Re: Indirect C-function calls not sibcall-optimised?

[Richard Henderson <rth at redhat dot com>]

On Tue, Jul 16, 2002 at 02:52:10PM +1000, Andreas Bauer wrote:
> > One has to allocate the address to a call-clobbered that does not
> > overlap the argument registers. 
> 
> Ok, but how does that differ from a "normal call"?  I guess, I still don't 
> see the rationale behind it.

In a normal call, one can use call-saved registers to hold the
address, since you'll be returning to the current function, and
can thus properly restore the call-saved register.

Think about it -- in a tail call you must have (1) all call-saved
registers restored to the state your caller expects (2) all
argument registers loaded as the called function expects and
(3) a place to hold the address of the called function.  And 
sometimes (4) scratch registers to accomplish nr 1.

For some targets, e.g. ARM, *all* call-clobbered registers are
used for argument passing, so when calling a function with four
arguments there is no space for nr 3.


r~