[CIR] Upstream proper function alias lowering #150520

andykaylor · 2025-07-24T20:46:24Z

This change implements correct lowering of function aliases to the LLVM dialect.

llvmbot · 2025-07-24T20:46:51Z

@llvm/pr-subscribers-clangir

@llvm/pr-subscribers-clang

Author: Andy Kaylor (andykaylor)

Changes

This change implements correct lowering of function aliases to the LLVM dialect.

Full diff: https://github.com/llvm/llvm-project/pull/150520.diff

4 Files Affected:

(modified) clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp (+70-6)
(modified) clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h (+5)
(modified) clang/test/CIR/CodeGen/ctor-alias.cpp (+21-4)
(modified) clang/test/CIR/CodeGen/dtor-alias.cpp (+2-4)

diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
index 3cd7de0a56bc3..baa0a5e326c4a 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
@@ -919,13 +919,46 @@ rewriteCallOrInvoke(mlir::Operation *op, mlir::ValueRange callOperands,
                            memoryEffects, noUnwind, willReturn);
 
   mlir::LLVM::LLVMFunctionType llvmFnTy;
+
+  // Temporary to handle the case where we need to prepend an operand if the
+  // callee is an alias.
+  SmallVector<mlir::Value> adjustedCallOperands;
+
   if (calleeAttr) { // direct call
-    mlir::FunctionOpInterface fn =
-        mlir::SymbolTable::lookupNearestSymbolFrom<mlir::FunctionOpInterface>(
-            op, calleeAttr);
-    assert(fn && "Did not find function for call");
-    llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(
-        converter->convertType(fn.getFunctionType()));
+    mlir::Operation *callee =
+        mlir::SymbolTable::lookupNearestSymbolFrom(op, calleeAttr);
+    if (auto fn = dyn_cast<mlir::FunctionOpInterface>(callee)) {
+      llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(
+          converter->convertType(fn.getFunctionType()));
+    } else if (auto alias = cast<mlir::LLVM::AliasOp>(callee)) {
+      // If the callee wasan alias. In that case,
+      // we need to prepend the address of the alias to the operands. The
+      // way aliases work in the LLVM dialect is a little counter-intuitive.
+      // The AliasOp itself is a pseudo-function that returns the address of
+      // the global value being aliased, but when we generate the call we
+      // need to insert an operation that gets the address of the AliasOp.
+      // This all gets sorted out when the LLVM dialect is lowered to LLVM IR.
+      auto symAttr = cast<mlir::FlatSymbolRefAttr>(calleeAttr);
+      auto addrOfAlias =
+          rewriter
+              .create<mlir::LLVM::AddressOfOp>(
+                  op->getLoc(),
+                  mlir::LLVM::LLVMPointerType::get(rewriter.getContext()),
+                  symAttr)
+              .getResult();
+      adjustedCallOperands.push_back(addrOfAlias);
+
+      // Now add the regular operands and assign this to the range value.
+      llvm::append_range(adjustedCallOperands, callOperands);
+      callOperands = adjustedCallOperands;
+
+      // Clear the callee attribute because we're calling an alias.
+      calleeAttr = {};
+      llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(alias.getType());
+    } else {
+      // Was this an ifunc?
+      return op->emitError("Unexpected callee type!");
+    }
   } else { // indirect call
     assert(!op->getOperands().empty() &&
            "operands list must no be empty for the indirect call");
@@ -1166,6 +1199,31 @@ void CIRToLLVMFuncOpLowering::lowerFuncAttributes(
   }
 }
 
+mlir::LogicalResult CIRToLLVMFuncOpLowering::matchAndRewriteAlias(
+    cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee, mlir::Type ty,
+    OpAdaptor adaptor, mlir::ConversionPatternRewriter &rewriter) const {
+  SmallVector<mlir::NamedAttribute, 4> attributes;
+  lowerFuncAttributes(op, /*filterArgAndResAttrs=*/false, attributes);
+
+  mlir::Location loc = op.getLoc();
+  auto aliasOp = rewriter.replaceOpWithNewOp<mlir::LLVM::AliasOp>(
+      op, ty, convertLinkage(op.getLinkage()), op.getName(), op.getDsoLocal(),
+      /*threadLocal=*/false, attributes);
+
+  // Create the alias body
+  mlir::OpBuilder builder(op.getContext());
+  mlir::Block *block = builder.createBlock(&aliasOp.getInitializerRegion());
+  builder.setInsertionPointToStart(block);
+  // The type of AddressOfOp is always a pointer.
+  assert(!cir::MissingFeatures::addressSpace());
+  mlir::Type ptrTy = mlir::LLVM::LLVMPointerType::get(ty.getContext());
+  auto addrOp =
+      builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee.getValue());
+  builder.create<mlir::LLVM::ReturnOp>(loc, addrOp);
+
+  return mlir::success();
+}
+
 mlir::LogicalResult CIRToLLVMFuncOpLowering::matchAndRewrite(
     cir::FuncOp op, OpAdaptor adaptor,
     mlir::ConversionPatternRewriter &rewriter) const {
@@ -1190,6 +1248,12 @@ mlir::LogicalResult CIRToLLVMFuncOpLowering::matchAndRewrite(
       resultType ? resultType : mlir::LLVM::LLVMVoidType::get(getContext()),
       signatureConversion.getConvertedTypes(),
       /*isVarArg=*/fnType.isVarArg());
+
+  // If this is an alias, it needs to be lowered to llvm::AliasOp.
+  std::optional<mlir::FlatSymbolRefAttr> aliasee = op.getAliaseeAttr();
+  if (aliasee && *aliasee)
+    return matchAndRewriteAlias(op, *aliasee, llvmFnTy, adaptor, rewriter);
+
   // LLVMFuncOp expects a single FileLine Location instead of a fused
   // ___location.
   mlir::Location loc = op.getLoc();
diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
index 2911ced66e58e..0f003f644d2fa 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
@@ -257,6 +257,11 @@ class CIRToLLVMFuncOpLowering : public mlir::OpConversionPattern<cir::FuncOp> {
       cir::FuncOp func, bool filterArgAndResAttrs,
       mlir::SmallVectorImpl<mlir::NamedAttribute> &result) const;
 
+  mlir::LogicalResult
+  matchAndRewriteAlias(cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee,
+                       mlir::Type ty, OpAdaptor adaptor,
+                       mlir::ConversionPatternRewriter &rewriter) const;
+
 public:
   using mlir::OpConversionPattern<cir::FuncOp>::OpConversionPattern;
 
diff --git a/clang/test/CIR/CodeGen/ctor-alias.cpp b/clang/test/CIR/CodeGen/ctor-alias.cpp
index a20e206c6d714..c4bf455e3a09b 100644
--- a/clang/test/CIR/CodeGen/ctor-alias.cpp
+++ b/clang/test/CIR/CodeGen/ctor-alias.cpp
@@ -11,6 +11,8 @@ struct B {
 B::B() {
 }
 
+// LLVM: @_ZN1BC1Ev = alias void (ptr), ptr @_ZN1BC2Ev
+
 // OGCG: @_ZN1BC1Ev = unnamed_addr alias void (ptr), ptr @_ZN1BC2Ev
 
 // CHECK: cir.func{{.*}} @_ZN1BC2Ev(%arg0: !cir.ptr<!rec_B>
@@ -25,15 +27,30 @@ B::B() {
 // LLVM:   store ptr %[[THIS_ARG]], ptr %[[THIS_ADDR]]
 // LLVM:   %[[THIS:.*]] = load ptr, ptr %[[THIS_ADDR]]
 
-// This should be an alias, like the similar OGCG alias above, but that's not
-// implemented yet.
-// LLVM: declare dso_local void @_ZN1BC1Ev(ptr)
-
 // OGCG: define{{.*}} @_ZN1BC2Ev(ptr{{.*}} %[[THIS_ARG:.*]])
 // OGCG:   %[[THIS_ADDR:.*]] = alloca ptr
 // OGCG:   store ptr %[[THIS_ARG]], ptr %[[THIS_ADDR]]
 // OGCG:   %[[THIS:.*]] = load ptr, ptr %[[THIS_ADDR]]
 
+void bar() {
+  B b;
+}
+
+// CHECK: cir.func{{.*}} @_Z3barv()
+// CHECK:   %[[B:.*]] = cir.alloca !rec_B, !cir.ptr<!rec_B>, ["b", init]
+// CHECK:   cir.call @_ZN1BC1Ev(%[[B]]) : (!cir.ptr<!rec_B>) -> ()
+// CHECK:   cir.return
+
+// LLVM: define{{.*}} void @_Z3barv()
+// LLVM:   %[[B:.*]] = alloca %struct.B, i64 1, align 1
+// LLVM:   call void @_ZN1BC1Ev(ptr %[[B]])
+// LLVM:   ret void
+
+// OGCG: define{{.*}} void @_Z3barv()
+// OGCG:   %[[B:.*]] = alloca %struct.B, align 1
+// OGCG:   call void @_ZN1BC1Ev(ptr{{.*}} %[[B]])
+// OGCG:   ret void
+
 // The constructor in this cases is handled by RAUW rather than aliasing.
 struct Struk {
   Struk() {}
diff --git a/clang/test/CIR/CodeGen/dtor-alias.cpp b/clang/test/CIR/CodeGen/dtor-alias.cpp
index e37ddabb5b4fd..f4d54dfd7da26 100644
--- a/clang/test/CIR/CodeGen/dtor-alias.cpp
+++ b/clang/test/CIR/CodeGen/dtor-alias.cpp
@@ -11,6 +11,8 @@ struct B {
 B::~B() {
 }
 
+// LLVM: @_ZN1BD1Ev = alias void (ptr), ptr @_ZN1BD2Ev
+
 // OGCG: @_ZN1BD1Ev = unnamed_addr alias void (ptr), ptr @_ZN1BD2Ev
 
 // CHECK: cir.func{{.*}} @_ZN1BD2Ev(%arg0: !cir.ptr<!rec_B>
@@ -25,10 +27,6 @@ B::~B() {
 // LLVM:   store ptr %[[THIS_ARG]], ptr %[[THIS_ADDR]]
 // LLVM:   %[[THIS:.*]] = load ptr, ptr %[[THIS_ADDR]]
 
-// This should be an alias, like the similar OGCG alias above, but that's not
-// implemented yet.
-// LLVM: declare dso_local void @_ZN1BD1Ev(ptr)
-
 // OGCG: define{{.*}} @_ZN1BD2Ev(ptr{{.*}} %[[THIS_ARG:.*]])
 // OGCG:   %[[THIS_ADDR:.*]] = alloca ptr
 // OGCG:   store ptr %[[THIS_ARG]], ptr %[[THIS_ADDR]]

xlauko · 2025-07-25T13:32:02Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+      llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(
+          converter->convertType(fn.getFunctionType()));


Suggested change

llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(

converter->convertType(fn.getFunctionType()));

llvmFnTy =

converter->convertType<mlir::LLVM::LLVMFunctionType>(fn.getFunctionType());

xlauko · 2025-07-25T13:32:25Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+    if (auto fn = dyn_cast<mlir::FunctionOpInterface>(callee)) {
+      llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(
+          converter->convertType(fn.getFunctionType()));
+    } else if (auto alias = cast<mlir::LLVM::AliasOp>(callee)) {


Suggested change

} else if (auto alias = cast<mlir::LLVM::AliasOp>(callee)) {

} else if (auto alias = mlir::cast<mlir::LLVM::AliasOp>(callee)) {

xlauko · 2025-07-25T13:32:35Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+      // the global value being aliased, but when we generate the call we
+      // need to insert an operation that gets the address of the AliasOp.
+      // This all gets sorted out when the LLVM dialect is lowered to LLVM IR.
+      auto symAttr = cast<mlir::FlatSymbolRefAttr>(calleeAttr);


Suggested change

auto symAttr = cast<mlir::FlatSymbolRefAttr>(calleeAttr);

auto symAttr = mlir::cast<mlir::FlatSymbolRefAttr>(calleeAttr);

xlauko · 2025-07-25T13:33:32Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+          rewriter
+              .create<mlir::LLVM::AddressOfOp>(


Suggested change

rewriter

.create<mlir::LLVM::AddressOfOp>(

mlir::LLVM::AddressOfOp::create(rewriter,

some formatting

xlauko · 2025-07-25T13:33:58Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+
+      // Clear the callee attribute because we're calling an alias.
+      calleeAttr = {};
+      llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(alias.getType());


Suggested change

llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(alias.getType());

llvmFnTy = mlir::cast<mlir::LLVM::LLVMFunctionType>(alias.getType());

xlauko · 2025-07-25T13:35:15Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+  auto addrOp =
+      builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee.getValue());
+  builder.create<mlir::LLVM::ReturnOp>(loc, addrOp);


Suggested change

auto addrOp =

builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee.getValue());

builder.create<mlir::LLVM::ReturnOp>(loc, addrOp);

auto addrOp =

mlir::LLVM::AddressOfOp::create(builder, loc, ptrTy, aliasee.getValue());

mlir::LLVM::ReturnOp::(builder, loc, addrOp);

xlauko · 2025-07-25T15:21:14Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+  std::optional<mlir::FlatSymbolRefAttr> aliasee = op.getAliaseeAttr();
+  if (aliasee && *aliasee)
+    return matchAndRewriteAlias(op, *aliasee, llvmFnTy, adaptor, rewriter);


Suggested change

std::optional<mlir::FlatSymbolRefAttr> aliasee = op.getAliaseeAttr();

if (aliasee && *aliasee)

return matchAndRewriteAlias(op, *aliasee, llvmFnTy, adaptor, rewriter);

if (std::optional<llvm::StringRef> aliasee = op.getAliasee())

return matchAndRewriteAlias(op, *aliasee, llvmFnTy, adaptor, rewriter);

xlauko · 2025-07-25T15:22:19Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -1166,6 +1199,31 @@ void CIRToLLVMFuncOpLowering::lowerFuncAttributes(
  }
 }

+mlir::LogicalResult CIRToLLVMFuncOpLowering::matchAndRewriteAlias(
+    cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee, mlir::Type ty,


Suggested change

cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee, mlir::Type ty,

cir::FuncOp op, llvm::StringRef aliasee, mlir::Type ty,

xlauko · 2025-07-25T15:22:29Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+  assert(!cir::MissingFeatures::addressSpace());
+  mlir::Type ptrTy = mlir::LLVM::LLVMPointerType::get(ty.getContext());
+  auto addrOp =
+      builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee.getValue());


Suggested change

builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee.getValue());

builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee);

xlauko · 2025-07-25T15:23:04Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h

@@ -257,6 +257,11 @@ class CIRToLLVMFuncOpLowering : public mlir::OpConversionPattern<cir::FuncOp> {
      cir::FuncOp func, bool filterArgAndResAttrs,
      mlir::SmallVectorImpl<mlir::NamedAttribute> &result) const;

+  mlir::LogicalResult
+  matchAndRewriteAlias(cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee,


Suggested change

matchAndRewriteAlias(cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee,

matchAndRewriteAlias(cir::FuncOp op, llvm::StringRef aliasee,

bcardosolopes

LGTM, minor nit

bcardosolopes · 2025-07-28T19:35:23Z

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

+          fn.getFunctionType());
+      assert(llvmFnTy && "Failed to convert function type");
+    } else if (auto alias = mlir::cast<mlir::LLVM::AliasOp>(callee)) {
+      // If the callee wasan alias. In that case,


wasan -> was an

xlauko

lgtm

[CIR] Upstream proper function alias lowering

88039a4

This change implements correct lowering of function aliases to the LLVM dialect.

andykaylor requested review from xlauko, bcardosolopes, mmha, erichkeane and lanza July 24, 2025 20:46

llvmbot added clang Clang issues not falling into any other category ClangIR Anything related to the ClangIR project labels Jul 24, 2025

xlauko reviewed Jul 25, 2025

View reviewed changes

Address review feedback

2e95edb

bcardosolopes approved these changes Jul 28, 2025

View reviewed changes

xlauko approved these changes Jul 29, 2025

View reviewed changes

Fix comment typo

4c4aeaf

andykaylor merged commit 8a1b252 into llvm:main Jul 29, 2025
9 checks passed

andykaylor deleted the cir-lower-fn-alias branch July 29, 2025 16:45

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[CIR] Upstream proper function alias lowering #150520

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		llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(
		converter->convertType(fn.getFunctionType()));

	} else if (auto alias = cast<mlir::LLVM::AliasOp>(callee)) {
	} else if (auto alias = mlir::cast<mlir::LLVM::AliasOp>(callee)) {

	auto symAttr = cast<mlir::FlatSymbolRefAttr>(calleeAttr);
	auto symAttr = mlir::cast<mlir::FlatSymbolRefAttr>(calleeAttr);

	rewriter
	.create<mlir::LLVM::AddressOfOp>(
	mlir::LLVM::AddressOfOp::create(rewriter,

	llvmFnTy = cast<mlir::LLVM::LLVMFunctionType>(alias.getType());
	llvmFnTy = mlir::cast<mlir::LLVM::LLVMFunctionType>(alias.getType());

	cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee, mlir::Type ty,
	cir::FuncOp op, llvm::StringRef aliasee, mlir::Type ty,

	builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee.getValue());
	builder.create<mlir::LLVM::AddressOfOp>(loc, ptrTy, aliasee);

	matchAndRewriteAlias(cir::FuncOp op, mlir::FlatSymbolRefAttr aliasee,
	matchAndRewriteAlias(cir::FuncOp op, llvm::StringRef aliasee,

[CIR] Upstream proper function alias lowering #150520

[CIR] Upstream proper function alias lowering #150520

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llvmbot commented Jul 24, 2025 •

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