Re: Remove noce_mem_write_may_trap_or_fault_p in ifcvt

[Bernd Schmidt <bschmidt@redhat.com>]

On 11/26/2015 10:46 AM, Richard Biener wrote:

> Ok with the change suggested by Micha for the asm()s.  Note that I
> originally used gimple_vuse () instead of gimple_vdef () as even
> reading random memory is a barrier for the compiler to move stores
> across it (not reads, of course).  Which is why I also considered
> pure (global memory reading) calls to be a barrier (for the stores).

Yes, but IIUC the stores aren't being moved, they may just be turned 
into unconditional ones. Being nontrapping is one necessary condition 
for that (which we already compute), but we also want to make sure that 
we don't introduce surprises for threaded programs.

> Of course as we don't consider regular assign statement reads (or stores)
> to be a "barrier" in the sense that matters here (we're not looking for
> memory optimization barriers!) this might be moot and then the
> middle-end will effectively require all synchronization barriers (which we
> are looking for(?)) to appear as clobbering memory.

If I read this correctly you have reached the same conclusions. Test 
results came back ok (with vdef tested for asms), and I've committed the 
change.


Bernd