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Re: [PATCH 0/8] eBPF support for GCC


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...


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