Hi,

Given the following test C++ file:

class Class
{
public:
	void func();

        float *buf;
        int size;
};

void Class::func()
{
        for (int i = 0; i < size; ++i) {
                buf[i] = 0;
	}
}

4.6 (see below for exact version) will generate larger code (36 vs. 30 bytes) than 4.5.1 (Debian 4.5.1-6) given -Os. The output is

00000000 <Class::func()>:
   0:	55                   	push   %ebp
   1:	31 c0                	xor    %eax,%eax
   3:	89 e5                	mov    %esp,%ebp
   5:	8b 4d 08             	mov    0x8(%ebp),%ecx
   8:	53                   	push   %ebx
   9:	8b 59 04             	mov    0x4(%ecx),%ebx
   c:	eb 10                	jmp    1e <Class::func()+0x1e>
   e:	8d 14 85 00 00 00 00 	lea    0x0(,%eax,4),%edx
  15:	40                   	inc    %eax
  16:	03 11                	add    (%ecx),%edx
  18:	c7 02 00 00 00 00    	movl   $0x0,(%edx)
  1e:	39 d8                	cmp    %ebx,%eax
  20:	7c ec                	jl     e <Class::func()+0xe>
  22:	5b                   	pop    %ebx
  23:	5d                   	pop    %ebp
  24:	c3                   	ret    

Basically the problem is that the lea is large (due to the zero immediate taking up 32 bits); 4.5 uses a variation where the address calculation takes both a base and an index register, which has a shorter form not requiring to store the zero. (The joys of x86; lea edx, [eax*4 + ecx] takes less space then lea edx, [eax*4]...)

===

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Thread model: posix
gcc version 4.6.0 20100828 (experimental) [trunk revision 163616] (Debian 20100828-1)