Re: [PATCH] perf: use __builtin_preserve_field_info for GCC compatibility

Yonghong Song <yonghong.song@xxxxxxxxx> · Wed, 6 Aug 2025 17:27:02 -0700

On 8/6/25 4:57 PM, Andrew Pinski wrote:

-----Original Message-----
From: Namhyung Kim <namhyung@xxxxxxxxxx>
Sent: Wednesday, August 6, 2025 4:34 PM
To: Sam James <sam@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Ingo Molnar
<mingo@xxxxxxxxxx>; Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx>; Mark Rutland
<mark.rutland@xxxxxxx>; Alexander Shishkin
<alexander.shishkin@xxxxxxxxxxxxxxx>; Jiri Olsa
<jolsa@xxxxxxxxxx>; Ian Rogers <irogers@xxxxxxxxxx>; Adrian
Hunter <adrian.hunter@xxxxxxxxx>; Liang, Kan
<kan.liang@xxxxxxxxxxxxxxx>; Andrew Pinski
<quic_apinski@xxxxxxxxxxx>; linux-perf-
users@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
bpf@xxxxxxxxxxxxxxx
Subject: Re: [PATCH] perf: use __builtin_preserve_field_info
for GCC compatibility

Hello,

On Wed, Aug 06, 2025 at 01:03:01AM +0100, Sam James
wrote:
When exploring building bpf_skel with GCC's BPF support,
there was a
buid failure because of bpf_core_field_exists vs the
mem_hops bitfield:
```
  In file included from util/bpf_skel/sample_filter.bpf.c:6:
util/bpf_skel/sample_filter.bpf.c: In function
'perf_get_sample':
tools/perf/libbpf/include/bpf/bpf_core_read.h:169:42:
error: cannot take address of bit-field 'mem_hops'
   169 | #define ___bpf_field_ref1(field)        (&(field))
       |                                          ^
tools/perf/libbpf/include/bpf/bpf_helpers.h:222:29: note: in
expansion of macro '___bpf_field_ref1'
   222 | #define ___bpf_concat(a, b) a ## b
       |                             ^
tools/perf/libbpf/include/bpf/bpf_helpers.h:225:29: note: in
expansion of macro '___bpf_concat'
   225 | #define ___bpf_apply(fn, n) ___bpf_concat(fn, n)
       |                             ^~~~~~~~~~~~~
tools/perf/libbpf/include/bpf/bpf_core_read.h:173:9: note:
in expansion of macro '___bpf_apply'
   173 |         ___bpf_apply(___bpf_field_ref,
___bpf_narg(args))(args)
       |         ^~~~~~~~~~~~
tools/perf/libbpf/include/bpf/bpf_core_read.h:188:39: note:
in expansion of macro '___bpf_field_ref'
   188 |
__builtin_preserve_field_info(___bpf_field_ref(field),
BPF_FIELD_EXISTS)
       |                                       ^~~~~~~~~~~~~~~~
util/bpf_skel/sample_filter.bpf.c:167:29: note: in expansion
of macro 'bpf_core_field_exists'
   167 |                         if (bpf_core_field_exists(data-
mem_hops))
       |                             ^~~~~~~~~~~~~~~~~~~~~
cc1: error: argument is not a field access ```

___bpf_field_ref1 was adapted for GCC in
12bbcf8e840f40b82b02981e96e0a5fbb0703ea9
but the trick added for compatibility in
3a8b8fc3174891c4c12f5766d82184a82d4b2e3e
isn't compatible with that as an address is used as an
argument.
Workaround this by calling __builtin_preserve_field_info
directly as
the bpf_core_field_exists macro does, but without the
___bpf_field_ref use.

IIUC GCC doesn't support bpf_core_fields_exists() for bitfield
members, right?  Is it gonna change in the future?
Let's discuss how __builtin_preserve_field_info is handled in the first place for BPF. Right now it seems it is passed some expression as the first argument is never evaluated.
The problem is GCC's implementation of __builtin_preserve_field_info is all in the backend and the front end does not understand of the special rules here.

GCC implements some "special" builtins in the front-end but not by the normal function call rules but parsing them separately; this is how __builtin_offsetof and a few others are implemented in both the C and C++ front-ends (and implemented separately). Now we could have add a hook to allow a backend to something similar and maybe that is the best way forward here.
But it won't be __builtin_preserve_field_info but rather `__builtin_preserve_field_type_info(type,field,kind)` instead.

__builtin_preserve_enum_type_value would also be added with the following:
__builtin_preserve_enum_type_value(enum_type, enum_value, kind)

And change all of the rest of the builtins to accept a true type argument rather than having to cast an null pointer to that type.

Will clang implement a similar builtin?

The clang only has one builtin for some related relocations:
   
   __builtin_preserve_field_info(..., BPF_FIELD_EXISTS)
   __builtin_preserve_field_info(..., BPF_FIELD_BYTE_OFFSET)
   ...
They are all used in bpf_core_read.h.


Note this won't be done until at least GCC 16; maybe not until GCC 17 depending on if I or someone else gets time to implement the front-end parts which is acceptable to both the C and C++ front-ends.

Thanks,
Andrew Pinski

Link: https://gcc.gnu.org/PR121420
Co-authored-by: Andrew Pinski
<quic_apinski@xxxxxxxxxxx>
Signed-off-by: Sam James <sam@xxxxxxxxxx>
---
  tools/perf/util/bpf_skel/sample_filter.bpf.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/bpf_skel/sample_filter.bpf.c
b/tools/perf/util/bpf_skel/sample_filter.bpf.c
index b195e6efeb8be..e5666d4c17228 100644
--- a/tools/perf/util/bpf_skel/sample_filter.bpf.c
+++ b/tools/perf/util/bpf_skel/sample_filter.bpf.c
@@ -164,7 +164,7 @@ static inline __u64
perf_get_sample(struct bpf_perf_event_data_kern *kctx,
               if (entry->part == 8) {
                       union perf_mem_data_src___new *data = (void
*)&kctx->data->data_src;

-                     if (bpf_core_field_exists(data->mem_hops))
+                     if
+ (__builtin_preserve_field_info(data->mem_hops,
BPF_FIELD_EXISTS))

I believe those two are equivalent (maybe worth a
comment?).  But it'd be great if BPF/clang folks can review if
it's ok.

Anyway, I can build it with clang.

Tested-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Thanks,
Namhyung


                               return data->mem_hops;

                       return 0;
--
2.50.1









