On Mon, May 31, 2021 at 7:18 PM Jose E. Marchesi via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> This patch changes the BPF GCC backend in order to use the DWARF debug
> hooks and therefore enables the user to generate BTF debugging
> information with -gbtf. Generating BTF is crucial when compiling BPF
> programs, since the CO-RE (compile-once, run-everwhere) mechanism
> used by the kernel BPF loader relies on it.
>
> Note that since in eBPF it is not possible to unwind frames due to the
> restrictive nature of the target architecture, we are disabling the
> generation of CFA in this target.
You want to CC the BPF maintainer here. Note that IIRC RTX_FRAME_RELATED_P
also prevents code-motion for some insns, so I'm not sure removing
that is 100% safe.
> 2021-05-14 David Faust <david.fa...@oracle.com>
>
> * config/bpf/bpf.c (bpf_expand_prologue): Do not mark insns as
> frame related.
> (bpf_expand_epilogue): Likewise.
> * config/bpf/bpf.h (DWARF2_FRAME_INFO): Define to 0.
> Do not define DBX_DEBUGGING_INFO.
> ---
> gcc/config/bpf/bpf.c | 4 ----
> gcc/config/bpf/bpf.h | 12 ++----------
> 2 files changed, 2 insertions(+), 14 deletions(-)
>
> diff --git a/gcc/config/bpf/bpf.c b/gcc/config/bpf/bpf.c
> index 126d4a2798d..e635f9edb40 100644
> --- a/gcc/config/bpf/bpf.c
> +++ b/gcc/config/bpf/bpf.c
> @@ -349,7 +349,6 @@ bpf_expand_prologue (void)
> hard_frame_pointer_rtx,
> fp_offset - 8));
> insn = emit_move_insn (mem, gen_rtx_REG (DImode, regno));
> - RTX_FRAME_RELATED_P (insn) = 1;
> fp_offset -= 8;
> }
> }
> @@ -364,7 +363,6 @@ bpf_expand_prologue (void)
> {
> insn = emit_move_insn (stack_pointer_rtx,
> hard_frame_pointer_rtx);
> - RTX_FRAME_RELATED_P (insn) = 1;
>
> if (size > 0)
> {
> @@ -372,7 +370,6 @@ bpf_expand_prologue (void)
> gen_rtx_PLUS (Pmode,
> stack_pointer_rtx,
> GEN_INT (-size))));
> - RTX_FRAME_RELATED_P (insn) = 1;
> }
> }
> }
> @@ -412,7 +409,6 @@ bpf_expand_epilogue (void)
> hard_frame_pointer_rtx,
> fp_offset - 8));
> insn = emit_move_insn (gen_rtx_REG (DImode, regno), mem);
> - RTX_FRAME_RELATED_P (insn) = 1;
> fp_offset -= 8;
> }
> }
> diff --git a/gcc/config/bpf/bpf.h b/gcc/config/bpf/bpf.h
> index 4c5b19e262b..55beecbcb36 100644
> --- a/gcc/config/bpf/bpf.h
> +++ b/gcc/config/bpf/bpf.h
> @@ -235,17 +235,9 @@ enum reg_class
>
> /**** Debugging Info ****/
>
> -/* We cannot support DWARF2 because of the limitations of eBPF. */
> +/* In eBPF it is not possible to unwind frames. Disable CFA. */
>
> -/* elfos.h insists in using DWARF. Undo that here. */
> -#ifdef DWARF2_DEBUGGING_INFO
> -# undef DWARF2_DEBUGGING_INFO
> -#endif
> -#ifdef PREFERRED_DEBUGGING_TYPE
> -# undef PREFERRED_DEBUGGING_TYPE
> -#endif
> -
> -#define DBX_DEBUGGING_INFO
> +#define DWARF2_FRAME_INFO 0
>
> /**** Stack Layout and Calling Conventions. */
>
> --
> 2.25.0.2.g232378479e
>
