While formally GP is not a callee-saved register, for static linking
the link time optimization described in section 3.2.3 of the "Calling
Standard for Alpha Systems" allows the caller to rely on GP being the
value needed within setjmp() - and not restore it after return if setjmp
and the call site share a GP value.
This fixes longjmp/setjmp for statically linked programs, e.g. /rescue/csh
or the static tcsh variant from pkgsrc.
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