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From: "urjaman at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/50749] Auto-inc-dec does not find subsequent contiguous mem accesses
Date: Thu, 17 Sep 2015 09:00:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50749

Urja Rannikko <urjaman at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |urjaman at gmail dot com

--- Comment #20 from Urja Rannikko <urjaman at gmail dot com> ---
I'll add a note here that this seems to also affect the AVR target when it is
under register pressure and cant use Z or Y registers which can do Z+n /Y+n
addressing, it ends up doing really stupid things with X register (aka r26:r27)
(which could post-inc):
adiw    r26, 0x01
ld      r24, X
sbiw    r26, 0x01
(followed by adiw r26, 2...)

This was my test case for showing this behaviour on AVR:
unsigned char test_0 (unsigned char* p1, unsigned char *p2, unsigned char *p3)
{
  unsigned char r = 0;
  r += *p1++;
  r += *p2++;
  r += *p3++;

  r += *p1++;
  r += *p2++;
  r += *p3++;

  r += *p1++;
  r += *p2++;
  r += *p3++;

  return r;
}

This note added for anyone later trying to google for this after seeing that X
reg stupidity on AVR, sorry for maybe-a-bit-noise ...