[PATCH 0/8] eBPF support for GCC

[Richard Sandiford]

"Jose E. Marchesi" <jose.marchesi@oracle.com> writes:
> . 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.  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.

Richard