[PATCH] D34025: [SCEV] Teach SCEVExpander to expand BinPow

[Daniel Neilson via Phabricator via llvm-commits]

dneilson added a comment.

Have you looked at what happens with addition? i.e.

  x = a;
  for (i = 0; i < N; i++) x = x + x;

Should still be an exponential number of additions, when it could be just (x * (N << 1)).



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Comment at: lib/Analysis/ScalarEvolutionExpander.cpp:766
+    // 1u << 31 at most to not deal with unsigned overflow.
+    while (E != OpsAndLoops.end() && *I == *E && Exponent != MaxExponent) {
+      ++Exponent; ++E;
----------------
How challenging would it be to enhance SCEVMulExpr to represent repeated multiplies by the same value as a pair (value, exponent) rather than as a list of the same value being repeated 'exponent' times?

This patch has addressed not having to expand a ^ (2 ^ N) at runtime, but we still have an exponential number of terms in the OpsAndLoops list, which should quite negatively affect compile time.


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Comment at: lib/Analysis/ScalarEvolutionExpander.cpp:773
+    // that are needed into the result.
+    Value *P = expandCodeFor(I->second, Ty);
+    Value *Result = nullptr;
----------------
If Ty is a float type and N isn't a trivial value (like 1 or 2) then it can be better (faster & more accurate) to generate a call to @llvm.powi rather than the tree of multiplies.


https://reviews.llvm.org/D34025