[LoongArch] Fix ABI mismatch with gcc/g++ about empty structs passing

[SixWeining]

How empty structs (not as fields of container struct) are passed in C++ is not explicitly documented in psABI. However, this patch fixes the mismatch with g++.

Note that the unnamed bitfield case struct { int : 1; } in C is also fixed. Previously clang regards it as an empty struct and then ignores it when passing. Now size of the struct is counted; since it's size is not 0, clang will not ignore it even in C.

While https://reviews.llvm.org/D156116 fixed the handling of empty struct when considering eligibility of the container struct for the FP calling convention ('flattening'), this patch fixes the handling of passing the empty struct itself.

Fix #70319

[llvmbot]

@llvm/pr-subscribers-clang
@llvm/pr-subscribers-backend-loongarch

@llvm/pr-subscribers-clang-codegen

Author: Lu Weining (SixWeining)

Changes

How empty structs (not as fields of container struct) are passed in C++ is not explicitly documented in psABI. However, this patch fixes the mismatch with g++.

Note that the unnamed bitfield case struct { int : 1; } in C is also fixed. Previously clang regards it as an empty struct and then ignores it when passing. Now size of the struct is counted; since it's size is not 0, clang will not ignore it even in C.

While https://reviews.llvm.org/D156116 fixed the handling of empty struct when considering eligibility of the container struct for the FP calling convention ('flattening'), this patch fixes the handling of passing the empty struct itself.

Fix #70319

---

Full diff: https://github.com/llvm/llvm-project/pull/70320.diff

2 Files Affected:

• (modified) clang/lib/CodeGen/Targets/LoongArch.cpp (+6-4)
• (modified) clang/test/CodeGen/LoongArch/abi-lp64d-empty-structs.c (+4-4)

diff --git a/clang/lib/CodeGen/Targets/LoongArch.cpp b/clang/lib/CodeGen/Targets/LoongArch.cpp
index 7483bf6d6d1e8e2..bc508a99da9cedf 100644
--- a/clang/lib/CodeGen/Targets/LoongArch.cpp
+++ b/clang/lib/CodeGen/Targets/LoongArch.cpp
@@ -308,12 +308,14 @@ ABIArgInfo LoongArchABIInfo::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 struct or union whose size is zero, e.g. `struct { }` in C or
+  // `struct { int a[0]; }` in C++. In C++, `struct { }` is empty but it's size
+  // is 1 byte and g++ doesn't ignore it; clang++ matches this behaviour.
+  if (isEmptyRecord(getContext(), Ty, true) && Size == 0)
+    return ABIArgInfo::getIgnore();
+
   // Pass floating point values via FARs if possible.
   if (IsFixed && Ty->isFloatingType() && !Ty->isComplexType() &&
       FRLen >= Size && FARsLeft) {
diff --git a/clang/test/CodeGen/LoongArch/abi-lp64d-empty-structs.c b/clang/test/CodeGen/LoongArch/abi-lp64d-empty-structs.c
index d0daafac336ec0c..281b7b15841a999 100644
--- a/clang/test/CodeGen/LoongArch/abi-lp64d-empty-structs.c
+++ b/clang/test/CodeGen/LoongArch/abi-lp64d-empty-structs.c
@@ -93,7 +93,7 @@ struct s9 test_s9(struct s9 a) {
 }
 
 // CHECK-C: define{{.*}} void @test_s10()
-// CHECK-CXX: define{{.*}} void @_Z8test_s103s10()
+// CHECK-CXX: define{{.*}} i64 @_Z8test_s103s10(i64 {{.*}})
 struct s10 { };
 struct s10 test_s10(struct s10 a) {
   return a;
@@ -128,14 +128,14 @@ struct s14 test_s14(struct s14 a) {
 }
 
 // CHECK-C: define{{.*}} void @test_s15()
-// CHECK-CXX: define{{.*}} void @_Z8test_s153s15()
+// CHECK-CXX: define{{.*}} i64 @_Z8test_s153s15(i64 {{.*}})
 struct s15 { int : 0; };
 struct s15 test_s15(struct s15 a) {
   return a;
 }
 
-// CHECK-C: define{{.*}} void @test_s16()
-// CHECK-CXX: define{{.*}} void @_Z8test_s163s16()
+// CHECK-C: define{{.*}} i64 @test_s16(i64 {{.*}})
+// CHECK-CXX: define{{.*}} i64 @_Z8test_s163s16(i64 {{.*}})
 struct s16 { int : 1; };
 struct s16 test_s16(struct s16 a) {
   return a;

[SixWeining]

Hi @xen0n @xry111, what do you think about this change? If it is ok, I'd like to checrry-pick to release/17.x (hope it's not too late...).

[xry111]

LGTM. I've no permission to make a formal ("GitHub style") approval.

[SixWeining]

LGTM. I've no permission to make a formal ("GitHub style") approval.

Thanks. You can request to join the llvm organization via https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access firstly. Then you can make formal ("Github style") approval.