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This isn't that conclusive of a test -- uninitialized memory is 0 relatively often.

Okay, that is conclusive. (And I'm pleasantly surprised by how readable GCC's source code is.)

This has nothing to do with memory. The x86_64 ABI returns values from functions in the RAX register.

Sure; same point applies though -- just because 0 happens to be in RAX doesn't mean it's defined behavior.