[PATCH] D47681: [DAGCombiner] Bug 31275- Extract a shift from a constant mul or udiv if a rotate can be formed

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:4831-4841
+///   (or (shrl (mul v c0) c1) (mul v c2)) ->
+///   (or (shrl (mul v c0) c1) (shl (mul v c0) c3))
+//
+///   (or (udiv v c0) (shl (udiv v c1) c2)) ->
+///   (or (shrl (udiv v c1) c3) (shl (udiv v c1) c2))
+//
+///   (or (shrl (shl v c0) c1) (shl v c2)) ->
----------------
This should be:
```
//   (or (shrl (mul v c0) c1)      (mul v c2)) ->
//   (or (shrl (mul v c0) c1) (shl (mul v c3) c4))
//
//   (or       (udiv v c0)     (shl (udiv v c1) c2)) ->
//   (or (shrl (udiv v c3) c4) (shl (udiv v c1) c2))
//
//   (or (shrl (shl v c0) c1)      (shl v c2)) ->
//   (or (shrl (shl v c0) c1) (shl (shl v c3) c4))
//
//   (or       (shrl v c0)     (shl (shrl v c1) c2)) ->
//   (or (shrl (shrl v c1) c3) (shl (shrl v c1) c2))
//
// NEW:
//   (or (shrl (shl v c0) c1)      (mul v c2)) ->
//   (or (shrl (shl v c0) c1) (shl (mul v c3) c4))
//
//   (or       (shrl v c0)     (shl (div v c1) c2)) ->
//   (or (shrl (shrl v c1) c3) (shl (div v c1) c2))
```
We may not always succeed in converting `mul`/`div` into a shift,



================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:4842-4843
+///   (or (shrl (shrl v c1) c3) (shl (shrl v c1) c2))
+static SDValue extractShift(SelectionDAG &DAG, SDValue OppShift,
+                            SDValue ExtractFrom, SDValue &Mask,
+                            const SDLoc &DL) {
----------------
You want to add a comment that it will be called for both the LHS and RHS,
so it does not have to worry about looking at the other side here.


https://reviews.llvm.org/D47681