[mlir][scf] Fix for-loop-peeling crash (#77697)

Before applying the peeling patterns, it can happen that the `ForOp`
gets a step of zero during folding. This leads to a division-by-zero
down the line.

This patch adds an additional check for a constant-zero step and a
 test.

Fix #75758

Author: ubfx

mlir/lib/Dialect/SCF/Transforms/LoopSpecialization.cpp

@@ -123,8 +123,9 @@ static LogicalResult peelForLoop(RewriterBase &b, ForOp forOp,
   auto ubInt = getConstantIntValue(forOp.getUpperBound());
   auto stepInt = getConstantIntValue(forOp.getStep());
 
-  // No specialization necessary if step size is 1.
-  if (getConstantIntValue(forOp.getStep()) == static_cast<int64_t>(1))
+  // No specialization necessary if step size is 1. Also bail out in case of an
+  // invalid zero or negative step which might have happened during folding.
+  if (stepInt && *stepInt <= 1)
     return failure();
 
   // No specialization necessary if step already divides upper bound evenly.

mlir/test/Dialect/SCF/for-loop-peeling.mlir

@@ -1,5 +1,5 @@
-// RUN: mlir-opt %s -scf-for-loop-peeling -canonicalize -split-input-file | FileCheck %s
-// RUN: mlir-opt %s -scf-for-loop-peeling=skip-partial=false -canonicalize -split-input-file | FileCheck %s -check-prefix=CHECK-NO-SKIP
+// RUN: mlir-opt %s -scf-for-loop-peeling -canonicalize -split-input-file -verify-diagnostics | FileCheck %s
+// RUN: mlir-opt %s -scf-for-loop-peeling=skip-partial=false -canonicalize -split-input-file -verify-diagnostics | FileCheck %s -check-prefix=CHECK-NO-SKIP
 
 //  CHECK-DAG: #[[MAP0:.*]] = affine_map<()[s0, s1, s2] -> (s1 - (-s0 + s1) mod s2)>
 //  CHECK-DAG: #[[MAP1:.*]] = affine_map<(d0)[s0] -> (-d0 + s0)>
@@ -289,3 +289,18 @@ func.func @regression(%arg0: memref<i64>, %arg1: index) {
   }
   return
 }
+
+// -----
+
+// Check that this doesn't crash but trigger the verifier.
+func.func @zero_step(%arg0: memref<i64>) {
+  %c0 = arith.constant 0 : index
+  %c1 = arith.constant 1 : index
+  %foldto0 = arith.subi %c1, %c1 : index
+  // expected-error @+1 {{'scf.for' op constant step operand must be positive}}
+  scf.for %arg2 = %c0 to %c1 step %foldto0 {
+    %2 = arith.index_cast %arg2 : index to i64
+    memref.store %2, %arg0[] : memref<i64>
+  }
+  return
+}