[PATCH] D41940: [InstSimplify] Missed optimization in math expression: log10(pow(10.0, x)) == x, log2(pow(2.0, x)) == x

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: lib/Analysis/InstructionSimplify.cpp:4944
+        (match(Op0, m_Intrinsic<Intrinsic::exp2>(m_Value(X))) ||
+         match(Op0, m_Intrinsic<Intrinsic::pow>(m_SpecificFP(2.0),
+                                                m_Value(X))))) return X;
----------------
spatel wrote:
> Quolyk wrote:
> > lebedev.ri wrote:
> > > Hmm, do we want to canonicalize `Intrinsic::pow(2.0, x)` to `exp2(x)` instead ?
> > If `Intrinsic::pow(2.0, x)` to `exp2(x)` transform is useful somewhere else or you have strong opinion about it we do. Otherwise I don't see the reason for this.
> It's a specialized form of the more general math function, so we can assume it has a faster implementation. I think we should add that fold and reduce this patch.
There is a counter-argument to that: (that i have thought of after asking, but did not post)
do we *know* that instcombine would have always been run after such an Intrinsic call was added,
and before this `simplifyUnaryIntrinsic()` was called?

I.e. is there some situation where we might end up with `simplifyUnaryIntrinsic()` being called
*before* instcombine had a chance to do that canonicalization?
If yes, do we care?



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  https://reviews.llvm.org/D41940/new/

https://reviews.llvm.org/D41940