message: [XLA:CPU] Replace woo-woo explanation for slowness of alternative exp implementation.
  I think the "nonlocal effects on the pipeline" explanation is wrong.  A simpler and more plausible explanation (per acbrown) is that we're simply manipulating subnormals, and this is what's slow.
diff: @@ -180,9 +180,8 @@ llvm::Value* GenerateVF32Exp(llvm::IRBuilder<>* b, llvm::Value* input,
  // e^a * 2^n = e^a * 2^(n1 + n2)
  // = (2^n1 * e^a) * 2^n2.
  //
  - // But it turns out this approach is quite slow. It's not clear why; our
  - // hypothesis is that the integer operations on the exponent `n` have nonlocal
  - // effects on the pipeline.
  + // But it turns out this approach is quite slow, probably because it
  + // manipulates subnormal values.
  //
  // The approach we use instead is to clamp n to [-127, 127]. Let n' be the
  // value of n clamped to [-127, 127]. In the case where n' = 127, `a` can grow
  