Re: [Bug c/22278] gcc -O2 discards cast to volatile

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

"neroden at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> writes:


[...]

| of an arbitrary memory address.  If the "underlying object" has  
| to be volatile, how do we make it volatile?  Can we simply create a new  
| "underlying object" by throwing in a temporary pointer variable?  Is something  
| like the following sufficient to force an access?  
|   
| static inline unsigned char myMmioIn8(volatile void* base,  
|                                       const unsigned long offset)  
| {  
|   volatile CARD8 * tmp = (CARD8*)base + offset;  
|   return *tmp;  
| }  

If is fragile.  If the compiler does more aggressive inline (which I
hope 4.0 doesn't, but 4.x may at some point) and discover that the
chains of conversion started with something it thinks really not
volatile, then it would behave the same way as with the macro.

| #  define MMIO_IN8(base, offset) myMmioIn8(base, offset)    
|   
| If that's not sufficient, I don't see any way to accomplish the required  
| behavior under gcc 4.0 (without massive rewriting of the X.org code), so I 
| hope it is.  

I've come to believe that GCC's current choice is needlessly
unhelpful.  I think  it is much more reasonable to have GCC leave its
optimizer fingers out of the access through volatile lvalue, whether
the object behind is really defined volatile of not -- GCC cannot
know and the programmer cannot if is the "object behind" is actually
never defined, but is mapped to a memory.  Unfortunately, I don't know
the extent of the GCC code that needs to be fixed so I don't know
whether it is fixable in 4.0.  

-- Gaby