Merging identical functions in GCC

[Joe Buck]

On Wed, Sep 20, 2006 at 02:52:40AM -0300, Alexandre Oliva wrote:
> The linker already has code to merge sections.  There's nothing to
> stop us from compiling every function into say:
> 
> foo:
>   jmp .foo.impl
> .section .gnu.impl.foo, "axM"
> .foo.impl:
>   [actual code for foo emitted]
> .previous
> 
> Then the linker doesn't need any smarts whatsoever to merge functions
> with identical code.  Hmm, maybe it does, since it must compare
> relocations as well, but this doesn't sound too hard at first.
> 
> For extra bonus points, it may short-circuit relocations that call foo
> (as opposed to taking its address) when it binds locally and skip the
> jmp altogether.  Ditto for *all* relocations that reference foo, when
> it doesn't merge .foo.impl with anything else.

Alternatively, the compiler could arrange that pointers to functions always
point to a jump that then leads to the function, while direct calls
go directly to the function.  That way pointers compare unequal
but the code is merged.