[clang] [RISCV] Handle empty structs/unions passing in C++ (PR #97315)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-risc-v

@llvm/pr-subscribers-clang-codegen

Author: Sudharsan Veeravalli (svs-quic)

<details>
<summary>Changes</summary>

According to RISC-V integer calling convention empty structs or union arguments or return values are ignored by C compilers which support them as a non-standard extension. This is not the case for C++, which requires them to be sized types.

Fixes #<!-- -->97285 

---
Full diff: https://github.com/llvm/llvm-project/pull/97315.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/Targets/RISCV.cpp (+5-4) 
- (modified) clang/test/CodeGen/RISCV/abi-empty-structs.c (+34) 


``````````diff
diff --git a/clang/lib/CodeGen/Targets/RISCV.cpp b/clang/lib/CodeGen/Targets/RISCV.cpp
index a4c5ec315b8df..f2add9351c03c 100644
--- a/clang/lib/CodeGen/Targets/RISCV.cpp
+++ b/clang/lib/CodeGen/Targets/RISCV.cpp
@@ -361,12 +361,13 @@ ABIArgInfo RISCVABIInfo::classifyArgumentType(QualType Ty, bool IsFixed,
                                            CGCXXABI::RAA_DirectInMemory);
   }
 
-  // Ignore empty structs/unions.
-  if (isEmptyRecord(getContext(), Ty, true))
-    return ABIArgInfo::getIgnore();
-
   uint64_t Size = getContext().getTypeSize(Ty);
 
+  // Ignore empty structs/unions whose size is zero. According to the calling
+  // convention empty structs/unions are required to be sized types in C++.
+  if (isEmptyRecord(getContext(), Ty, true) && Size == 0)
+    return ABIArgInfo::getIgnore();
+
   // Pass floating point values via FPRs if possible.
   if (IsFixed && Ty->isFloatingType() && !Ty->isComplexType() &&
       FLen >= Size && ArgFPRsLeft) {
diff --git a/clang/test/CodeGen/RISCV/abi-empty-structs.c b/clang/test/CodeGen/RISCV/abi-empty-structs.c
index c48a2891627d4..877ed4962d4e8 100644
--- a/clang/test/CodeGen/RISCV/abi-empty-structs.c
+++ b/clang/test/CodeGen/RISCV/abi-empty-structs.c
@@ -167,6 +167,40 @@ struct s9 {
 //
 void test_s9(struct s9 a) {}
 
+struct s10 { };
+// CHECK-C-LABEL: define dso_local void @test_s10
+// CHECK-C-SAME: () #[[ATTR0]] {
+// CHECK-C:  entry:
+//
+// CHECK32-CXX-LABEL: define dso_local i32 @_Z8test_s103s10
+// CHECK32-CXX-SAME: (i32 [[A_COERCE:%.*]]) #[[ATTR0]] {
+// CHECK32-CXX:  entry:
+//
+// CHECK64-CXX-LABEL: define dso_local i64 @_Z8test_s103s10
+// CHECK64-CXX-SAME: (i64 [[A_COERCE:%.*]]) #[[ATTR0]] {
+// CHECK64-CXX:  entry:
+//
+struct s10 test_s10(struct s10 a) {
+  return a;
+}
+
+struct s11 { int : 0; };
+// CHECK-C-LABEL: define dso_local void @test_s11
+// CHECK-C-SAME: () #[[ATTR0]] {
+// CHECK-C:  entry:
+//
+// CHECK32-CXX-LABEL: define dso_local i32 @_Z8test_s113s11
+// CHECK32-CXX-SAME: (i32 [[A_COERCE:%.*]]) #[[ATTR0]] {
+// CHECK32-CXX:  entry:
+//
+// CHECK64-CXX-LABEL: define dso_local i64 @_Z8test_s113s11
+// CHECK64-CXX-SAME: (i64 [[A_COERCE:%.*]]) #[[ATTR0]] {
+// CHECK64-CXX:  entry:
+//
+struct s11 test_s11(struct s11 a) {
+  return a;
+}
+
 //// NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
 // CHECK32-C: {{.*}}
 // CHECK64-C: {{.*}}

``````````

</details>


https://github.com/llvm/llvm-project/pull/97315