It's not about registers -- registers aren't (or shouldn't be) permitted
by the spinlock asm constraints anyway.
The asm spec says "substite an operand representing this VALUE". It's
possible for the compiler to /copy/ the VALUE and use an operand
referring to that.
The type cast simply affects the typing system in C. The value can
still be copied -- even 100 words if the compiler decides that's a good
idea.
Object based alias inference means the compiler can decide it only needs
to copy the original object words despite the type cast, so it might
actually be efficient to do so in some circumstances. We might hope
that any solution to the existing -fstrict-aliasing problems would apply
equally here, but object-based alias inference is a different fish to
type-based alias inference, and current work (and -fno-strict-aliasing)
apply only to type-based aliasing. (E.g. the __typealias work).
Work on putting structure elements in registers will presumably lead to
GCC being good at analysing the use of structure members in detail. I
would not be surprised to find GCC is good at noticing the type cast is
meaningless as a result.
-- Jamie
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