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

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
Herald added a subscriber: mzolotukhin.

SCEV tends to keep its expressions as flat as possible, preferring (a * b * c * d) over ((a * b) * (c * d)).
As result, if we have something like

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

which actually calculates a ^ (2 ^ N), its SCEV will look like (a * a * a * ... * a), the amount of
operands is 2 ^ N. If we try to expand such an expression, we will naively produce approx. 2 ^ N
mul instructions. To avoid this situation, this patch teaches SCEV Expander to deal with such
expressions generating a Binary Pow instruction sequence:

Let N be https://reviews.llvm.org/P1 + https://reviews.llvm.org/P2 + ... + PK, where all P are powers of 2. Then a ^ (2 ^ N) = (a ^ https://reviews.llvm.org/P1) * (a ^ https://reviews.llvm.org/P2) * ... * (a ^ PK),
so the number of instructions becomes O(log(N)).


https://reviews.llvm.org/D34025

Files:
  lib/Analysis/ScalarEvolutionExpander.cpp
  test/Transforms/LoopStrengthReduce/X86/big_power.ll

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