[PATCH] D24833: [LoopDataPrefetch/AArch64] Allow selective prefetching of symbolic strided accesses

[Balaram Makam via llvm-commits]

bmakam added inline comments.

================
Comment at: lib/Target/AArch64/AArch64Subtarget.cpp:74-75
@@ -73,3 +73,4 @@
     PrefetchDistance = 740;
+    PrefetchDegree = 1;
     MinPrefetchStride = 1024;
     MaxPrefetchIterationsAhead = 11;
----------------
anemet wrote:
> So you are saying on one hand (MinPrefetchStride = 1024) that we shouldn't bother prefetching unless the stride is at least 1K but then you say (PrefetchDegree = 1) that you want to prefetch the very next cache line anytime the stride is not known at compile time.
> 
> I feel that there is a contradiction here.  The former suggest that you have a pretty powerful HW prefetcher, the latter that you don't and willing to speculate aggressively to compensate for it.
> 
> It seems that something is wrong with the model here.
Thanks for the review, Adam.

You are right, If the stride is unknown at compile time we speculate and prefetch the next cache line and if the stride is known we do not need to speculate so we conservatively prefetch for strides > 1K. 

Sorry, I do not see a contradiction here. Are you suggesting to insert runtime checks to determine that the unknown stride is > 1K and only then prefetch? I just do not see a justification for the additional complexity and probably it might hurt performance due to the runtime checks.


https://reviews.llvm.org/D24833