[Bug rtl-optimization/50749] Auto-inc-dec does not find subsequent contiguous mem accesses

[amker.cheng at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50749

bin.cheng <amker.cheng at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amker.cheng at gmail dot com

--- Comment #15 from bin.cheng <amker.cheng at gmail dot com> ---
There must be another scenario for the example, and in this case example:

int test_0 (char* p, int c)
{
  int r = 0;
  r += *p++;
  r += *p++;
  r += *p++;
  return r;
}

should be translated into sth like:
  //...
  ldrb [rx]
  ldrb [rx+1]
  ldrb [rx+2]
  add rx, rx, #3
  //...
This way all loads are independent and can be issued on super scalar machine. 
Actuall for targets like arm which supports post-increment constant (other than
size of memory access), it can be further changed into:
  //...
  ldrb [rx], #3
  ldrb [rx-2]
  ldrb [rx-1]
  //...
For now auto-increment pass can't do this optimization.  I once have a patch
for this but benchmark shows the case is not common.

This case is common especially after loop unrolling and rtl passes deliberately
break down long dependence of RX, which I think is right.