[PATCH] D28535: Scale frequencies of a set of blocks

Chandler Carruth via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Wed Jan 18 17:56:24 PST 2017

chandlerc accepted this revision.
chandlerc added a comment.

In https://reviews.llvm.org/D28535#649966, @eraman wrote:

> Chandler, if you strongly prefer the use of ScaledNumber to APInt, I have a patch ready and will happily upload that. Even after this change, the multiply and divide operations allocate memory. Multiply seems similar in complexity (since we always multiply two 64 bit words). I don't know how Knuth's algorithm used in APInt compare with ScaledNumber's long division in the case when the dividend is more than 64 bits.  The tradeoff here is the precision loss with ScaledNumber - whose effects in practice is unknown. I personally prefer to keep the APInt, but as I said above ready to switch to ScaledNumber to get this patch in.

Sorry I missed your earlier update.

This isn't about using ScaledNumber to *directly* implement the algorithm you have, this is about using a different algorithm. Specifically, if you look at how BFI computes things, it goes to great lengths to accumulate and normalize the frequency for each block without doing division. Instead, it uses shifts and other techniques to scale things. But this isn't really compatible with the style of update you are using here.

Now that the memory allocation is gone, the part I am concerned with is the 'udiv'. Knuth's algorithm is still frighteningly expensive. But we can wait and revisit this once it shows up on a profile. Please comment clearly that this udiv is slow and that there are ways to remove it if it shows up for the future maintainer of the code.

I see three fundamental options that would eliminate this. Two are simpler but lose precision more rapidly, the other is more complex but retains precision:

1. Factor the division out of the loop into a pre-computed scale, and shift that scale and the input frequencies to maximize the precision that remains.
2. Reduce the precision a-priori to 32-bits so that a 64-bit multiply and divide suffice and then re-scale to the 64-bit values.
3. Recast this using the core BFI algorithm for normalizing block mass after scaling due to backedges (or inlining in this case). There might be a way to do this locally, I'm not sure. This would essentially mean adding live updates to the core BFI algorithm used.

#3 seems like it will have the best results but will be *very* hard to implement. Both #1 and #2 seem possible but I think its fine to wait and do that in a follow-up patch. I particularly like #1 because it isn't just hiding the cost in the hardware divide instruction which, for example, won't exist on 32-bit ARM hosts.

Anyways, feel free to submit now that the memory allocation is handled separately and with a comment that the udiv is a known likely hot spot.


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