Re: [boost] std::map::find() wrapper

Subject: Re: [boost] std::map::find() wrapper
From: Vicente BOTET (vicente.botet_at_[hidden])
Date: 2011-05-05 17:55:17

> Message du 05/05/11 23:50
> De : "John Bytheway" 
> A : boost_at_[hidden]
> Copie à : 
> Objet : Re: [boost] std::map::find() wrapper
> 
> On 05/05/11 19:35, Marsh Ray wrote:
> 
> > So I'm interpreting that as "Can you give an example of an actual C++
> > program where the compiler does not emit the same code for references as
> > the analogous code using plain pointers."
> > 
> > .... I played around with g++-4.6.0 and was not able to make it emit
> > different code in the short time I'm willing to try. G++ seems to miss
> > some opportunities for optimization of references, and seems to be smart
> > about tracking pointer values.
> 
> There's this:
> 
> struct A { int x; };
> struct B { int y; };
> struct C : A, B {};
> 
> C& f(B& b) {
> return static_cast(b);
> }
> 
> C* g(B* b) {
> return static_cast(b);
> }
> 
> which compiles to:
> 
> 0000000000000000 <_Z1fR1B>:
> 0: 48 8d 47 fc lea -0x4(%rdi),%rax
> 4: c3 retq
> 
> 0000000000000010 <_Z1gP1B>:
> 10: 48 8d 57 fc lea -0x4(%rdi),%rdx
> 14: 31 c0 xor %eax,%eax
> 16: 48 85 ff test %rdi,%rdi
> 19: 48 0f 45 c2 cmovne %rdx,%rax
> 1d: c3 retq
> 
> The compiler doesn't have to do a null-check for the reference case, so
> it's shorter.

I would be interested in knowing why the compiler need to check for 0 when you apply a static_cast? 

Best,
Vicente