Re: [PATCH dwarves v2 0/3] pahole: Fix arm64 argument register allocation

[Alan Maguire <alan.maguire@oracle.com>]

On 17/09/2026 17:40, Yonghong Song wrote:
> 
> The arm64 ABI starts an argument twice the register size on an even-numbered
> argument register, leaving a hole when the next free register is an odd one:
> 
>   u64 f(u64 a, __int128 v, u64 b);
> 
> passes a in x0, v in x2:x3 -- skipping x1 -- and b in x4.  pahole did not
> model the hole, so b looked like it was in an unexpected register and the
> whole function was dropped from BTF.  Patch 1 skips the register the ABI
> leaves behind, patch 3 adds a test for it.  Only arm64 has this rule; the
> other architectures are unaffected.
> 
> Patch 2 is a small follow-up.  Floating point and vector arguments are
> allocated from a register bank of their own -- v0-v7 on arm64, xmm0-xmm7 on
> x86-64 -- so a wide one never occupies a general purpose register pair and
> must not trigger the even-register rule either.  It does not change the BTF
> emitted today, because such an argument still advances the general purpose
> register index by its own size, so it comes with no test of its own; it
> keeps the alignment rule scoped to the bank it applies to.
>

one small issue here; we record alignment for structs from DW_AT_align
in associated tags; we should ensure that we utilize that alignment
where available. However it's a bit broken in that we don't appear
to record alignment for typedefs, just structs.

I think we can tackle that with a followup though; unless anyone objects I'll
land these patches later today.

 
> Changelog:
>   v1 -> v2:
>     - v1: https://lore.kernel.org/bpf/20260911040955.339939-1-yonghong.song@linux.dev/
>     - add commit "Do not align the argument register index for FP arguments".
> 
> Yonghong Song (3):
>   dwarf_loader: Skip the argument register the arm64 ABI leaves as a
>     hole
>   dwarf_loader: Do not align the argument register index for FP
>     arguments
>   tests: tests: Add test for 16-byte aligned arguments on arm64
> 
>  dwarf_loader.c              | 72 ++++++++++++++++++++++++++++++++--
>  dwarves.h                   | 11 ++++++
>  tests/clang_parm_align16.sh | 77 +++++++++++++++++++++++++++++++++++++
>  tests/test_lib.sh           |  3 +-
>  4 files changed, 158 insertions(+), 5 deletions(-)
>  create mode 100755 tests/clang_parm_align16.sh
> 
> -- 
> 2.53.0-Meta
>