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Re: avoid unnecessary register saves for setjmp
On Fri, 2003-11-21 at 19:08, Daniel Jacobowitz wrote:
> Here's the difficulty: changing the size of jmp_buf.
That is a good point. Still, we shouldn't penalize targets that have
defined setjmp correctly. So I suggest using NON_SAVING_SETJMP for
this. This will do what you want if you define it for your target.
I'll go ahead and fix NON_SAVING_SETJMP to make it usable.
> Sure - but that assumes that all unused call-saved registers are
> restored by the setjmp. So then it is required to save registers to
> get reasonable behavior.
You are confusing two different issues here. setjmp is not required to
restore call-used registers. Thus any value stored in a register in
this function may not be restored when longjmp is called. This is why
the C language standard says that local automatic non-volatile variables
may not be restored by longjmp. However, setjmp is required to restore
the original context, and that means call-saved registers must be
restored. A setjmp call is not allowed to clobber locals in the parent
function, which is what would happen if call-saved registers were not
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com