Hi Richard.
> . Dynamic stack allocation (alloca and VLAs) is achieved by using what
> otherwise would be a perfectly normal general register, %r9, as a
> pseudo stack pointer. This has the disadvantage of making the
> register "fixed" and therefore not available for general register
> allocation. Hopefully there is a way to conditionalize this, since
> both alloca and VLAs are relatively uncommon; I haven't found it
> yet.
In principle it's possible to define register eliminations for
target-specific registers as well as the usual
FRAME/ARG_POINTER_REGNUM crowd.
Yeah, before I started using %r9 as a stack pointer, I was indeed
"eliminating" a fake stack pointer hard register to the frame register,
i.e. the opposite of what is usually done.
That seemed to work well, but as soon as __builtin_alloca and/or VLAs
were used, lra-eliminations would enter into an infinite loop: it didn't
like the stack pointer being eliminated.
So you could have a fake fixed register to represent the pseudo
stack pointer, then allow that to be "eliminated" to %r9 in
functions that need it. Functions that don't need it can continue
(not) using the fake register and leave %r9 free for general use.
Interesting idea... but wouldn't that require to have %r9 declared as a
fixed register, in functions that cfun->calls_alloca?
After reading your reply I investigated a bit, and found out that
CONDITIONAL_REGISTER_USAGE can indeed be called at pleasure, via
reinit_regs(). The i386 port calls reinit_regs in set_current_function,
for example.
So it should be possible to declare %r9 as fixed or non-fixed, in
bpf_set_current_function, depending on the value of
cfun->calls_alloca...
