Compile error when not using -ftree-ter

[Job Noorman]

On Tuesday 17 August 2010 16:15:40 you wrote:
> Job Noorman <jobnoorman@gmail.com> writes:
> > Consider the following code snippet:
> >     struct Foo
> >     {
> >     
> >         void foobar() {}
> >     
> >     };
> >     
> >     typedef void (*plain_foobar_t)(Foo*);
> >     
> >     void test()
> >     {
> >     
> >         asm("push %0;"
> >         
> >             : "i"((plain_foobar_t)&Foo::foobar));
> >     
> >     }
> >     
> >     int main(){}
> > 
> > If I compile this with
> > 
> >     g++ -Wno-pmf-conversions main.cpp
> > 
> > I get the following;
> > 
> >     main.cpp: In function ‘void test()’:
> >     main.cpp:12: warning: asm operand 0 probably doesn’t match
> >     constraints main.cpp:12: error: impossible constraint in ‘asm’
> 
> I would not expect this kind of thing to work when not optimizing.  You
> are requiring the compiler to see that the value is an integer constant,
> which the compiler can't reliably do when not optimizing.
> 
> It should work reliably with or without optimization if you use an "r"
> constraint, though of course that will introduce an additional
> instruction.
> 
> Ian
When I use the "r" constraint, it indeed works and GCC emits the following 
code:

    movl $_ZN3Foo6foobarEv, %eax
    push %eax;

What I find strange about this is that GCC has converted 
"(plain_foobar_t)&Foo::foobar" in a constant value ($_ZN3Foo6foobarEv) without 
using optimizations. So why can't it do the same when using the "i" 
constraint?

Job