Re: [PATCH v2 0/1] Add BPF callx support to objdump and as

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

Hi Will.

[Adding Yonghong and Eduard in CC]

> After additional consideration and discussion with Jose and Dave,
> it seems like we have determined the way that clang, gcc and binutils
> need to handle the callx/callr:
>
> 1. callr remains with the register holding the target of the jump stored
> in the dst_reg.
> 2. callx is added with the register holding the target of the jump stored
> in the imm32.
> 3. We have to remove the pseudoc syntax because it is no longer possible
> to disambiguate between versions of call by simply looking at the
> parameter.

I don't recall reaching any agreement on the above.  What is the point
of having both callr and callx?

The existing callr is generated by GCC in -mxbpf mode.  It is an
experimental extension that we use in order to be able to run more of
the GCC testsuite, so it is always possible to change it to use imm32
instead of dst_reg.

I wouldn't personally welcome that change and would much prefer if clang
starts using either reg_src or reg_dst, because compromising/reserving
endian-dependent 32 whole bits for a register number that only requires
4 bits seems like a waste of insn space that will complicate future ISA
extensions.

In either case, if we all use the same encoding for the indirect call
instruction (I fail to see any reason for not doing so) then point
3. becomes moot.

>
> Tests are added/refactored to meet the above. 
>
> I am more than happy to resend as a separate mailing to the list but
> sending first as a reply in order to keep list traffic manageable.
>
> As I said before, I sincerely appreciate all that you are doing for
> the community and how welcoming you have been to a first-time contributor.
>
> Sincerely,
> Will
>
> Will Hawkins (1):
>   objdump, as: Add callx support for BPF CPU v1
>
>  gas/config/tc-bpf.c                           | 25 ++++++++++++++++++-
>  gas/testsuite/gas/bpf/bpf.exp                 |  4 +--
>  .../gas/bpf/{indcall-1.d => callr.d}          |  4 +--
>  .../gas/bpf/{indcall-1.s => callr.s}          |  2 +-
>  gas/testsuite/gas/bpf/indcall-1-pseudoc.d     | 23 -----------------
>  gas/testsuite/gas/bpf/indcall-1-pseudoc.s     | 13 ----------
>  include/opcode/bpf.h                          |  3 ++-
>  opcodes/bpf-dis.c                             |  6 +++++
>  opcodes/bpf-opc.c                             |  4 ++-
>  sim/bpf/bpf-sim.c                             |  4 +++
>  10 files changed, 44 insertions(+), 44 deletions(-)
>  rename gas/testsuite/gas/bpf/{indcall-1.d => callr.d} (90%)
>  rename gas/testsuite/gas/bpf/{indcall-1.s => callr.s} (90%)
>  delete mode 100644 gas/testsuite/gas/bpf/indcall-1-pseudoc.d
>  delete mode 100644 gas/testsuite/gas/bpf/indcall-1-pseudoc.s