[Mlir-commits] [mlir] [MLIR][SCF] Add canonicalization pattern to fold away iter args of scf.forall (PR #90189)

[Matthias Springer]

================
@@ -1509,6 +1522,177 @@ class ForallOpControlOperandsFolder : public OpRewritePattern<ForallOp> {
   }
 };
 
+/// The following canonicalization pattern folds the iter arguments of
+/// scf.forall op if :-
+/// 1. The corresponding result has zero uses.
+/// 2. The iter argument is NOT being modified within the loop body.
+/// uses.
+///
+/// Example of first case :-
+///  INPUT:
+///   %res:3 = scf.forall ... shared_outs(%arg0 = %a, %arg1 = %b, %arg2 = %c)
+///            {
+///                ...
+///                <SOME USE OF %arg0>
+///                <SOME USE OF %arg1>
+///                <SOME USE OF %arg2>
+///                ...
+///                scf.forall.in_parallel {
+///                    <STORE OP WITH DESTINATION %arg1>
+///                    <STORE OP WITH DESTINATION %arg0>
+///                    <STORE OP WITH DESTINATION %arg2>
+///                }
+///             }
+///   return %res#1
+///
+///  OUTPUT:
+///   %res:3 = scf.forall ... shared_outs(%new_arg0 = %b)
+///            {
+///                ...
+///                <SOME USE OF %a>
+///                <SOME USE OF %new_arg0>
+///                <SOME USE OF %c>
+///                ...
+///                scf.forall.in_parallel {
+///                    <STORE OP WITH DESTINATION %new_arg0>
+///                }
+///             }
+///   return %res
+///
+/// NOTE: 1. All uses of the folded shared_outs (iter argument) within the
+///          scf.forall is replaced by their corresponding operands.
+///       2. Even if there are <STORE OP WITH DESTINATION *> ops within the body
+///          of the scf.forall besides within scf.forall.in_parallel terminator,
+///          this canonicalization remains valid. For more details, please refer
+///          to :
+///          https://github.com/llvm/llvm-project/pull/90189#discussion_r1589011124
+///       3. TODO(avarma): Generalize it for other store ops. Currently it
+///          handles tensor.parallel_insert_slice ops only.
+///
+/// Example of second case :-
+///  INPUT:
+///   %res:2 = scf.forall ... shared_outs(%arg0 = %a, %arg1 = %b)
+///            {
+///                ...
+///                <SOME USE OF %arg0>
+///                <SOME USE OF %arg1>
+///                ...
+///                scf.forall.in_parallel {
+///                    <STORE OP WITH DESTINATION %arg1>
+///                }
+///             }
+///   return %res#0, %res#1
+///
+///  OUTPUT:
+///   %res = scf.forall ... shared_outs(%new_arg0 = %b)
+///            {
+///                ...
+///                <SOME USE OF %a>
+///                <SOME USE OF %new_arg0>
+///                ...
+///                scf.forall.in_parallel {
+///                    <STORE OP WITH DESTINATION %new_arg0>
+///                }
+///             }
+///   return %a, %res
+struct ForallOpIterArgsFolder : public OpRewritePattern<ForallOp> {
+  using OpRewritePattern<ForallOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(ForallOp forallOp,
+                                PatternRewriter &rewriter) const final {
+    // Step 1: For a given i-th result of scf.forall, check the following :-
+    //         a. If it has any use.
+    //         b. If the corresponding iter argument is being modified within
+    //            the loop, i.e. has at least one store op with the iter arg as
+    //            its destination operand. For this we use
+    //            ForallOp::getStoreOpUser(iter_arg).
+    //
+    //         Based on the check we maintain the following :-
+    //         a. `resultToDelete` - i-th result of scf.forall that'll be
+    //            deleted.
+    //         b. `resultToReplace` - i-th result of the old scf.forall
+    //            whose uses will be replaced by the new scf.forall.
+    //         c. `newOuts` - the shared_outs' operand of the new scf.forall
+    //            corresponding to the i-th result with at least one use.
+    SetVector<OpResult> resultToDelete;
+    SmallVector<Value> resultToReplace;
+    SmallVector<Value> newOuts;
+    for (OpResult result : forallOp.getResults()) {
+      OpOperand *opOperand = forallOp.getTiedOpOperand(result);
+      BlockArgument blockArg = forallOp.getTiedBlockArgument(opOperand);
+      if (result.use_empty() || forallOp.getStoreOpUser(blockArg).empty()) {
+        resultToDelete.insert(result);
+      } else {
+        resultToReplace.push_back(result);
+        newOuts.push_back(opOperand->get());
+      }
+    }
+
+    // Return early if all results of scf.forall has at least one use and being
+    // modified within the loop.
+    if (resultToDelete.empty()) {
+      return failure();
+    }
+
+    // Step 2: For the the i-th result, do the following :-
+    //         a. Fetch the corresponding BlockArgument.
+    //         b. Look for store ops (currently tensor.parallel_insert_slice)
+    //            with the BlockArgument as its destination operand.
+    //         c. Remove the operations fetched in b.
+    for (OpResult result : resultToDelete) {
+      OpOperand *opOperand = forallOp.getTiedOpOperand(result);
+      BlockArgument blockArg = forallOp.getTiedBlockArgument(opOperand);
+      SmallVector<Operation *> storeOps = forallOp.getStoreOpUser(blockArg);
+      for (Operation *storeOp : storeOps) {
+        rewriter.eraseOp(storeOp);
+      }
+    }
+
+    // Step 3. Create a new scf.forall op with the new shared_outs' operands
+    //         fetched earlier
+    auto newforallOp = rewriter.create<scf::ForallOp>(
----------------
matthias-springer wrote:

nit: `newForallOp`

https://github.com/llvm/llvm-project/pull/90189