[PATCH] [SLSR] handle candidate form &B[i * S]

[hfinkel at anl.gov]

================
Comment at: lib/Transforms/Scalar/StraightLineStrengthReduce.cpp:366
@@ +365,3 @@
+      ConstantInt *RHS = nullptr;
+      // TODO: handle shl. e.g., we could treat (S << 2) as (S * 4).
+      if (match(Idx, m_Mul(m_Value(LHS), m_ConstantInt(RHS)))) {
----------------
jingyue wrote:
> hfinkel wrote:
> > Can't you use SCEV to do that here? If you do, then you get the shift case for free. If the problem here is the loss of the information on the NSW, please comment on that.
> > 
> NSW is one problem. The other problem, as I replied earlier, is that this may complicate the rewriting phase because the rewriting would have to translate SCEVs back to IR instructions. For example, suppose `S = a + b` and the immediate basis of `X = S * 4` is `Y = S * 3`. The current code simply rewrites `X` as `Y + S` without tracing into how S is computed. However, if we use SCEV, we would represent X as SCEV `(a + b) * 4` and Y as SCEV `(a + b) * 3`, and rewrite Y as SCEV `X + (a + b)`. After that, we would need to translate SCEV `X + (a + b)` back into IR instructions `X + (a + b)` and further into `X + S`. While I think this is doable, given the relatively simple pattern matching this pass currently has, I don't feel it's worthwhile leveraging SCEVExpander or such to rewrite SCEV into IR instructions. 
Okay, please add a detailed comment about this in the code.

http://reviews.llvm.org/D7459

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/