Builtins-vs-libcalls-vs-optimised-away-vs-still-emitting-an-undefined-symbol-reference

["Dave Korn" <dave dot korn at artimi dot com>]

    Hi everyone, 

  Although I'm seeing this on 3.3.3, it appears to be determined by the
backend, so I think it's still reasonable to ask:

  I have some code that calls __builtin_ffs, but entirely on compile-time
constants.  When I compile it (using an inhouse custom ELF-target backend)
with -O2, it gets totally optimised away, but the compiler still emits an
undefined symbol reference to the library ffs() function:

---------------------------------<snip!>---------------------------------
/tmp $ ${libexecdir}/3.3.3/cc1.exe -O2 - -o -

int foo ()
{
  return __builtin_ffs (32);
}

        .file   "<stdin>"
 foo    .global _ffs

        .text
        .align 2
        .proc   _foo
        .global _foo
        .type   _foo, @function
_foo:
        addi    r1,r0,0x6
        jr      r15
        .endproc _foo
        .size   _foo, .-_foo

        .ident  "GCC: (GNU) 3.3.3 (artimi-1.19)"
---------------------------------<snip!>---------------------------------

  When I retarget the same gcc at x86/cygwin, no such reference is emitted:

---------------------------------<snip!>---------------------------------
/tmp $ /repository/gcc-build/gcc-x86/gcc/cc1.exe    -O2  - -o -
int foo ()
{
  return __builtin_ffs (32);
}

        .file   "<stdin>"
 foo    .text
        .p2align 4,,15
.globl _foo
        .def    _foo;   .scl    2;      .type   32;     .endef
_foo:
        pushl   %ebp
        movl    $6, %eax
        movl    %esp, %ebp
        popl    %ebp
        ret
---------------------------------<snip!>---------------------------------

  Does anyone know off the top of their head if this is simply a side-effect
of the fact that i386.md supplies an rtl insn for ffs, where my custom target
doesn't?  Or is it perhaps an elf-vs-PE difference?  I couldn't find anything
to explain this behaviour in the manual.

  I can kind-of work around it by tricking the compiler into thinking
__builtin_ffs can be elided (as no body is ever required by a static
inline[*])

---------------------------------<snip!>---------------------------------
/tmp $ ./cc1.exe -O2 - -o -
static inline int __builtin_ffs (int);
int foo ()
{
  return __builtin_ffs (32);
}

        .file   "<stdin>"
 foo
        .text
        .align 2
        .proc   _foo
        .global _foo
        .type   _foo, @function
_foo:
        ; 010000000000000000000000000000000000000000000000000000000000000
        ; frame size 0: local_vars_size 0 out_args_size 0 pretend_args_size 0
        ; mul_save_size 0 gpr_save_size 0 lr_save_ 0 iar_save_ 0 fp_save_ 0
        addi    r1,r0,0x20
        j       ___builtin_ffs ; sibcall_value insn ;
        .endproc _foo
        .size   _foo, .-_foo
?

<stdin>:1: warning: `__builtin_ffs' used but never defined
        .ident  "GCC: (GNU) 3.3.3 (artimi-1.19)"
---------------------------------<snip!>---------------------------------

but that leaves me with an ugly warning.  Does anyone know a better way to
eliminate the undefined reference?  (I'm currently just supplying a dummy
version of _ffs() in my source solely in order to satisfy the linker).

    cheers,
      DaveK

[*] - before the recent changes for C99 compatibility, of course!
-- 
Can't think of a witty .sigline today....