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

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...


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