[Mlir-commits] [mlir] [mlir][scf] Fix crash in extractFixedOuterLoops with iter_args loops (PR #184106)

[Mehdi Amini]

https://github.com/joker-eph created https://github.com/llvm/llvm-project/pull/184106

The stripmineSink helper splices loop body operations into a new inner scf.for that has no iter_args. When the target loop carries iter_args, values yielded by the spliced body are moved inside the inner loop, but the outer loop's yield terminator still references those values, creating an SSA invariant violation. In debug builds this triggers the assertion
  use_empty() && "Cannot destroy a value that still has uses\!"
when the outer RewriterBase tries to erase the now-broken operations.

Fix: in extractFixedOuterLoops, skip the strip-mining transformation if any of the collected perfectly-nested loops have iter_args.
w
Add a regression test to parametric-tiling.mlir.

Fixes #129044

>From d9a19497643bd227cd2b2bfb0f4c72f6f34b4c04 Mon Sep 17 00:00:00 2001
From: Mehdi Amini <joker.eph at gmail.com>
Date: Mon, 2 Mar 2026 03:42:38 -0800
Subject: [PATCH] [mlir][scf] Fix crash in extractFixedOuterLoops with
 iter_args loops

The stripmineSink helper splices loop body operations into a new inner
scf.for that has no iter_args. When the target loop carries iter_args,
values yielded by the spliced body are moved inside the inner loop, but
the outer loop's yield terminator still references those values, creating
an SSA invariant violation. In debug builds this triggers the assertion
  use_empty() && "Cannot destroy a value that still has uses\!"
when the outer RewriterBase tries to erase the now-broken operations.

Fix: in extractFixedOuterLoops, skip the strip-mining transformation
if any of the collected perfectly-nested loops have iter_args.

Add a regression test to parametric-tiling.mlir.

Fixes #129044
---
 mlir/lib/Dialect/SCF/Utils/Utils.cpp        |  9 +++++++++
 mlir/test/Transforms/parametric-tiling.mlir | 19 +++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/mlir/lib/Dialect/SCF/Utils/Utils.cpp b/mlir/lib/Dialect/SCF/Utils/Utils.cpp
index 2dd7df09e0f22..e495b52b1a1ee 100644
--- a/mlir/lib/Dialect/SCF/Utils/Utils.cpp
+++ b/mlir/lib/Dialect/SCF/Utils/Utils.cpp
@@ -1349,6 +1349,15 @@ TileLoops mlir::extractFixedOuterLoops(scf::ForOp rootForOp,
   if (forOps.size() < sizes.size())
     sizes = sizes.take_front(forOps.size());
 
+  // The strip-mining transformation splices loop bodies into a new inner loop
+  // without threading iter_args.  If any of the collected loops carries
+  // iter_args, the splice would produce invalid IR (yielded values from the
+  // inner scope used in the outer terminator).  Skip the transformation in
+  // that case.
+  if (llvm::any_of(forOps,
+                   [](scf::ForOp op) { return !op.getInitArgs().empty(); }))
+    return {};
+
   // Compute the tile sizes such that i-th outer loop executes size[i]
   // iterations.  Given that the loop current executes
   //   numIterations = ceildiv((upperBound - lowerBound), step)
diff --git a/mlir/test/Transforms/parametric-tiling.mlir b/mlir/test/Transforms/parametric-tiling.mlir
index f6cace5397def..515d0af6c06db 100644
--- a/mlir/test/Transforms/parametric-tiling.mlir
+++ b/mlir/test/Transforms/parametric-tiling.mlir
@@ -127,3 +127,22 @@ func.func @triangular(%arg0: memref<?x?xf32>) {
   }
   return
 }
+
+// Verify that extractFixedOuterLoops silently skips loops with iter_args
+// instead of producing invalid IR (regression test for
+// https://github.com/llvm/llvm-project/issues/129044).
+
+// COMMON-LABEL: @loop_with_iter_args
+// COMMON: scf.for {{.*}} iter_args({{.*}}) -> (f32)
+func.func @loop_with_iter_args(%buffer: memref<1024xf32>, %lb: index,
+                               %ub: index, %step: index) -> f32 {
+  %initial_sum = arith.constant 0.0 : f32
+  // This loop has iter_args; strip-mining must not be applied.
+  %final_sum = scf.for %iv = %lb to %ub step %step
+      iter_args(%sum_iter = %initial_sum) -> (f32) {
+    %element = memref.load %buffer[%iv] : memref<1024xf32>
+    %updated_sum = arith.addf %sum_iter, %element : f32
+    scf.yield %updated_sum : f32
+  }
+  return %final_sum : f32
+}