[MLIR][SCFToGPU] Guard operands before AffineApplyOp::create to avoid crash (#167959)

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

Signed-off-by: Shashi Shankar <[email protected]>

Author: shashforge

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()) {

mlir/test/Conversion/SCFToGPU/parallel_loop.mlir

@@ -673,3 +673,51 @@ func.func @nested_parallel_with_side_effect() {
 
 // CHECK: gpu.launch
 // CHECK-NOT: scf.parallel
+
+// -----
+
+func.func @scf2gpu_index_creation_2d() {
+  %c0  = arith.constant 0 : index
+  %c1  = arith.constant 1 : index
+  %c32 = arith.constant 32 : index
+
+  // Single 2-D scf.parallel mapped to block_x and thread_x.
+  // Use both IVs so the conversion must compute 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
+  } {
+    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 @scf2gpu_index_creation_2d
+//       CHECK:   gpu.launch
+//       CHECK:     %[[IDX:.*]] = affine.apply
+//       CHECK:     arith.addi %[[IDX]],
+
+// -----
+
+func.func @scf2gpu_index_creation_1d() {
+  %c0  = arith.constant 0 : index
+  %c1  = arith.constant 1 : index
+  %c64 = arith.constant 64 : index
+
+  scf.parallel (%t) = (%c0) to (%c64) step (%c1) {
+    %w = arith.addi %t, %c0 : index
+  } {
+    mapping = [
+      #gpu.loop_dim_map<processor = thread_x, map = (d0) -> (d0), bound = (d0) -> (d0)>
+    ]
+  }
+  return
+}
+
+// CHECK-LABEL: func @scf2gpu_index_creation_1d
+//       CHECK:   gpu.launch
+//       CHECK:     %[[IDX:.*]] = affine.apply
+//       CHECK:     arith.addi %[[IDX]],