bpf: facilitate constant propagation of function addresses

bpf: facilitate constant propagation of function addresses

eBPF effectively supports two kind of call instructions:

- The so called pseudo-calls ("bpf to bpf").
- External calls ("bpf to kernel").

The BPF call instruction always gets an immediate argument, whose
interpretation varies depending on the purpose of the instruction:

- For pseudo-calls, the immediate argument is interpreted as a
  32-bit PC-relative displacement measured in number of 64-bit words
  minus one.

- For external calls, the immediate argument is interpreted as the
  identification of a kernel helper.

In order to differenciate both flavors of CALL instructions the SRC
field of the instruction (otherwise unused) is abused as an opcode;
if the field holds 0 the instruction is an external call, if it holds
BPF_PSEUDO_CALL the instruction is a pseudo-call.

C-to-BPF toolchains, including the GNU toolchain, use the following
practical heuristic at assembly time in order to determine what kind
of CALL instruction to generate: call instructions requiring a fixup
at assembly time are interpreted as pseudo-calls.  This means that in
practice a call instruction involving symbols at assembly time (such
as `call foo') is assembled into a pseudo-call instruction, whereas
something like `call 12' is assembled into an external call
instruction.

In both cases, the argument of CALL is an immediate: at the time of
writing eBPF lacks support for indirect calls, i.e. there is no
call-to-register instruction.

This is the reason why BPF programs, in practice, rely on certain
optimizations to happen in order to generate calls to immediates.
This is a typical example involving a kernel helper:

  static void * (*bpf_map_lookup_elem)(void *map, const void *key)
    = (void *) 1;

  int foo (...)
  {
    char *ret;

    ret = bpf_map_lookup_elem (args...);
    if (ret)
      return 1;
    return 0;
  }

Note how the code above relies on the compiler to do constant
propagation so the call to bpf_map_lookup_elem can be compiled to a
`call 1' instruction.

While GCC provides a kernel_helper function declaration attribute that
can be used in a robust way to tell GCC to generate an external call
despite of optimization level and any other consideration, the Linux
kernel bpf_helpers.h file relies on tricks like the above.

This patch modifies the BPF backend to avoid SSA sparse constant
propagation to be "undone" by the expander loading the function
address into a register.  A new test is also added.

Tested in bpf-unknown-linux-gnu.
No regressions.

gcc/ChangeLog:

	PR target/106733
	* config/bpf/bpf.cc (bpf_legitimate_address_p): Recognize integer
	constants as legitimate addresses for functions.
	(bpf_small_register_classes_for_mode_p): Define target hook.

gcc/testsuite/ChangeLog:

	PR target/106733
	* gcc.target/bpf/constant-calls.c: Rename to ...
	* gcc.target/bpf/constant-calls-1.c: and modify to not expect
	failure anymore.
	* gcc.target/bpf/constant-calls-2.c: New test.