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Unified Diff: src/arm/full-codegen-arm.cc

Issue 8479034: Reapply r9870 "Remove some initialization checks based on source positions.". (Closed) Base URL: https://v8.googlecode.com/svn/branches/bleeding_edge
Patch Set: Created 9 years, 1 month ago
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Index: src/arm/full-codegen-arm.cc
diff --git a/src/arm/full-codegen-arm.cc b/src/arm/full-codegen-arm.cc
index adc1b26c6f8f35c64e2455549f2e99dd3d2eca34..20c10c926abeaf4ce1255559a90b7797f7c93d80 100644
--- a/src/arm/full-codegen-arm.cc
+++ b/src/arm/full-codegen-arm.cc
@@ -1283,28 +1283,64 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
Comment cmnt(masm_, var->IsContextSlot()
? "Context variable"
: "Stack variable");
- if (!var->binding_needs_init()) {
- context()->Plug(var);
- } else {
- // Let and const need a read barrier.
- GetVar(r0, var);
- __ CompareRoot(r0, Heap::kTheHoleValueRootIndex);
- if (var->mode() == LET || var->mode() == CONST_HARMONY) {
- // Throw a reference error when using an uninitialized let/const
- // binding in harmony mode.
- Label done;
- __ b(ne, &done);
- __ mov(r0, Operand(var->name()));
- __ push(r0);
- __ CallRuntime(Runtime::kThrowReferenceError, 1);
- __ bind(&done);
+ if (var->binding_needs_init()) {
+ // var->scope() may be NULL when the proxy is located in eval code and
+ // refers to a potential outside binding. Currently those bindings are
+ // always looked up dynamically, i.e. in that case
+ // var->location() == LOOKUP.
+ // always holds.
+ ASSERT(var->scope() != NULL);
+
+ // Check if the binding really needs an initialization check. The check
+ // can be skipped in the following situation: we have a LET or CONST
+ // binding in harmony mode, both the Variable and the VariableProxy have
+ // the same declaration scope (i.e. they are both in global code, in the
+ // same function or in the same eval code) and the VariableProxy is in
+ // the source physically located after the initializer of the variable.
+ //
+ // We cannot skip any initialization checks for CONST in non-harmony
+ // mode because const variables may be declared but never initialized:
+ // if (false) { const x; }; var y = x;
+ //
+ // The condition on the declaration scopes is a conservative check for
+ // nested functions that access a binding and are called before the
+ // binding is initialized:
+ // function() { f(); let x = 1; function f() { x = 2; } }
+ //
+ bool skip_init_check;
+ if (var->scope()->DeclarationScope() != scope()->DeclarationScope()) {
+ skip_init_check = false;
} else {
- // Uninitalized const bindings outside of harmony mode are unholed.
- ASSERT(var->mode() == CONST);
- __ LoadRoot(r0, Heap::kUndefinedValueRootIndex, eq);
+ // Check that we always have valid source position.
+ ASSERT(var->initializer_position() != RelocInfo::kNoPosition);
+ ASSERT(proxy->position() != RelocInfo::kNoPosition);
+ skip_init_check = var->mode() != CONST &&
+ var->initializer_position() < proxy->position();
+ }
+
+ if (!skip_init_check) {
+ // Let and const need a read barrier.
+ GetVar(r0, var);
+ __ CompareRoot(r0, Heap::kTheHoleValueRootIndex);
+ if (var->mode() == LET || var->mode() == CONST_HARMONY) {
+ // Throw a reference error when using an uninitialized let/const
+ // binding in harmony mode.
+ Label done;
+ __ b(ne, &done);
+ __ mov(r0, Operand(var->name()));
+ __ push(r0);
+ __ CallRuntime(Runtime::kThrowReferenceError, 1);
+ __ bind(&done);
+ } else {
+ // Uninitalized const bindings outside of harmony mode are unholed.
+ ASSERT(var->mode() == CONST);
+ __ LoadRoot(r0, Heap::kUndefinedValueRootIndex, eq);
+ }
+ context()->Plug(r0);
+ break;
}
- context()->Plug(r0);
}
+ context()->Plug(var);
break;
}
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