Comment by gpderetta
12 days ago
I think that named register variables (a GCC extension) are meant to be live in asm block by design, so they shouldn't be optimized away.
Still I would use extended asm.
edit: from the docs: "The only supported use for [Specifying Registers for Local Variables] is to specify registers for input and output operands when calling Extended asm".
So the example is UB.
It's not UB, it's documented behaviour of a vendor extension.
It's not UB because it's defined as outside the scope of the language standard. The vendor (in this case, GCC) does document how to use its inline assembly extension in quite a lot of detail, including how to use clobber lists to prevent exactly the kind of thing these failures demonstrate.
GCC says that register extensions are not supported with basic asm. What happens if you do it anyway is not documented. Ergo UB.
GCC says that register extensions are not supported with basic inline asm. If you do it anyway it doesn't work it's not undefined behaviour, it's behaving as documented. Once you've ventured into vendorland you're outside the realm of undefined behaviour to start with, but following the vendor's documentation on how to use the vendor's extension is the minimum requirement for meeting your expectations that the feature will work.
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