[mlir][emitc] Fix creating pointer from constant array

When creating a pointer from a constant emitc array,
check if it is constant. If it is, create the pointer as
opaque<"const {type}">>.

Move out C type string creation logic from TranslateToCpp.cpp
to getCTypeString in EmitC.cpp as a shared utility function.

Author: Jimmy2027

mlir/include/mlir/Dialect/EmitC/IR/EmitC.h

@@ -27,6 +27,7 @@
 #include "mlir/Dialect/EmitC/IR/EmitCDialect.h.inc"
 #include "mlir/Dialect/EmitC/IR/EmitCEnums.h.inc"
 
+#include <string>
 #include <variant>
 
 namespace mlir {
@@ -49,6 +50,10 @@ bool isSupportedFloatType(mlir::Type type);
 /// Determines whether \p type is a emitc.size_t/ssize_t type.
 bool isPointerWideType(mlir::Type type);
 
+/// Convert an MLIR type to its C type string representation.
+/// Returns an empty string if the type cannot be represented as a C type.
+std::string getCTypeString(Type type);
+
 // Either a literal string, or an placeholder for the fmtArgs.
 struct Placeholder {};
 using ReplacementItem = std::variant<StringRef, Placeholder>;

mlir/lib/Conversion/MemRefToEmitC/MemRefToEmitC.cpp

@@ -19,6 +19,7 @@
 #include "mlir/IR/BuiltinTypes.h"
 #include "mlir/IR/Diagnostics.h"
 #include "mlir/IR/PatternMatch.h"
+#include "mlir/IR/SymbolTable.h"
 #include "mlir/IR/TypeRange.h"
 #include "mlir/IR/Value.h"
 #include "mlir/Transforms/DialectConversion.h"
@@ -134,8 +135,26 @@ createPointerFromEmitcArray(Location loc, OpBuilder &builder,
   llvm::SmallVector<mlir::Value> indices(arrayType.getRank(), zeroIndex);
   emitc::SubscriptOp subPtr =
       emitc::SubscriptOp::create(builder, loc, arrayValue, ValueRange(indices));
+
+  // Determine the pointer type
+  Type pointerElementType = arrayType.getElementType();
+
+  // Check if the array comes from a const global
+  if (auto getGlobalOp = arrayValue.getDefiningOp<emitc::GetGlobalOp>()) {
+    auto globalOp = SymbolTable::lookupNearestSymbolFrom<emitc::GlobalOp>(
+        getGlobalOp, getGlobalOp.getNameAttr());
+    if (globalOp && globalOp.getConstSpecifier()) {
+      // Create a const pointer type using opaque type
+      std::string cTypeString = emitc::getCTypeString(pointerElementType);
+      if (!cTypeString.empty()) {
+        pointerElementType = emitc::OpaqueType::get(builder.getContext(),
+                                                    "const " + cTypeString);
+      }
+    }
+  }
+
   emitc::ApplyOp ptr = emitc::ApplyOp::create(
-      builder, loc, emitc::PointerType::get(arrayType.getElementType()),
+      builder, loc, emitc::PointerType::get(pointerElementType),
       builder.getStringAttr("&"), subPtr);
 
   return ptr;

mlir/lib/Dialect/EmitC/IR/EmitC.cpp

@@ -139,6 +139,43 @@ bool mlir::emitc::isFundamentalType(Type type) {
          isa<emitc::PointerType>(type);
 }
 
+std::string mlir::emitc::getCTypeString(Type type) {
+  if (auto intType = dyn_cast<IntegerType>(type)) {
+    switch (intType.getWidth()) {
+    case 1:
+      return "bool";
+    case 8:
+      return intType.isUnsigned() ? "uint8_t" : "int8_t";
+    case 16:
+      return intType.isUnsigned() ? "uint16_t" : "int16_t";
+    case 32:
+      return intType.isUnsigned() ? "uint32_t" : "int32_t";
+    case 64:
+      return intType.isUnsigned() ? "uint64_t" : "int64_t";
+    default:
+      return "";
+    }
+  }
+  if (auto floatType = dyn_cast<FloatType>(type)) {
+    if (floatType.getWidth() == 16) {
+      if (isa<Float16Type>(type))
+        return "_Float16";
+      if (isa<BFloat16Type>(type))
+        return "__bf16";
+      return "";
+    }
+    if (floatType.getWidth() == 32)
+      return "float";
+    if (floatType.getWidth() == 64)
+      return "double";
+    return "";
+  }
+  if (auto opaqueType = dyn_cast<emitc::OpaqueType>(type))
+    return opaqueType.getValue().str();
+
+  return "";
+}
+
 /// Check that the type of the initial value is compatible with the operations
 /// result type.
 static LogicalResult verifyInitializationAttribute(Operation *op,

mlir/lib/Target/Cpp/TranslateToCpp.cpp

@@ -1792,40 +1792,15 @@ LogicalResult CppEmitter::emitVariableDeclaration(Location loc, Type type,
 }
 
 LogicalResult CppEmitter::emitType(Location loc, Type type) {
-  if (auto iType = dyn_cast<IntegerType>(type)) {
-    switch (iType.getWidth()) {
-    case 1:
-      return (os << "bool"), success();
-    case 8:
-    case 16:
-    case 32:
-    case 64:
-      if (shouldMapToUnsigned(iType.getSignedness()))
-        return (os << "uint" << iType.getWidth() << "_t"), success();
-      else
-        return (os << "int" << iType.getWidth() << "_t"), success();
-    default:
-      return emitError(loc, "cannot emit integer type ") << type;
-    }
-  }
-  if (auto fType = dyn_cast<FloatType>(type)) {
-    switch (fType.getWidth()) {
-    case 16: {
-      if (llvm::isa<Float16Type>(type))
-        return (os << "_Float16"), success();
-      if (llvm::isa<BFloat16Type>(type))
-        return (os << "__bf16"), success();
-      else
-        return emitError(loc, "cannot emit float type ") << type;
-    }
-    case 32:
-      return (os << "float"), success();
-    case 64:
-      return (os << "double"), success();
-    default:
-      return emitError(loc, "cannot emit float type ") << type;
-    }
-  }
+  std::string cTypeString = emitc::getCTypeString(type);
+  if (!cTypeString.empty())
+    return (os << cTypeString), success();
+
+  // Handle integer and float cases that failed above
+  if (isa<IntegerType>(type))
+    return emitError(loc, "cannot emit integer type ") << type;
+  if (isa<FloatType>(type))
+    return emitError(loc, "cannot emit float type ") << type;
   if (auto iType = dyn_cast<IndexType>(type))
     return (os << "size_t"), success();
   if (auto sType = dyn_cast<emitc::SizeTType>(type))
@@ -1854,10 +1829,6 @@ LogicalResult CppEmitter::emitType(Location loc, Type type) {
   }
   if (auto tType = dyn_cast<TupleType>(type))
     return emitTupleType(loc, tType.getTypes());
-  if (auto oType = dyn_cast<emitc::OpaqueType>(type)) {
-    os << oType.getValue();
-    return success();
-  }
   if (auto aType = dyn_cast<emitc::ArrayType>(type)) {
     if (failed(emitType(loc, aType.getElementType())))
       return failure();

mlir/test/Conversion/MemRefToEmitC/memref-to-emitc.mlir

@@ -58,3 +58,25 @@ module @globals {
     return
   }
 }
+
+// -----
+
+// CHECK-LABEL: const_global_copy
+module @const_global_copy {
+  memref.global "private" constant @const_data : memref<4xi8> = dense<[1, 2, 3, 4]>
+  // CHECK: emitc.global static const @const_data : !emitc.array<4xi8> = dense<[1, 2, 3, 4]>
+
+  func.func @copy_from_const_global() {
+    // CHECK: get_global @const_data : !emitc.array<4xi8>
+    %0 = memref.get_global @const_data : memref<4xi8>
+    // CHECK: "emitc.variable"() <{value = #emitc.opaque<"">}> : () -> !emitc.array<4xi8>
+    %1 = memref.alloca() : memref<4xi8>
+
+    // Verify that pointer from const global has const qualifier
+    // CHECK: apply "&"({{.*}}) : (!emitc.lvalue<i8>) -> !emitc.ptr<!emitc.opaque<"const int8_t">>
+    // CHECK: apply "&"({{.*}}) : (!emitc.lvalue<i8>) -> !emitc.ptr<i8>
+    // CHECK: call_opaque "memcpy"({{.*}}, {{.*}}, {{.*}}) : (!emitc.ptr<i8>, !emitc.ptr<!emitc.opaque<"const int8_t">>, !emitc.size_t) -> ()
+    memref.copy %0, %1 : memref<4xi8> to memref<4xi8>
+    return
+  }
+}