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From: "sgunderson at bigfoot dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/49872] New: Missed optimization: Could coalesce neighboring memsets
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49872

           Summary: Missed optimization: Could coalesce neighboring
                    memsets
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: sgunderson@bigfoot.com


Given the following code:

#include <string.h>

struct S {
    int f[1024];
    int g[1024];
};

void func(struct S* s)
{
    memset(s->f, 0, sizeof(s->f));
    memset(s->g, 0, sizeof(s->g));
}

GCC currently generates two memsets. The code with -O2 is a bit hard to read,
so I'm just pasting the -Os assembly for clarity:

00000000 <func>:
   0:    55                       push   %ebp
   1:    31 c0                    xor    %eax,%eax
   3:    89 e5                    mov    %esp,%ebp
   5:    b9 00 04 00 00           mov    $0x400,%ecx
   a:    57                       push   %edi
   b:    8b 7d 08                 mov    0x8(%ebp),%edi
   e:    f3 ab                    rep stos %eax,%es:(%edi)
  10:    8b 55 08                 mov    0x8(%ebp),%edx
  13:    66 b9 00 04              mov    $0x400,%cx
  17:    81 c2 00 10 00 00        add    $0x1000,%edx
  1d:    89 d7                    mov    %edx,%edi
  1f:    f3 ab                    rep stos %eax,%es:(%edi)
  21:    5f                       pop    %edi
  22:    5d                       pop    %ebp
  23:    c3                       ret    

Ideally GCC should also be able to coalesce this together with memsets not
written as memset, e.g. s->g[0] = 0;.