[Bug tree-optimization/85330] New: -Os generates bigger code than -O2 due to disabled strlen pass

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85330

            Bug ID: 85330
           Summary: -Os generates bigger code than -O2 due to disabled
                    strlen pass
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When the test case below is compiled with -O2, GCC emits object code that's 
optimally efficient both in space and runtime.  But when the same test case is
compiled with -Os it results in much bigger object code that will run far
slower than the -O2 equivalent.  That's because the strlen optimization doesn't
run with -Os.

At the same time, it's possible to create test cases involving string functions
where the opposite is true (often involving long strings).  I wonder if
enabling the strlen optimization to a limited extent at -Os (to at least track
string lengths without other transformations) would give closer to optimal
results.

$ cat z.c && gcc -O2 -fdump-tree-optimized=/dev/stdout -c z.c && objdump -d z.o
&& gcc -Os -fdump-tree-optimized=/dev/stdout -c z.c && objdump -d z.o
#define S "012345678"

int f (void)
{
  char a[sizeof S];
  __builtin_strcpy (a, S);
  return __builtin_strlen (a);
}

;; Function f (f, funcdef_no=0, decl_uid=1958, cgraph_uid=0, symbol_order=0)

f ()
{
  <bb 2> [local count: 1073741825]:
  return 9;

}



z.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <f>:
   0:   b8 09 00 00 00          mov    $0x9,%eax
   5:   c3                      retq   

;; Function f (f, funcdef_no=0, decl_uid=1958, cgraph_uid=0, symbol_order=0)

f ()
{
  char a[10];
  long unsigned int _1;
  int _4;

  <bb 2> [local count: 1073741825]:
  __builtin_strcpy (&a, "012345678");
  _1 = __builtin_strlen (&a);
  _4 = (int) _1;
  a ={v} {CLOBBER};
  return _4;

}



z.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <f>:
   0:   48 83 ec 18             sub    $0x18,%rsp
   4:   be 00 00 00 00          mov    $0x0,%esi
   9:   48 8d 54 24 06          lea    0x6(%rsp),%rdx
   e:   48 89 d7                mov    %rdx,%rdi
  11:   e8 00 00 00 00          callq  16 <f+0x16>
  16:   48 83 c9 ff             or     $0xffffffffffffffff,%rcx
  1a:   48 89 c2                mov    %rax,%rdx
  1d:   31 c0                   xor    %eax,%eax
  1f:   48 89 d7                mov    %rdx,%rdi
  22:   f2 ae                   repnz scas %es:(%rdi),%al
  24:   48 83 c4 18             add    $0x18,%rsp
  28:   48 89 c8                mov    %rcx,%rax
  2b:   48 f7 d0                not    %rax
  2e:   48 ff c8                dec    %rax
  31:   c3                      retq