[Patch, avr] Relax LDS/STS to IN/OUT if symbol is in I/O address range

[Georg-Johann Lay]

Senthil Kumar Selvaraj schrieb:
> On Mon, Sep 29, 2014 at 08:08:37PM +0400, Denis Chertykov wrote:
>> 2014-09-29 14:45 GMT+04:00 Senthil Kumar Selvaraj
>>> The patch below adds linker relaxation support for rewriting LDS/STS
>>> instructions to IN/OUT where appropriate. The IN/OUT instruction is
>>> shorter by a couple of bytes, and executes a cycle quicker. The
>>> compiler already does this optimization for addresses known at
>>> compile time - this linker patch does it for the rest.
>>>
>>> The patch looks for R_AVR_16 relocations, and if it finds an
>>> LDS/STS instruction with the symbol value (i.e. address) in I/O range,
>>> rewrites it to use IN/OUT instead, adjusting the address for SFR offsets.
>>>
>>> The patch also includes a couple of test cases to verify that it works
>>> for tiny, mega and xmega archs, and to ensure I/O range check is
>>> implemented correctly.
>>>
>>> If ok, could someone commit please? I don't have commit access.
>> Generally, I don't like this.
>> The compiler already does this optimization.
>> May be somebody want to have LDS/STS instead of IN/OUT.
>> (Calc delay)
> 
> Hmm, but can't you make that argument for all linker relaxations (jmp ->
> rjmp, for e.g.)? Besides, this relaxation kicks in only if there is a
> relocation - a plain lds <register> <constant_address> won't get rewritten
> unless a .reloc directive is used to forcibly emit a relocation. And of
> course, the user can simply choose to NOT pass --relax to the linker.
> 
> The reason this patch came about was some (inline assembly) code in 
> avr-libc (wdt.h) that has conditional compilation branches for 
> emitting LDS/STS vs IN/OUT based on the device name. We figured it 
> would be simpler to let the linker deal with it instead.

Instead of mentioning each and every device in #if defined(), did you 
consider using an always inline function and just write

   if (addr < A0)
     {
        __asm __volatile__ (...
     }
   else if (addr < A1)
     {
        __asm __volatile__ (...
     }

GCC kicks out the dead alternatives even at -O0 because both addr and A0 
etc. are known at compile time.

You'll still have to write down all possible OUT / STS combinations, but 
once you've accomplished that there's no need to touch wdt.h for new 
devices -- except in the case when the device needs a complete different 
scheme for WDT which means you have to extend wdt.h anyway...

Johann