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SubjectRe: [RFC PATCH tip 5/5] tracing filter examples in BPF
From
On Tue, Dec 3, 2013 at 4:35 PM, Jonathan Corbet <corbet@lwn.net> wrote:
> On Mon, 2 Dec 2013 20:28:50 -0800
> Alexei Starovoitov <ast@plumgrid.com> wrote:
>
>> GCC-BPF backend is available on github
>> (since gcc plugin infrastructure doesn't allow for out-of-tree backends)
>
> Do you have a pointer to where this backend can be found? I've
> done a bit of digging around but seem to be unable to find it…

here it is:
https://github.com/iovisor/bpf_gcc/commit/9e7223f8f09c822ecc6e18309e89a574a23dbf63

It's quite small comparing to normal backend:
config.me | 3 +
config.sub | 12 +-
gcc/common/config/bpf/bpf-common.c | 39 ++
gcc/config.gcc | 18 +
gcc/config/bpf/bpf-modes.def | 22 +
gcc/config/bpf/bpf-protos.h | 60 ++
gcc/config/bpf/bpf.c | 1063 ++++++++++++++++++++++++++++++++++++
gcc/config/bpf/bpf.h | 746 +++++++++++++++++++++++++
gcc/config/bpf/bpf.md | 895 ++++++++++++++++++++++++++++++
gcc/config/bpf/linux.h | 67 +++
10 files changed, 2922 insertions(+), 3 deletions(-)

Instruction emission is in bpf.md in functions like:
(define_insn "adddi3"
[(set (match_operand:DI 0 "int_reg_operand" "=r, r")
(plus:DI (match_operand:DI 1 "int_reg_operand" "0, 0")
(match_operand:DI 2 "int_reg_or_const_operand" "r, K")))]
""
"@
BPF_INSN_ALU(BPF_ADD, %0, %2), // %0 += %2
BPF_INSN_ALU_IMM(BPF_ADD, %0, %2), // %0 += %2"
[(set_attr "type" "arith,arith") ])

So it takes C and emits BPF_* macros from include/linux/bpf.h
tools/bpf/trace/ examples were generated by it.
Eventually it will be changed to emit binary hex directly, but macros
are much easier to understand for now.

Thanks
Alexei
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