[Mlir-commits] [mlir] [mlir][scf] Extend consumer fuse to nested loop structure (PR #94190)

[Prashant Kumar]

================
@@ -1289,28 +1351,108 @@ getUntiledConsumerFromSlice(tensor::ParallelInsertSliceOp candidateSliceOp) {
   return getConsumerFromUses(resultingValue, containingOp->getBlock());
 }
 
-/// This utility currently checks whether the loop either :-
-/// 1. Yields exactly one result.
-/// 2. Has consumer op as its first user and other users to be in the same
-/// containing block as that of consumer op's. Currently we clone the loop op
-/// right before the consumer op in order to maintain a valid def-use chain.
-/// This utility thus helps ensuring that no invalid IR is formed due to the
-/// same.
+/// This utility currently checks whether the first userOp of loop is NOT before
+/// the last defineOp of consumer. Currently we clone the loop op right before
+/// a certain op in order to maintain a valid def-use chain. This utility thus
+/// helps ensuring that no invalid IR is formed due to the same. E.g.
+///
+/// ```
+/// %0 = scf.for() {
+///
+/// }
+/// ...
+/// %1 = firstUserOfLoop(%0)
+/// ...
+/// %2 = lastDefOfConsumer
+/// ...
+/// %3 = consumerOp(%2)
+/// ```
+///
+/// If the `firstUserOfLoop`is before `lastDefOfConsumer`, then it would be
+/// invalid to clone the loop op right before the `firstUserOfLoop`:
+///
+/// ```
+/// %0:2 = scf.for() {
+///    %3 = tiledConsumerOp(%2)
+/// }
+/// %1 = firstUserOfLoop(%0)
+/// ...
+/// %2 = lastDefOfConsumer
+/// ```
+///
+/// To address this issue, this utility would double-check there is no user of
+/// `firstUserOfLoop` before `lastDefOfConsumer`. If so, moving
+/// `firstUserOfLoop` after `lastDefOfConsumer`. Then, it turns out valid as
+/// follow:
+///
+/// ```
+/// %2 = lastDefOfConsumer
+/// %0:2 = scf.for() {
+///    %3 = tiledConsumerOp(%2)
+/// }
+/// %1 = firstUserOfLoop(%0)
+/// ```
+///
+/// @param loopOp: loop operation
----------------
pashu123 wrote:

https://github.com/iree-org/iree/blob/16f7ebf3883fd319152518c067121fdabad00b03/compiler/src/iree/compiler/Dialect/Flow/Transforms/FuseHorizontalContractions.cpp#L324 

https://github.com/llvm/llvm-project/pull/94190