turbo cfg graph (for C1 visualizer) to a given file name")
 DEFINE_BOOL(trace_turbo_types, true, "trace TurboFan's types")
 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")
 DEFINE_BOOL(turbo_verify, DEBUG_BOOL, "verify TurboFan graphs at each phase")
 DEFINE_BOOL(turbo_stats, false, "print TurboFan statistics")
 DEFINE_BOOL(turbo_stats_nvp, false,
             "print TurboFan statistics in machine-readable format")
 DEFINE_BOOL(turbo_splitting, true, "split nodes during scheduling in TurboFan")
 DEFINE_BOOL(turbo_type_feedback, false,
             "use typed feedback for representation inference in Turbofan")
@@ skipping 732 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA