[PATCH] D75982: [DAGCombine] Respect the uses when combine FMA for a*b+/-c*d

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

This doesn't look unreasonable to me now.



================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:11900-11919
   // fold (fsub (fmul x, y), z) -> (fma x, y, (fneg z))
-  if (isContractableFMUL(N0) && (Aggressive || N0->hasOneUse())) {
-    return DAG.getNode(PreferredFusedOpcode, SL, VT,
-                       N0.getOperand(0), N0.getOperand(1),
-                       DAG.getNode(ISD::FNEG, SL, VT, N1), Flags);
-  }
+  auto tryToFoldXYSubZ = [&](SDValue XY, SDValue Z) {
+    if (isContractableFMUL(XY) && (Aggressive || XY->hasOneUse())) {
+      return DAG.getNode(PreferredFusedOpcode, SL, VT, XY.getOperand(0),
+                         XY.getOperand(1), DAG.getNode(ISD::FNEG, SL, VT, Z),
+                         Flags);
+    }
----------------
I suggest to land this NFC refactoring first, to make the diff more understandable.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:11925
+      (N0.getNode()->use_size() > N1.getNode()->use_size())) {
+    // fold (fsub x, (fmul y, z)) -> (fma (fneg y), z, x)
+    if (SDValue V = tryToFoldXSubYZ(N0, N1))
----------------
```
// fold (fsub (fmul a, b), (fmul c, d)) -> (fma (fneg c), d, (fmul a, b))
```


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:11928
+      return V;
+    // fold (fsub (fmul x, y), z) -> (fma x, y, (fneg z))
+    if (SDValue V = tryToFoldXYSubZ(N0, N1))
----------------
```
// fold (fsub (fmul a, b), (fmul c, d)) -> (fma a, b, (fneg (fmul c, d)))
```



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  https://reviews.llvm.org/D75982/new/

https://reviews.llvm.org/D75982