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

[Ian Lance Taylor <iant at google dot com>]

"Dave Korn" <dave.korn@artimi.com> writes:

>   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

Is this really a reference?  It just looks like a declaration.  A
typical ELF assembler will not generate an undefined symbol merely
because it sees a .global pseudo-op.

It looks like this comes from an inappropriate call to
assemble_external.  You should find out what is calling that for _ffs.

Ian