dwarf_loader: Help spotting functions with optimized-out parameters
Compilation generates DWARF at several stages, and often the later DWARF
representations more accurately represent optimizations that have
occurred during compilation.
In particular, parameter representations can be spotted by their
abstract origin references to the original parameter, but they often
have more accurate location information. In most cases, the parameter
locations will match calling conventions, and be registers for the first
6 parameters on x86_64, first 8 on ARM64 etc. If the parameter is not a
register when it should be however, it is likely passed via the stack
or the compiler has used a constant representation instead. The latter
can often be spotted by checking for a DW_AT_const_value attribute, as
noted by Eduard.
In addition, absence of a location tag (either across the abstract
origin reference and the original parameter, or in the standalone
parameter description) is evidence of an optimized-out parameter.
Presence of a location tag is stored in the parameter description and
shared between abstract tags and their original referents.
This change adds a field to parameters and their associated ftype to
note if a parameter has been optimized out. Having this information
allows us to skip such functions, as their presence in CUs makes BTF
encoding impossible.
Committer notes:
Changed the NR_REGISTER_PARAMS definition from a if/elif/endif for the
native architecture into a function that uses the ELF header e_machine
to find the target architecture, to allow for cross builds.
Also avoided looking at location expression in parameter__new() when the
param_idx argument is -1, as is the case when creating 'struct
parameter' instances for DW_TAG_subroutine_type, since we don't have
such info, only for the 'struct parameter' instances created from
DW_TAG_subprogram.
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Hao Luo <haoluo@google.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>
Cc: Kui-Feng Lee <sinquersw@gmail.com>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Timo Beckers <timo@incline.eu>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/1675088985-20300-2-git-send-email-alan.maguire@oracle.com
Link: https://lore.kernel.org/r/1675790102-23037-2-git-send-email-alan.maguire@oracle.com
Link: https://lore.kernel.org/r/9c330c78-e668-fa4c-e0ab-52aa445ccc00@oracle.com # DW_OP_reg0 is the first register on aarch64
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
2 files changed
