[Mlir-commits] [mlir] [MLIR][ArithToLLVM] Fix index_cast on memref types generating invalid LLVM IR (PR #189227)

[Mehdi Amini]

https://github.com/joker-eph created https://github.com/llvm/llvm-project/pull/189227

`arith.index_cast` and `arith.index_castui` accept memref operands (via `IndexCastTypeConstraint`), but `IndexCastOpLowering::matchAndRewrite` did not handle this case. When the operand was a memref, the conversion framework substituted the converted LLVM struct type, and the lowering incorrectly attempted to emit `llvm.sext`/`llvm.zext`/`llvm.trunc` on a struct value, producing invalid LLVM IR.

Since LLVM uses opaque pointers, all memrefs with integer or index element types lower to the same `\!llvm.struct<(ptr, ptr, i64, ...)>` type, making `arith.index_cast` on memrefs a no-op at the LLVM level. Add a check that treats the memref case as an identity conversion (same as the same-bit-width path).

Fixes #92377

Assisted-by: Claude Code

>From bae2325037ce5ab29e137cf903b8d29380d90bc1 Mon Sep 17 00:00:00 2001
From: Mehdi Amini <joker.eph at gmail.com>
Date: Sat, 28 Mar 2026 05:43:59 -0700
Subject: [PATCH] [MLIR][ArithToLLVM] Fix index_cast on memref types generating
 invalid LLVM IR

`arith.index_cast` and `arith.index_castui` accept memref operands (via
`IndexCastTypeConstraint`), but `IndexCastOpLowering::matchAndRewrite` did
not handle this case. When the operand was a memref, the conversion
framework substituted the converted LLVM struct type, and the lowering
incorrectly attempted to emit `llvm.sext`/`llvm.zext`/`llvm.trunc` on a
struct value, producing invalid LLVM IR.

Since LLVM uses opaque pointers, all memrefs with integer or index element
types lower to the same `\!llvm.struct<(ptr, ptr, i64, ...)>` type, making
`arith.index_cast` on memrefs a no-op at the LLVM level. Add a check that
treats the memref case as an identity conversion (same as the
same-bit-width path).

Fixes #92377

Assisted-by: Claude Code
---
 .../Conversion/ArithToLLVM/ArithToLLVM.cpp    |  8 +++++++
 .../Conversion/ArithToLLVM/arith-to-llvm.mlir | 24 +++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/mlir/lib/Conversion/ArithToLLVM/ArithToLLVM.cpp b/mlir/lib/Conversion/ArithToLLVM/ArithToLLVM.cpp
index 11aae4de26c1e..f9ea8dba105a4 100644
--- a/mlir/lib/Conversion/ArithToLLVM/ArithToLLVM.cpp
+++ b/mlir/lib/Conversion/ArithToLLVM/ArithToLLVM.cpp
@@ -368,6 +368,14 @@ LogicalResult IndexCastOpLowering<OpTy, ExtCastTy>::matchAndRewrite(
     return success();
   }
 
+  // Memref index_cast is a no-op at the LLVM level since LLVM uses opaque
+  // pointers and memrefs of different integer/index element types all convert
+  // to the same LLVM struct type.
+  if (isa<MemRefType>(op.getIn().getType())) {
+    rewriter.replaceOp(op, adaptor.getIn());
+    return success();
+  }
+
   bool isNonNeg = false;
   if constexpr (std::is_same_v<ExtCastTy, LLVM::ZExtOp>)
     isNonNeg = op.getNonNeg();
diff --git a/mlir/test/Conversion/ArithToLLVM/arith-to-llvm.mlir b/mlir/test/Conversion/ArithToLLVM/arith-to-llvm.mlir
index 6a6016c4f5b16..75601e215744c 100644
--- a/mlir/test/Conversion/ArithToLLVM/arith-to-llvm.mlir
+++ b/mlir/test/Conversion/ArithToLLVM/arith-to-llvm.mlir
@@ -160,6 +160,30 @@ func.func @index_castui_nneg_not_set(%arg0: i1) {
 
 // -----
 
+// Memref index_cast is a no-op at the LLVM level since LLVM uses opaque
+// pointers and all memrefs with integer or index element types convert to the
+// same struct type. Verify that no sext/zext/trunc is generated.
+
+// CHECK-LABEL: @memref_index_cast
+// CHECK-NOT: llvm.sext
+// CHECK-NOT: llvm.trunc
+func.func @memref_index_cast(%arg0: memref<3xi32>) -> memref<3xindex> {
+  %0 = arith.index_cast %arg0 : memref<3xi32> to memref<3xindex>
+  return %0 : memref<3xindex>
+}
+
+// -----
+
+// CHECK-LABEL: @memref_index_castui
+// CHECK-NOT: llvm.zext
+// CHECK-NOT: llvm.trunc
+func.func @memref_index_castui(%arg0: memref<3xi32>) -> memref<3xindex> {
+  %0 = arith.index_castui %arg0 : memref<3xi32> to memref<3xindex>
+  return %0 : memref<3xindex>
+}
+
+// -----
+
 // Checking conversion of signed integer types to floating point.
 // CHECK-LABEL: @sitofp
 func.func @sitofp(%arg0 : i32, %arg1 : i64) {