Message-ID: <3B3A1E84.A12C5FCA@sensor.com>
From: Ron Natalie <ron AT sensor DOT com>
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Newsgroups: comp.os.msdos.djgpp
Subject: Re: DJGPP reserves wrong int size
References: <9dde68b7 DOT 0106241053 DOT 2a385311 AT posting DOT google DOT com> <3B3A1B39 DOT F4D0F58B AT stud DOT ntnu DOT no>
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Date: Wed, 27 Jun 2001 13:57:24 -0400
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To: djgpp AT delorie DOT com
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Kent Dahl wrote:

> > 00000000 55                          push ebp
> > 00000001 89E5                        mov ebp,esp
> > 00000003 83EC04                      sub esp,byte +0x18
> > 00000006 C745FC78563412              mov dword [ebp-0x4],0x12345678
> > 0000000D C9                          leave
> > 0000000E C3                          ret
> >
> > The third line reserves 18 bytes.
> 
> Does it? First of all, its 0x18 bytes, aka 24 bytes.
> Are you sure it isn't just padding for alignment?

If you read before posting, you'll see the third line allocates 4
bytes.  83 is the opcode for immediate subtract, where the EC indicates
the register ESP operand and 04 is the value 4 being s subtracted.
> 
> If I add yet another int, the number doesn't increase...
> 
> <CODE>
> int main ()
> {
> int i;
> i = 0x12345678;
> int j;
> j = 0xCAFEBABE;
> }
> </CODE>
> 
Gee, my G++ 2.91.66 does no such thing.  It allocates
exactly 4 (with the similar code as the original poster) and
8 with the above code.  Of course, this is really all quite
a silly discusion, if you turn on the optimizer, it emits

	main:
	pushl %ebp
	movl %esp,%ebp
	xorl %eax, %eax
	leave
	ret

I.e., it doesn't allocate anything.  It realizes you aren't doing
anything with either one of those variables and optimizes them
both away.