[PATCH] D105432: [Analysis] Add simple cost model for strict (in-order) reductions

[David Sherwood via Phabricator via llvm-commits]

david-arm added inline comments.


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Comment at: llvm/include/llvm/Analysis/TargetTransformInfo.h:1163
+  /// FastMathFlags parameter \p FPFlags together indicate what type of
+  /// reduction we are performing:
+  ///   1. Tree-wise. This is the typical 'fast' reduction performed that
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dmgreen wrote:
> This comment looks useful. I'm wondering if it's worth emphasizing a bit that these are the default lowerings, and the cost should be whatever the fastest way the target can legally lower the intrinsic would be. 
> Maybe spelling out that that float operations without Reassoc require ordered reductions that look like `((((init + v0) + v1) + v2) + ..`. And otherwise the reduction can happen in any order, which by default will follow a treewise reduction.
Hi @dmgreen, thanks for the suggestions. In the 1) case below I do state this is the default for integer operations and FP when reassociation is allowed. I was trying to list the two types of reduction, then explain which is the default. What I could do is emphasise early on that "Tree-wise" is the default, i.e

  1. Tree-wise. This is the default, 'fast' reduction ...

I can also mention after 2) that the cost should correspond to the fastest way the target can lower the intrinsic?


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  https://reviews.llvm.org/D105432/new/

https://reviews.llvm.org/D105432