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From: "evstupac at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/48052] loop not vectorized if index is "unsigned int"
Date: Tue, 23 Jun 2015 13:23:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48052

Stupachenko Evgeny <evstupac at gmail dot com> changed:

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

--- Comment #15 from Stupachenko Evgeny <evstupac at gmail dot com> ---
The commit caused regressions on some benchmarks. Test to reproduce:
(compilations flags: -Ofast)

int foo (int flag, char *a)                                                    
{                                                                              
  short i, j;                                                                  
  short l = 0;                                                                 
  if (flag == 1)                                                               
    l = 3;                                                                     

  for (i = 0; i < 4; i++)                                                      
    {                                                                          
      for (j = l - 1; j > 0; j--)                                              
        a[j] = a[j - 1];                                                       
      a[0] = i;                                                                
    }                                                                          
}

Here value of l is between 0 and 3, and therefore value of the innermost loop
bound (l - 1) is between -1 and 2.

After the commit the innermost loop is replaced with memmove call. This is
obviously not optimal as amount of memory to move is not greater than 2.