[Bug target/109068] New: bpf: "error: too many function arguments for eBPF" for always_inline function

[Bug target/109068] New: bpf: "error: too many function arguments for eBPF" for always_inline function

[david.faust at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109068

            Bug ID: 109068
           Summary: bpf: "error: too many function arguments for eBPF" for
                    always_inline function
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david.faust at oracle dot com
                CC: cupertino.miranda at oracle dot com, jemarch at gcc dot gnu.org
  Target Milestone: ---
            Target: bpf

BPF calling convention does not support function calls with more than
5 arguments. But if the function is inlined this restriction should be
relaxed.

Many convenience macros for BPF in the kernel make use of always_inline
functions, and the expansion of the macro uses can result in an
always_inline function with >5 args. There are several examples of this
in the BPF selftests, so GCC fails to compile those tests.
e.g. on bpf-next:
    linux/tools/testing/selftests/bpf/progs/bpf_syscall_macro.c
 and corresponding BPF_KSYSCALL macro def in
    linux/tools/testing/selftests/bpf/tools/include/bpf/bpf_tracing.h
 duplicated from
    linux/tools/lib/bpf/bpf_tracing.h

clang compiles these tests without issue.

We should fix the BPF backend check for function arguments to allow
calls to inline functions with >5 args.


$ cat args.c
inline __attribute__((always_inline))
int foo (int a, int b, int c, int d, int e, int f)
{
  return a + b + c + d + e + f;
}

int bar (int x)
{
  return foo (x, x*2, x*3, x*4, x*5, x*6);
}

$ bpf-unknown-none-gcc -c args.c -o args.o
args.c: In function ‘foo’:
args.c:2:5: error: too many function arguments for eBPF
    2 | int foo (int a, int b, int c, int d, int e, int f)
      |     ^~~

[Bug target/109068] bpf: "error: too many function arguments for eBPF" for always_inline function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109068

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-03-30
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |diagnostic

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug target/109068] bpf: "error: too many function arguments for eBPF" for always_inline function

[jemarch at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109068

Jose E. Marchesi <jemarch at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Jose E. Marchesi <jemarch at gcc dot gnu.org> ---
Fixed by:

commit 6103df1e4fae5192c507484b1d32f00c42c70b54
Author: Jose E. Marchesi <jose.marchesi@oracle.com>
Date:   Thu Aug 10 10:53:16 2023 +0200

    bpf: allow exceeding max num of args in BPF when always_inline

    BPF currently limits the number of registers used to pass arguments to
    functions to five registers.  There is a check for this at function
    expansion time.  However, if a function is guaranteed to be always
    inlined (and its body never generated) by virtue of the always_inline
    attribute, it can "receive" any number of arguments.