Re: Register allocation in special circumstancies

Andrey S. Bokhanko <ruff@mcst.ru> wrote:
: As we all know, C language has nasty (for compiler developers)
: feature: setjmp/longjmp. If there is "setjmp" point in procedure, than
: to explicitly express control flow we have to create control flow
: edges from ALMOST EVERY reachable (from "setjmp" point) operation to
: that "setjmp" point. This is TOO costly (both in memory requirements
: and compilation time), so we decided not to make such edges. Of
: course, this decision prohibit application of wide class of
: optimizations in such procedures, but this is inevitable cost (AFAIK,
: gcc compiler works exactly same way).


: But we can't disable one special optimization: register
: allocation. So, that's where the problem is: if r.a. based on
: classical graph coloring approach, we must construct Register
: Interference Graph (RIG), and hence we must take into account this
: nasty feature of "setjmp" points. But how to do it? I have some ideas,
: but they all are less than ideal, so any help will be gratefully
: appreciated.


Hmmmm. If you implement setjmp by saving every register and longjmp
by restoring every register, then you have the problem that
longjmp'ing might reset some variables to obsolete values. The C
standard allows for that in the case of variables that are local to
the function in which the corresponding setjmp occurred, but that
still leaves a problem with any global that is register resident at
some setjmp point. Would it work to give each problematic global a
"home" memory location and, just before each occurrence of setjmp or
longjmp, to spill each register-resident problematic global to its
home location?


Tom Payne

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