[PATCH] D59252: [TTI] getMemcpyCost

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


================
Comment at: include/llvm/Analysis/TargetTransformInfoImpl.h:144
+  unsigned getMemcpyCost(const Instruction *I) {
+    return TTI::TCC_Basic;
+  }
----------------
SjoerdMeijer wrote:
> t.p.northover wrote:
> > That seems optimistic, even as a default. I'd probably bump it to TCC_Expensive if just one load/store is needed, and a size-based estimate would be even better (is there a DataLayout handy to calculate length in terms of pointer-words?)
> Thanks for looking into this.  Agreed that TCC_Basic doesn't make much sense.
> 
> As a default size based estimate, I am thinking of this default:
> 
>    (2 * TCC_Basic)  * (memcpy-count / pointer-size)
> 
> where:
> (2 * TCC_Basic) is a load/store pair, and 
> (memcpy-count / pointer-size) models the number of load/stores needed
> 
> Since this will change behaviour, I probably need a test somewhere. :-)
A pointer size isn't relevant. We already have things like getMemcpyLoopLoweringType for when memcpy is expanded. The cost should also be much higher for unknown amounts


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  https://reviews.llvm.org/D59252/new/

https://reviews.llvm.org/D59252