[mlir][tensor] Fix runtime verification for tensor.extract_slice for empty tensor slices (llvm#166569)

I hit another runtime verification issue (similar to
llvm#164878) while working with
TFLite models. The verifier is incorrectly rejecting
`tensor.extract_slice` operations when extracting an empty slice
(size=0) that starts exactly at the tensor boundary.

The current runtime verification unconditionally enforces `offset <
dim_size`. This makes sense for non-empty slices, but it's too strict
for empty slices, causing false positives that lead to spurious runtime
assertions.

**Simple example that demonstrates the issue:**

```mlir
func.func @extract_empty_slice(%tensor: tensor<?xf32>, %offset: index, %size: index) {
  // When called with: tensor size=10, offset=10, size=0
  // Runtime verification fails: "offset 0 is out-of-bounds"
  %slice = tensor.extract_slice %tensor[%offset] [%size] [1] 
    : tensor<?xf32> to tensor<?xf32>
  return
}
```

For the above example, the check evaluates `10 < 10` which is false, so
verification fails. However, I believe this operation should be valid -
we're extracting zero elements, so there's no actual out-of-bounds
access.

**Real-world repro from the TensorFlow Lite models:**

This issue manifests while lowering TFLite models and a lot of our
system tests are failing due to this. Here's a simplified version
showing the problematic pattern:

In this code, `%extracted_slice_0` becomes an empty tensor when SSA
value `%15` reaches 10 (on the final loop iteration), making `%16 = 0`.
The operation extracts zero elements along dimension 0, which is
semantically valid but fails runtime verification.

```mlir
func.func @simplified_repro_from_tensorflowlite_model(%arg0: tensor<10x4x1xf32>) -> tensor<10x4x1xf32> {
  %c0 = arith.constant 0 : index
  %c1 = arith.constant 1 : index
  %c2 = arith.constant 2 : index
  %c10 = arith.constant 10 : index
  %c-1 = arith.constant -1 : index
  
  %0 = "tosa.const"() <{values = dense<0> : tensor<i32>}> : () -> tensor<i32>
  %1 = "tosa.const"() <{values = dense<1> : tensor<i32>}> : () -> tensor<i32>
  %2 = "tosa.const"() <{values = dense<10> : tensor<i32>}> : () -> tensor<i32>
  %3 = "tosa.const"() <{values = dense<-1> : tensor<2xi32>}> : () -> tensor<2xi32>
  %4 = "tosa.const"() <{values = dense<0> : tensor<2xi32>}> : () -> tensor<2xi32>
  %5 = "tosa.const"() <{values = dense<0.000000e+00> : tensor<1x4x1xf32>}> : () -> tensor<1x4x1xf32>
  %c4_1 = tosa.const_shape  {values = dense<1> : tensor<1xindex>} : () -> !tosa.shape<1>
  
  %6:2 = scf.while (%arg1 = %0, %arg2 = %arg0) 
    : (tensor<i32>, tensor<10x4x1xf32>) -> (tensor<i32>, tensor<10x4x1xf32>) {
    %7 = tosa.greater %2, %arg1 : (tensor<i32>, tensor<i32>) -> tensor<i1>
    %extracted = tensor.extract %7[] : tensor<i1>
    scf.condition(%extracted) %arg1, %arg2 : tensor<i32>, tensor<10x4x1xf32>
  } do {
  ^bb0(%arg1: tensor<i32>, %arg2: tensor<10x4x1xf32>):
    %7 = tosa.add %arg1, %1 : (tensor<i32>, tensor<i32>) -> tensor<i32>
    
    // First slice
    %8 = tosa.reshape %arg1, %c4_1 : (tensor<i32>, !tosa.shape<1>) -> tensor<1xi32>
    %9 = tosa.concat %8, %3 {axis = 0 : i32} : (tensor<1xi32>, tensor<2xi32>) -> tensor<3xi32>
    
    %extracted_0 = tensor.extract %9[%c0] : tensor<3xi32>
    %10 = index.casts %extracted_0 : i32 to index
    %11 = arith.cmpi eq, %10, %c-1 : index
    %12 = arith.select %11, %c10, %10 : index
    
    %extracted_slice = tensor.extract_slice %arg2[0, 0, 0] [%12, 4, 1] [1, 1, 1] 
      : tensor<10x4x1xf32> to tensor<?x4x1xf32>
    
    // Second slice - this is where the failure occurs
    %13 = tosa.reshape %7, %c4_1 : (tensor<i32>, !tosa.shape<1>) -> tensor<1xi32>
    %14 = tosa.concat %13, %4 {axis = 0 : i32} : (tensor<1xi32>, tensor<2xi32>) -> tensor<3xi32>
    
    %extracted_1 = tensor.extract %14[%c0] : tensor<3xi32>
    %15 = index.castu %extracted_1 : i32 to index
    %16 = arith.subi %c10, %15 : index  // size = 10 - offset
    
    %extracted_2 = tensor.extract %14[%c1] : tensor<3xi32>
    %17 = index.castu %extracted_2 : i32 to index
    
    %extracted_3 = tensor.extract %14[%c2] : tensor<3xi32>
    %18 = index.castu %extracted_3 : i32 to index
    
    // On the last loop iteration: %15=10, %16=0
    // %extracted_slice_0 becomes an empty tensor
    // Runtime verification fails: "offset 0 is out-of-bounds"
    %extracted_slice_0 = tensor.extract_slice %arg2[%15, %17, %18] [%16, 4, 1] [1, 1, 1] 
      : tensor<10x4x1xf32> to tensor<?x4x1xf32>
    
    %19 = tosa.concat %extracted_slice, %5, %extracted_slice_0 {axis = 0 : i32} 
      : (tensor<?x4x1xf32>, tensor<1x4x1xf32>, tensor<?x4x1xf32>) -> tensor<10x4x1xf32>
    
    scf.yield %7, %19 : tensor<i32>, tensor<10x4x1xf32>
  }
  
  return %6#1 : tensor<10x4x1xf32>
}
```
**The fix:**

Make the offset check conditional on slice size:
- Empty slice (size == 0): allow `0 <= offset <= dim_size`
- Non-empty slice (size > 0): require `0 <= offset < dim_size`


**Question for reviewers:**
Should we also relax the static verifier to allow this edge case?
Currently, the static verifier rejects the following IR:

```mlir
%tensor = arith.constant dense<1.0> : tensor<10xf32>
%slice = tensor.extract_slice %tensor[10] [0] [1] : tensor<10xf32> to tensor<0xf32>
```
Since we're allowing it at runtime for dynamic shapes, it seems
inconsistent to reject it statically. However, I wanted to get feedback
before making that change - this PR focuses only on the runtime
verification fix for dynamic shapes.

P.S. We have a similar issue with `memref.subview`. I will send a
separate patch for the issue.

Co-authored-by: Hanumanth Hanumantharayappa <[email protected]>

Author: Hanumanth

mlir/lib/Dialect/Tensor/Transforms/RuntimeOpVerification.cpp

@@ -155,13 +155,15 @@ struct ExtractSliceOpInterface
     RankedTensorType sourceType = extractSliceOp.getSource().getType();
 
     // For each dimension, assert that:
-    // 0 <= offset < dim_size
-    // 0 <= offset + (size - 1) * stride < dim_size
+    // For empty slices (size == 0)   : 0 <= offset <= dim_size
+    // For non-empty slices (size > 0): 0 <= offset < dim_size
+    //                                  0 <= offset + (size - 1) * stride <
+    //                                  dim_size
     Value zero = arith::ConstantIndexOp::create(builder, loc, 0);
     Value one = arith::ConstantIndexOp::create(builder, loc, 1);
 
     for (int64_t i : llvm::seq<int64_t>(0, sourceType.getRank())) {
-      // Reset insertion point to before the operation for each dimension
+
       builder.setInsertionPoint(extractSliceOp);
 
       Value offset = getValueOrCreateConstantIndexOp(
@@ -170,46 +172,63 @@ struct ExtractSliceOpInterface
           builder, loc, extractSliceOp.getMixedSizes()[i]);
       Value stride = getValueOrCreateConstantIndexOp(
           builder, loc, extractSliceOp.getMixedStrides()[i]);
-
-      // Verify that offset is in-bounds.
       Value dimSize = builder.createOrFold<tensor::DimOp>(
           loc, extractSliceOp.getSource(), i);
-      Value offsetInBounds =
-          generateInBoundsCheck(builder, loc, offset, zero, dimSize);
-      cf::AssertOp::create(builder, loc, offsetInBounds,
+
+      // Verify that offset is in-bounds (conditional on slice size).
+      Value sizeIsZero = arith::CmpIOp::create(
+          builder, loc, arith::CmpIPredicate::eq, size, zero);
+      auto offsetCheckIf = scf::IfOp::create(
+          builder, loc, sizeIsZero,
+          [&](OpBuilder &b, Location loc) {
+            // For empty slices, offset can be at the boundary: 0 <= offset <=
+            // dimSize.
+            Value offsetGEZero = arith::CmpIOp::create(
+                b, loc, arith::CmpIPredicate::sge, offset, zero);
+            Value offsetLEDimSize = arith::CmpIOp::create(
+                b, loc, arith::CmpIPredicate::sle, offset, dimSize);
+            Value emptyOffsetValid =
+                arith::AndIOp::create(b, loc, offsetGEZero, offsetLEDimSize);
+            scf::YieldOp::create(b, loc, emptyOffsetValid);
+          },
+          [&](OpBuilder &b, Location loc) {
+            // For non-empty slices, offset must be a valid index: 0 <= offset <
+            // dimSize.
+            Value offsetInBounds =
+                generateInBoundsCheck(b, loc, offset, zero, dimSize);
+            scf::YieldOp::create(b, loc, offsetInBounds);
+          });
+
+      Value offsetCondition = offsetCheckIf.getResult(0);
+      cf::AssertOp::create(builder, loc, offsetCondition,
                            generateErrorMessage(op, "offset " +
                                                         std::to_string(i) +
                                                         " is out-of-bounds"));
 
-      // Only verify if size > 0
+      // Verify that the slice endpoint is in-bounds (only for non-empty
+      // slices).
       Value sizeIsNonZero = arith::CmpIOp::create(
           builder, loc, arith::CmpIPredicate::sgt, size, zero);
+      auto ifOp = scf::IfOp::create(
+          builder, loc, sizeIsNonZero,
+          [&](OpBuilder &b, Location loc) {
+            // Verify that slice does not run out-of-bounds.
+            Value sizeMinusOne = arith::SubIOp::create(b, loc, size, one);
+            Value sizeMinusOneTimesStride =
+                arith::MulIOp::create(b, loc, sizeMinusOne, stride);
+            Value lastPos =
+                arith::AddIOp::create(b, loc, offset, sizeMinusOneTimesStride);
+            Value lastPosInBounds =
+                generateInBoundsCheck(b, loc, lastPos, zero, dimSize);
+            scf::YieldOp::create(b, loc, lastPosInBounds);
+          },
+          [&](OpBuilder &b, Location loc) {
+            Value trueVal =
+                arith::ConstantOp::create(b, loc, b.getBoolAttr(true));
+            scf::YieldOp::create(b, loc, trueVal);
+          });
 
-      auto ifOp = scf::IfOp::create(builder, loc, builder.getI1Type(),
-                                    sizeIsNonZero, /*withElseRegion=*/true);
-
-      // Populate the "then" region (for size > 0).
-      builder.setInsertionPointToStart(&ifOp.getThenRegion().front());
-
-      // Verify that slice does not run out-of-bounds.
-      Value sizeMinusOne = arith::SubIOp::create(builder, loc, size, one);
-      Value sizeMinusOneTimesStride =
-          arith::MulIOp::create(builder, loc, sizeMinusOne, stride);
-      Value lastPos =
-          arith::AddIOp::create(builder, loc, offset, sizeMinusOneTimesStride);
-      Value lastPosInBounds =
-          generateInBoundsCheck(builder, loc, lastPos, zero, dimSize);
-      scf::YieldOp::create(builder, loc, lastPosInBounds);
-
-      // Populate the "else" region (for size == 0).
-      builder.setInsertionPointToStart(&ifOp.getElseRegion().front());
-      Value trueVal =
-          arith::ConstantOp::create(builder, loc, builder.getBoolAttr(true));
-      scf::YieldOp::create(builder, loc, trueVal);
-
-      builder.setInsertionPointAfter(ifOp);
       Value finalCondition = ifOp.getResult(0);
-
       cf::AssertOp::create(
           builder, loc, finalCondition,
           generateErrorMessage(

mlir/test/Integration/Dialect/Tensor/extract_slice-runtime-verification.mlir

@@ -39,6 +39,11 @@ func.func @extract_slice_zero_size_dim(%arg0: tensor<10x4x1xf32>, %dim_0: index,
     return
 }
 
+func.func @extract_slice_empty_tensor(%arg0: tensor<10x4x1xf32>, %dim_0: index, %dim_1: index, %dim_2: index, %offset: index) {
+    tensor.extract_slice %arg0[%offset, 0, 0] [%dim_0, %dim_1, %dim_2] [1, 1, 1] : tensor<10x4x1xf32> to tensor<?x?x?xf32>
+    return
+}
+
 
 func.func @main() {
   %0 = arith.constant 0 : index
@@ -115,5 +120,9 @@ func.func @main() {
   %dim_2 = arith.constant 1 : index
   func.call @extract_slice_zero_size_dim(%cst10x4x1xf32, %dim_0, %dim_1, %dim_2) : (tensor<10x4x1xf32>, index, index, index) -> ()
 
+  // CHECK-NOT: ERROR: Runtime op verification failed
+  %offset = arith.constant 10 : index  
+  func.call @extract_slice_empty_tensor(%cst10x4x1xf32, %dim_0, %dim_1, %dim_2, %offset) : (tensor<10x4x1xf32>, index, index, index, index) -> ()
+
   return
 }