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...
Re: [PATCH 0/8] eBPF support for GCC
"Jose E. Marchesi" 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
Re: [PATCH 0/8] eBPF support for GCC
> The second patch adds the new GCC port proper. Machine description, > implementation of target hooks and macros, command-line options and > the like. Looks like [PATCH 2/8] didn't make it to the mailing list for some reason (maybe it exceeded a size limit?) Yeah the Oracle smtpmail server seems to not be routing 2/8... how annoying. I just sent it again using my GNU account.
Re: [PATCH 0/8] eBPF support for GCC
On Wed, 2019-08-14 at 23:36 +0200, Jose E. Marchesi wrote: > Hi people! > [...] > The second patch adds the new GCC port proper. Machine description, > implementation of target hooks and macros, command-line options and > the like. Looks like [PATCH 2/8] didn't make it to the mailing list for some reason (maybe it exceeded a size limit?) [...] Dave
