Volatile Memory accesses in Branch Delay Slots

[Oleg Endo]

On Tue, 2017-07-25 at 10:47 +0200, Jakob Wenzel wrote:
>Â 
> jr's delay slot is not filled. However, if the declaration of a is 
> changed to `extern int a`, the delay slot is filled with the sw.
> 
> The function responsible for this behavior seems to be 
> resource_conflicts_p in reorg.c. Sadly, I could not find any
> comments 
> explaining why volatile accesses cannot be put into delay slots.
> 
> What is the reason for this behavior? I am unable to think of any 
> situation where allowing volatile memory accesses in branch delay
> slots  leads to problems. Am I missing a case? Or are negative
> effects limited  to other architectures?

Maybe because the code that does the delay slot stuffing does not do
sophisticated checks whether such instruction reordering would not
violate anything? Â So it's playing safe and bails out if it sees
"volatile mem". Â Same thing happens also with insns that have multiple
sets. Â Ideally it should do some more fine grained checks and give the
backend an option to opt-in or opt-out.

Cheers,
Oleg