[LLVMdev] static functions and optimization

[Reed Kotler]

On 07/24/2013 03:45 PM, reed kotler wrote:
> I have some stub functions that are essentially static, but they cannot
> be removed.
>
> What linkage type should I use in that case. Internal linkage appears to
> get the functions discarded if they are not referenced under certain
> optimizations.
>
> This is part of the gcc mips16 hack for floating point.
>
> For example, a function like floor from the math library will be called
> with an external reference to function floor.
>
> At that time, the compiler does not know whether floor was compiled as
> mips16 or mips32.
>
This description below is not completely correct. But the issue remains 
nevertheless that some of the static function stubs are being optimized 
below.

I have filed a bug against myself to finally document this mips16/32 
floating point interoperability scheme. Even after implementing it, I 
forget parts of how it works if I have not worked on it for a while.

> It generates the call to floor as if it is a mips16 compiled function.
>
> It also generates a stub that the linker can use if during link time it
> is discovered that "floor" is a mips32 function.
>
> The stubs, which are mips32 code, will move the first argument register
> into a floating point register, call floor, and upon return will move
> the integer return value into a floating point register.
>
> If I designate this function as having internal linkage, then in some
> cases, the optimizer will throw it away.
>
> In that case, at link time it will call floor, and if floor is compiled
> as mips32, this will break a mips16 compiled program.
>
> The linker does not know there is an issue because only functions with
> certain kinds of signatures need a helper function for the call.