[MLIR][SCFToGPU] Guard operands before AffineApplyOp::create to avoid crash

[shashforge]

This fixes a crash in SCF→GPU when building the per‑dim index for mapped scf.parallel.

Change:

• Map step/lb through cloningMap, then run ensureLaunchIndependent.
• If either is still unavailable at launch scope, emit a match‑failure; otherwise build the affine.apply.

Why this is correct:

• Matches how the pass already handles launch bounds; avoids creating an op with invalid operands and replaces a segfault with a clear diagnostic.

Tests:

• Added two small regressions that lower to gpu.launch and exercise the affine.apply path.

Fixes : #167654

[llvmbot]

@llvm/pr-subscribers-mlir

@llvm/pr-subscribers-mlir-gpu

Author: Shashi Shankar (shashforge)

Changes

Description
This patch fixes a crash in the SCF -> GPU conversion when building the per‑dimension index for GPU‑mapped scf.parallel loops.

What was wrong

We called AffineApplyOp::create(...) with operands coming from ensureLaunchIndependent(...).

When step or lowerBound weren’t available at the gpu.launch scope, ensureLaunchIndependent returned an empty Value.

Passing that empty value into AffineApplyOp::create caused a crash.

What I changed

Map step and lowerBound through cloningMap.lookupOrDefault(...) first.

Then run ensureLaunchIndependent(...) on the mapped values.

If either is still not available, emit a clear notifyMatchFailure and return failure() (no crash).

Otherwise, build AffineApplyOp::create with the validated operands

Fixes : #167654

---

Full diff: https://github.com/llvm/llvm-project/pull/167959.diff

3 Files Affected:

• (modified) mlir/lib/Conversion/SCFToGPU/SCFToGPU.cpp (+16-2)
• (added) mlir/test/Conversion/SCFToGPU/parallel-to-gpu-crash-regression.mlir (+31)
• (added) mlir/test/Conversion/SCFToGPU/parallel-to-gpu-index-creation.mlir (+24)

diff --git a/mlir/lib/Conversion/SCFToGPU/SCFToGPU.cpp b/mlir/lib/Conversion/SCFToGPU/SCFToGPU.cpp
index 76a822b05a652..309121f520811 100644
--- a/mlir/lib/Conversion/SCFToGPU/SCFToGPU.cpp
+++ b/mlir/lib/Conversion/SCFToGPU/SCFToGPU.cpp
@@ -453,10 +453,24 @@ static LogicalResult processParallelLoop(
           1, 2,
           rewriter.getAffineDimExpr(0) * rewriter.getAffineSymbolExpr(0) +
               rewriter.getAffineSymbolExpr(1));
+      // Map through cloningMap first so we use values valid at the launch
+      // scope, then ensure they are launch-independent (or cloned constants).
+      Value mappedStep = cloningMap.lookupOrDefault(step);
+      Value mappedLowerBound = cloningMap.lookupOrDefault(lowerBound);
+
+      mappedStep = ensureLaunchIndependent(mappedStep);
+      mappedLowerBound = ensureLaunchIndependent(mappedLowerBound);
+
+      // If either cannot be made available above the launch, fail gracefully.
+      if (!mappedStep || !mappedLowerBound) {
+        return rewriter.notifyMatchFailure(
+            parallelOp, "lower bound / step must be constant or defined above "
+                        "the gpu.launch");
+      }
+
       newIndex = AffineApplyOp::create(
           rewriter, loc, annotation.getMap().compose(lowerAndStep),
-          ValueRange{operand, ensureLaunchIndependent(step),
-                     ensureLaunchIndependent(lowerBound)});
+          ValueRange{operand, mappedStep, mappedLowerBound});
       // If there was also a bound, insert that, too.
       // TODO: Check that we do not assign bounds twice.
       if (annotation.getBound()) {
diff --git a/mlir/test/Conversion/SCFToGPU/parallel-to-gpu-crash-regression.mlir b/mlir/test/Conversion/SCFToGPU/parallel-to-gpu-crash-regression.mlir
new file mode 100644
index 0000000000000..01daed159b6fd
--- /dev/null
+++ b/mlir/test/Conversion/SCFToGPU/parallel-to-gpu-crash-regression.mlir
@@ -0,0 +1,31 @@
+// RUN: mlir-opt %s --convert-parallel-loops-to-gpu | FileCheck %s
+
+// Goal: exercise the per-dim index computation
+//        newIndex = hardware_id * step + lowerBound
+// and ensure we see a gpu.launch and an affine.apply (no crash).
+
+module {
+  func.func @two_dim_parallel_mapped() {
+    %c0  = arith.constant 0 : index
+    %c1  = arith.constant 1 : index
+    %c32 = arith.constant 32 : index
+
+    // Single 2‑D scf.parallel. Each dimension is mapped to a GPU dim.
+    // We *use* both IVs so the conversion must build indices.
+    scf.parallel (%bx, %tx) = (%c0, %c0) to (%c32, %c32) step (%c1, %c1) {
+      %u = arith.addi %bx, %c0 : index
+      %v = arith.addi %tx, %c0 : index
+      // No explicit terminator: the parser inserts an empty scf.reduce.
+    } {
+      mapping = [
+        #gpu.loop_dim_map<processor = block_x,  map = (d0) -> (d0), bound = (d0) -> (d0)>,
+        #gpu.loop_dim_map<processor = thread_x, map = (d0) -> (d0), bound = (d0) -> (d0)>
+      ]
+    }
+    return
+  }
+}
+
+// CHECK-LABEL: func.func @two_dim_parallel_mapped
+// CHECK:       gpu.launch
+// CHECK:       affine.apply
diff --git a/mlir/test/Conversion/SCFToGPU/parallel-to-gpu-index-creation.mlir b/mlir/test/Conversion/SCFToGPU/parallel-to-gpu-index-creation.mlir
new file mode 100644
index 0000000000000..55e425a77c18f
--- /dev/null
+++ b/mlir/test/Conversion/SCFToGPU/parallel-to-gpu-index-creation.mlir
@@ -0,0 +1,24 @@
+// RUN: mlir-opt %s --convert-parallel-loops-to-gpu | FileCheck %s
+
+module {
+  func.func @one_dim_parallel_mapped() {
+    %c0  = arith.constant 0 : index
+    %c1  = arith.constant 1 : index
+    %c64 = arith.constant 64 : index
+
+    // 1‑D loop mapped to thread_x; use the IV to force index computation.
+    scf.parallel (%t) = (%c0) to (%c64) step (%c1) {
+      %w = arith.addi %t, %c0 : index
+      // Implicit empty scf.reduce terminator.
+    } {
+      mapping = [
+        #gpu.loop_dim_map<processor = thread_x, map = (d0) -> (d0), bound = (d0) -> (d0)>
+      ]
+    }
+    return
+  }
+}
+
+// CHECK-LABEL: func.func @one_dim_parallel_mapped
+// CHECK:       gpu.launch
+// CHECK:       affine.apply

[linuxlonelyeagle]

Some test-related suggestions.

[shashforge]

@linuxlonelyeagle @joker-eph — I’ve moved the tests to mlir/test/Conversion/SCFToGPU/parallel_loop.mlir and simplified the checks per the Testing Guide. When you have a chance, could you please take another look? Happy to iterate on any additional feedback. Thanks!

[github-actions]

🐧 Linux x64 Test Results

• 7099 tests passed
• 594 tests skipped

[linuxlonelyeagle]

Some changes required to the formatting.

[joker-eph]

LGTM, but please wait for @linuxlonelyeagle to have another look