RE: [PATCH] Fix stack red zone bug (PR38644)

["Jiangning Liu" <jiangning dot liu at arm dot com>]

> Think of it this way.  What the IR says is there is no barrier between
> those moves.  You either have an implicit barrier (which is what you
> are proposing) or you have it explicitly.  I think we all rather have
> more things explicit rather than implicit in the IR.  And that has
> been the overall feeling for a few years now.
> 

Sorry, I'm afraid I can't agree with you. Instead, I think using barrier to describe this kind of dependence is a kind of implicit method. Please note that this is not an usual data dependence issue. The stack pointer change doesn't have any dependence with memory access at all.

No matter what solution itself is, the problem itself is a quite a common one on ISA level, so we should solve it in middle-end, because middle-end is shared for all ports. 

My proposal avoids problems in future. Any new ports and new back-end implementations needn't explicitly define this hook in a very safe way. But if one port wants to "unsafely" introduce red zone, we can explicitly define this hook in back-end. This way, we would avoid the bug in the earliest time. Do you really want to hide this problem in back-end silently? And wait others to report bug and then waste time to get it fixed again?

The facts I see is over the years, for different ports reported the similar issue around this, and somebody tried to fix them. Actually, this kind of problem should be fixed in design level. If the first people solve this bug can give solution in middle end, we would not need to waste time on filing those bugs in bug zilla and fixing them around this problem.

Thanks,
-Jiangning