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

[Denis Chertykov <chertykov at gmail dot com>]

2014-09-30 6:10 GMT+04:00 Senthil Kumar Selvaraj
<senthil_kumar.selvaraj@atmel.com>:
> On Mon, Sep 29, 2014 at 08:08:37PM +0400, Denis Chertykov wrote:
>> 2014-09-29 14:45 GMT+04:00 Senthil Kumar Selvaraj
>> <senthil_kumar.selvaraj@atmel.com>:
>> >
>> > 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.

I don't have a strong objections.
I'm worry about some code (i.e. inline assembly) which can be
surprisingly changed by linker and this code isn't a jump relocations
(which is predictable).

Denis.