[openib-general] Re: uverbs events

[Grant Grundler]

On Mon, Apr 11, 2005 at 08:14:12PM -0700, Roland Dreier wrote:
>     Grant> doorbell[] is a local variable and mthca_write64() is
>     Grant> static inline.  I don't see a problem with the assignments
>     Grant> to doorbell getting optimized out since the scope of that
>     Grant> variable is completely visible to gcc. A smart compiler
>     Grant> would just use registers and reduce the 32-bit stores.
> 
> Actually, what is that the compiler sees that we write to doorbell[]
> as a uint32_t but then read from it by dereferencing a uint64_t*.  -O2
> turns on -fstrict-aliasing, which allows the compiler to assume that
> pointers of different types never alias each other.  So gcc says, hey,
> all you do is write to that local doorbell[] variable and never do
> anything with the values you write, so I'll just throw away that dead
> code.  So gcc ends up only generating code for the store in
> mthca_write64() without any code to initialize doorbell[].

Yup - I saw your followup right after I posted.
But this is a better explanation...thanks!

grant