Don't optimize away static field initialization when precompiling.

runtime/vm/intermediate_language.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/constant_propagator.h"
 #include "vm/cpu.h"
 #include "vm/dart_entry.h"
@@ skipping 19 matching lines @@
 
 DEFINE_FLAG(bool, ic_range_profiling, true,
     "Generate special IC stubs collecting range information "
     "for binary and unary arithmetic operations");
 DEFINE_FLAG(bool, propagate_ic_data, true,
     "Propagate IC data from unoptimized to optimized IC calls.");
 DEFINE_FLAG(bool, two_args_smi_icd, true,
     "Generate special IC stubs for two args Smi operations");
 DEFINE_FLAG(bool, unbox_numeric_fields, true,
     "Support unboxed double and float32x4 fields.");
+DEFINE_FLAG(bool, fields_may_be_reset, false,
+            "Don't optimize away static field initialization");

[hausner, 2015/09/10 22:55:12]

The flag name and the description are quite different. It would be nice to have
a comment explaining what is going on. (Why can't the initializers be optimized
away?)

OTOH, do we really need a separate flag for this? I'm not familiar with all the
different flags we have that affect the VM when pre-compilation is used, but
maybe this can be combined with one of the other flags.

[rmacnak, 2015/09/11 00:07:13]

Added:

  // When precompiling, the fact that a field is currently initialized does not
  // make it safe to omit code that checks if the field needs initialization
  // because the field will be reset so it starts uninitialized in the process
  // running the precompiled code. We must be prepared to reinitialize fields.
--precompile is a flag handler that flips a number of other flags. Having
separate flags lets us test alternate compilations independent of
precompilation.

 DECLARE_FLAG(bool, eliminate_type_checks);
 DECLARE_FLAG(bool, trace_optimization);
 DECLARE_FLAG(bool, throw_on_javascript_int_overflow);
 
 Definition::Definition(intptr_t deopt_id)
     : Instruction(deopt_id),
       range_(NULL),
       type_(NULL),
       temp_index_(-1),
       ssa_temp_index_(-1),
@@ skipping 328 matching lines @@
   }
   return (other_load->field() == NULL) &&
       (offset_in_bytes() == other_load->offset_in_bytes());
 }
 
 
 Instruction* InitStaticFieldInstr::Canonicalize(FlowGraph* flow_graph) {
   const bool is_initialized =
       (field_.StaticValue() != Object::sentinel().raw()) &&
       (field_.StaticValue() != Object::transition_sentinel().raw());
-  return is_initialized ? NULL : this;
+  return is_initialized && !FLAG_fields_may_be_reset ? NULL : this;
 }
 
 
 EffectSet LoadStaticFieldInstr::Dependencies() const {
   return StaticField().is_final() ? EffectSet::None() : EffectSet::All();
 }
 
 
 bool LoadStaticFieldInstr::AttributesEqual(Instruction* other) const {
   LoadStaticFieldInstr* other_load = other->AsLoadStaticField();
@@ skipping 3240 matching lines @@
     case Token::kTRUNCDIV: return 0;
     case Token::kMOD: return 1;
     default: UNIMPLEMENTED(); return -1;
   }
 }
 
 
 #undef __
 
 }  // namespace dart