I'm trying to find out why in windows has so much more instructions for the same program than linux.
So I just used int a=0xbeef; and printf("test\n"); in C and compiled in Linux and Windows. When I debug and disassembly the main frame I got this:
On linux:
0x080483e4 <+0>: push %ebp
0x080483e5 <+1>: mov %esp,%ebp
0x080483e7 <+3>: and $0xfffffff0,%esp
0x080483ea <+6>: sub $0x20,%esp
0x080483ed <+9>: movl $0xbeef,0x1c(%esp)
0x080483f5 <+17>: movl $0x80484d0,(%esp)
0x080483fc <+24>: call 0x8048318 <puts@plt>
0x08048401 <+29>: leave
0x08048402 <+30>: ret
Ok that's nice. I see the movl 0x1c offset of esp to put the value there.
But in windows I got this:
0x401290 <main>: push %ebp
0x401291 <main+1>: mov %esp,%ebp
0x401293 <main+3>: sub $0x18,%esp
0x401296 <main+6>: and $0xfffffff0,%esp
0x401299 <main+9>: mov $0x0,%eax
0x40129e <main+14>: add $0xf,%eax
0x4012a1 <main+17>: add $0xf,%eax
0x4012a4 <main+20>: shr $0x4,%eax
0x4012a7 <main+23>: shl $0x4,%eax
0x4012aa <main+26>: mov %eax,0xfffffff8(%ebp)
0x4012ad <main+29>: mov 0xfffffff8(%ebp),%eax
0x4012b0 <main+32>: call 0x401720 <_alloca>
0x4012b5 <main+37>: call 0x4013c0 <__main>
0x4012ba <main+42>: movl $0xbeef,0xfffffffc(%ebp)
0x4012c1 <main+49>: movl $0x403000,(%esp,1)
0x4012c8 <main+56>: call 0x401810 <printf>
0x4012cd <main+61>: mov $0x0,%eax
0x4012d2 <main+66>: leave
0x4012d3 <main+67>: ret
First of all, I don't know why the windows compiler (mingw) generate so much code. Is 2x more than Linux... this makes me thinking. And another thing: from main+9 to main+37 I can't see the point of that code.
I would thank if someone answer to this, I'm just curious :)
edit: With -O3 argument on linux I got the same and in windows something like magic happends:
0x401290 <main>: push %ebp
0x401291 <main+1>: mov $0x10,%eax
0x401296 <main+6>: mov %esp,%ebp
0x401298 <main+8>: sub $0x8,%esp
0x40129b <main+11>: and $0xfffffff0,%esp
0x40129e <main+14>: call 0x401700 <_alloca>
0x4012a3 <main+19>: call 0x4013a0 <__main>
0x4012a8 <main+24>: movl $0x403000,(%esp,1)
0x4012af <main+31>: call 0x4017f0 <puts>
0x4012b4 <main+36>: leave
0x4012b5 <main+37>: xor %eax,%eax
0x4012b7 <main+39>: ret
leave then xor then ret. ok :D the call _alloca and call __main still there. and I don't know what is 0x401291 <main+1>: mov $0x10,%eax doing here :D