[openib-general] Re: uverbs events

[Roland Dreier]

    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[].

 - R.