[PATCH] D159499: [InstCombine] Fold or(phi1,phi2) into or(icmp1,icmp2)

[Biplob Mishra via Phabricator via llvm-commits]

bipmis added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:2030
+                      m_ICmp(EqPred, m_Specific(operand->get()), m_Zero())) &&
+                EqPred == ICmpInst::ICMP_NE))))
+          return nullptr;
----------------
goldstein.w.n wrote:
> This seems very much so like a DAG type fold where you are basing the decision on existing expressions.
> `phi` doesn't exist in backend (dagcombiner) so its a bit tricky.
> @nikic, any place this can be put where it doesn't req iterating the use-list?
> 
> generally in instcombine we get our duplication by just blindly canoniclizing to a standard format.
> 
> Is there a regression if you blindly do the transform?
I am seeing quite some benefit with this Transform as we are able to reuse instructions which happens to be the other use of phi.
However just the direct transform 
or(i64 a,i64 b) -> or((icmp ne a ,0), (icmp ne b ,0)) is expensive. On the contrary the reverse is true in InstCombine.

I can check if the transform or(i64 phia,i64 phib) regresses.


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  https://reviews.llvm.org/D159499/new/

https://reviews.llvm.org/D159499