Optimization flags not working (GCC 4.2.4)

[slnc <s at slnc dot me>]

I have a file called "main.c":

-- main.c ------------------------------------------
int main(void) {
	int i, j;

	for (i = 0; i < 10; i ++)
		j = 5;

	return 0;
}
----------------------------------------------------

When I compile it with "gcc -O1 -c -o main.o main.c" and I do an 
"objdump -d main.o" I get:

------------------------------------------------------
main.o:     file format elf64-x86-64

Disassembly of section .text:

0000000000000000 <main>:
   0:	31 c0                	xor    %eax,%eax
   2:	c3                   	retq
------------------------------------------------------


But when I try to use just only one optimization flag by compiling with 
"gcc -ftree-dce -c -o main.o main.c" the optimization flag seems to do 
nothing at all:

------------------------------------------------------
main.o:     file format elf64-x86-64

Disassembly of section .text:

0000000000000000 <main>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	c7 45 fc 00 00 00 00 	movl   $0x0,-0x4(%rbp)
   b:	eb 0b                	jmp    18 <main+0x18>
   d:	c7 45 f8 05 00 00 00 	movl   $0x5,-0x8(%rbp)
  14:	83 45 fc 01          	addl   $0x1,-0x4(%rbp)
  18:	83 7d fc 09          	cmpl   $0x9,-0x4(%rbp)
  1c:	7e ef                	jle    d <main+0xd>
  1e:	b8 00 00 00 00       	mov    $0x0,%eax
  23:	c9                   	leaveq
  24:	c3                   	retq
------------------------------------------------------


And even if I specify all the optimization flags that are enabled by -O1 
I get the same previous objdump. What am I doing wrong?

Thank you,

--
Juan Alonso