[CIR][ABI][Lowering] Fixes calls with union type

clang/include/clang/CIR/MissingFeatures.h

@@ -261,7 +261,6 @@ struct MissingFeatures {
   static bool X86TypeClassification() { return false; }
 
   static bool ABIClangTypeKind() { return false; }
-  static bool ABIEnterStructForCoercedAccess() { return false; }
   static bool ABIFuncPtr() { return false; }
   static bool ABIInRegAttribute() { return false; }
   static bool ABINestedRecordLayout() { return false; }

clang/lib/CIR/Dialect/IR/CIRTypes.cpp

@@ -519,13 +519,12 @@ void StructType::computeSizeAndAlignment(
 
     // Found a nested union: recurse into it to fetch its largest member.
     auto structMember = mlir::dyn_cast<StructType>(ty);
-    if (structMember && structMember.isUnion()) {
-      auto candidate = structMember.getLargestMember(dataLayout);
-      if (dataLayout.getTypeSize(candidate) > largestMemberSize) {
-        largestMember = candidate;
-        largestMemberSize = dataLayout.getTypeSize(largestMember);
-      }
-    } else if (dataLayout.getTypeSize(ty) > largestMemberSize) {
+    if (!largestMember ||
+        dataLayout.getTypeABIAlignment(ty) >
+            dataLayout.getTypeABIAlignment(largestMember) ||
+        (dataLayout.getTypeABIAlignment(ty) ==
+             dataLayout.getTypeABIAlignment(largestMember) &&
+         dataLayout.getTypeSize(ty) > largestMemberSize)) {
       largestMember = ty;
       largestMemberSize = dataLayout.getTypeSize(largestMember);
     }

clang/lib/CIR/Dialect/Transforms/TargetLowering/LowerFunction.cpp

@@ -53,6 +53,12 @@ mlir::Value createCoercedBitcast(mlir::Value Src, mlir::Type DestTy,
                                           CastKind::bitcast, Src);
 }
 
+// FIXME(cir): Create a custom rewriter class to abstract this away.
+mlir::Value createBitcast(mlir::Value Src, mlir::Type Ty, LowerFunction &LF) {
+  return LF.getRewriter().create<CastOp>(Src.getLoc(), Ty, CastKind::bitcast,
+                                         Src);
+}
+
 /// Given a struct pointer that we are accessing some number of bytes out of it,
 /// try to gep into the struct to get at its inner goodness.  Dive as deep as
 /// possible without entering an element with an in-memory size smaller than
@@ -67,6 +73,9 @@ mlir::Value enterStructPointerForCoercedAccess(mlir::Value SrcPtr,
 
   mlir::Type FirstElt = SrcSTy.getMembers()[0];
 
+  if (SrcSTy.isUnion())
+    FirstElt = SrcSTy.getLargestMember(CGF.LM.getDataLayout().layout);
+
   // If the first elt is at least as large as what we're looking for, or if the
   // first element is the same size as the whole struct, we can enter it. The
   // comparison must be made on the store size and not the alloca size. Using
@@ -76,10 +85,26 @@ mlir::Value enterStructPointerForCoercedAccess(mlir::Value SrcPtr,
       FirstEltSize < CGF.LM.getDataLayout().getTypeStoreSize(SrcSTy))
     return SrcPtr;
 
-  cir_cconv_assert_or_abort(
-      !cir::MissingFeatures::ABIEnterStructForCoercedAccess(), "NYI");
-  return SrcPtr; // FIXME: This is a temporary workaround for the assertion
-                 // above.
+  auto &rw = CGF.getRewriter();
+  auto *ctxt = rw.getContext();
+  auto ptrTy = PointerType::get(ctxt, FirstElt);
+  if (mlir::isa<StructType>(SrcPtr.getType())) {
+    auto addr = SrcPtr;
+    if (auto load = mlir::dyn_cast<LoadOp>(SrcPtr.getDefiningOp()))
+      addr = load.getAddr();
+    cir_cconv_assert(mlir::isa<PointerType>(addr.getType()));
+    // we can not use getMemberOp here since we need a pointer to the first
+    // element. And in the case of unions we pick a type of the largest elt,
+    // that may or may not be the first one. Thus, getMemberOp verification
+    // may fail.
+    auto cast = createBitcast(addr, ptrTy, CGF);
+    SrcPtr = rw.create<LoadOp>(SrcPtr.getLoc(), cast);
+  }
+
+  if (auto sty = mlir::dyn_cast<StructType>(SrcPtr.getType()))
+    return enterStructPointerForCoercedAccess(SrcPtr, sty, DstSize, CGF);
+
+  return SrcPtr;
 }
 
 /// Convert a value Val to the specific Ty where both
@@ -141,12 +166,6 @@ static mlir::Value coerceIntOrPtrToIntOrPtr(mlir::Value val, mlir::Type typ,
   return val;
 }
 
-// FIXME(cir): Create a custom rewriter class to abstract this away.
-mlir::Value createBitcast(mlir::Value Src, mlir::Type Ty, LowerFunction &LF) {
-  return LF.getRewriter().create<CastOp>(Src.getLoc(), Ty, CastKind::bitcast,
-                                         Src);
-}
-
 AllocaOp createTmpAlloca(LowerFunction &LF, mlir::Location loc, mlir::Type ty) {
   auto &rw = LF.getRewriter();
   auto *ctxt = rw.getContext();
@@ -302,7 +321,7 @@ mlir::Value createCoercedValue(mlir::Value Src, mlir::Type Ty,
   // extension or truncation to the desired type.
   if ((mlir::isa<IntType>(Ty) || mlir::isa<PointerType>(Ty)) &&
       (mlir::isa<IntType>(SrcTy) || mlir::isa<PointerType>(SrcTy))) {
-    cir_cconv_unreachable("NYI");
+    return coerceIntOrPtrToIntOrPtr(Src, Ty, CGF);
   }
 
   // If load is legal, just bitcast the src pointer.

clang/test/CIR/CallConvLowering/AArch64/union.c

@@ -38,4 +38,34 @@ void foo(U u) {}
 U init() {
   U u;
   return u;
-}
+}
+
+typedef union {
+
+  struct {
+    short a;
+    char b;
+    char c;
+  };
+
+  int x;
+} A;
+
+void passA(A x) {}
+
+// CIR: cir.func {{.*@callA}}()
+// CIR:   %[[#V0:]] = cir.alloca !ty_A, !cir.ptr<!ty_A>, ["x"] {alignment = 4 : i64}
+// CIR:   %[[#V1:]] = cir.cast(bitcast, %[[#V0:]] : !cir.ptr<!ty_A>), !cir.ptr<!s32i>
+// CIR:   %[[#V2:]] = cir.load %[[#V1]] : !cir.ptr<!s32i>, !s32i
+// CIR:   %[[#V3:]] = cir.cast(integral, %[[#V2]] : !s32i), !u64i
+// CIR:   cir.call @passA(%[[#V3]]) : (!u64i) -> ()
+
+// LLVM: void @callA()
+// LLVM:   %[[#V0:]] = alloca %union.A, i64 1, align 4
+// LLVM:   %[[#V1:]] = load i32, ptr %[[#V0]], align 4
+// LLVM:   %[[#V2:]] = sext i32 %[[#V1]] to i64
+// LLVM:   call void @passA(i64 %[[#V2]])
+void callA() {
+  A x;
+  passA(x);
+}

clang/test/CIR/Lowering/unions.cir

@@ -16,7 +16,7 @@ module {
   cir.global external @u2 = #cir.zero : !ty_U2_
   cir.global external @u3 = #cir.zero : !ty_U3_
   // CHECK: llvm.mlir.global external @u2() {addr_space = 0 : i32} : !llvm.struct<"union.U2", (f64)>
-  // CHECK: llvm.mlir.global external @u3() {addr_space = 0 : i32} : !llvm.struct<"union.U3", (i32)>
+  // CHECK: llvm.mlir.global external @u3() {addr_space = 0 : i32} : !llvm.struct<"union.U3", (struct<"union.U1", (i32)>)>
 
   // CHECK: llvm.func @test
   cir.func @test(%arg0: !cir.ptr<!ty_U1_>) {