[clang] [LoongArch] Fix ABI mismatch with gcc/g++ about empty structs passing (PR #70320)

[Lu Weining via cfe-commits]

https://github.com/SixWeining created https://github.com/llvm/llvm-project/pull/70320

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 https://github.com/llvm/llvm-project/issues/70319

>From 3ed0605126e2cfd94e58dd38c2f58764e32e2057 Mon Sep 17 00:00:00 2001
From: Weining Lu <luweining at loongson.cn>
Date: Thu, 26 Oct 2023 13:40:27 +0800
Subject: [PATCH] [LoongArch] Fix ABI mismatch with gcc/g++ about empty structs
 passing

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 https://github.com/llvm/llvm-project/issues/70319
---
 clang/lib/CodeGen/Targets/LoongArch.cpp                | 10 ++++++----
 clang/test/CodeGen/LoongArch/abi-lp64d-empty-structs.c |  8 ++++----
 2 files changed, 10 insertions(+), 8 deletions(-)

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;