Status: Accepted
Owner: [email protected]
Labels: Type-Bug Priority-Medium
New issue 830 by [email protected]: Avoid using enums as opaque types,
it confuses GCC value range propagation.
http://code.google.com/p/v8/issues/detail?id=830
We define StackFrame::Id as
enum Id { NO_ID = 0 };
according to section 7.2/6 of C++ Standard range of values for this
enumeration contains a single value: 0.
GCC exploits this fact when Value Range Propagation optimization is enabled
and optimizes away certain functions, e.g.
static Smi* WrapFrameId(StackFrame::Id id) {
return Smi::FromInt(id >> 2);
}
becomes
static Smi* WrapFrameId(StackFrame::Id id) {
return 0;
}
which leads to incorrect behavior because we use StackFrame::Id to store
values outside of it's range.
Currently we disable VRP optimization to make V8 work when compiled with
GCC 4.4. Instead we should comply with Standard and fix our usage of
enumeration types.
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