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Unified Diff: src/x64/full-codegen-x64.cc

Issue 8422010: Remove some initialization checks based on source positions. (Closed) Base URL: https://v8.googlecode.com/svn/branches/bleeding_edge
Patch Set: Created 9 years, 2 months ago
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Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index d1565cca3567873f451f87ed7eec912a77a2d707..7f571d7aad6bbd395c9b3eefed2d887090d5a41d 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -1251,27 +1251,59 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
case Variable::LOCAL:
case Variable::CONTEXT: {
Comment cmnt(masm_, var->IsContextSlot() ? "Context slot" : "Stack slot");
- if (!var->binding_needs_init()) {
- context()->Plug(var);
- } else {
- // Let and const need a read barrier.
- Label done;
- GetVar(rax, var);
- __ CompareRoot(rax, Heap::kTheHoleValueRootIndex);
- __ j(not_equal, &done, Label::kNear);
- if (var->mode() == LET || var->mode() == CONST_HARMONY) {
- // Throw a reference error when using an uninitialized let/const
- // binding in harmony mode.
- __ Push(var->name());
- __ CallRuntime(Runtime::kThrowReferenceError, 1);
- } else {
- // Uninitalized const bindings outside of harmony mode are unholed.
- ASSERT(var->mode() == CONST);
- __ LoadRoot(rax, Heap::kUndefinedValueRootIndex);
+ 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; } }
+ //
+ // Check that we always have valid source position.
+ ASSERT(var->initializer_position() != RelocInfo::kNoPosition);
+ ASSERT(proxy->position() != RelocInfo::kNoPosition);
+ bool skip_init_check =
+ var->mode() != CONST &&
+ var->scope()->DeclarationScope() == scope()->DeclarationScope() &&
+ var->initializer_position() < proxy->position();
+ if (!skip_init_check) {
+ // Let and const need a read barrier.
+ Label done;
+ GetVar(rax, var);
+ __ CompareRoot(rax, Heap::kTheHoleValueRootIndex);
+ __ j(not_equal, &done, Label::kNear);
+ if (var->mode() == LET || var->mode() == CONST_HARMONY) {
+ // Throw a reference error when using an uninitialized let/const
+ // binding in harmony mode.
+ __ Push(var->name());
+ __ CallRuntime(Runtime::kThrowReferenceError, 1);
+ } else {
+ // Uninitalized const bindings outside of harmony mode are unholed.
+ ASSERT(var->mode() == CONST);
+ __ LoadRoot(rax, Heap::kUndefinedValueRootIndex);
+ }
+ __ bind(&done);
+ context()->Plug(rax);
+ break;
}
- __ bind(&done);
- context()->Plug(rax);
}
+ context()->Plug(var);
break;
}
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