Clang frontend generates unnecessary memcpy (that is optimized to load/store) for an empty class

[strimo378]

Hi all,

I found out that Clang codegen emits an unnecessary memcpy for an empty C++ class with an empty struct. I don't know if other (more common) cases are effected.

The following code

struct a {
  struct {};
} b;
a c;
int main() { b = c; }

generates the LLVM IR (see https://godbolt.org/z/4hjx8xxqf)

define dso_local noundef i32 @main() #0 {
  call void @llvm.memcpy.p0.p0.i64(ptr align 1 @b, ptr align 1 @c, i64 1, i1 false), !tbaa.struct !6
  ret i32 0
}

...

!6 = !{}

Interestingly, the memcpy is generated with an empty tbaa.struct metadata, indicating that the memcpy is copying nothing.

During IR optimization the memcpy is optimized to a load/store.

define dso_local noundef i32 @main() local_unnamed_addr #0 {
  %1 = load i8, ptr @c, align 1
  store i8 %1, ptr @b, align 1
  ret i32 0
}

The problem is within the CodeGenFunction::EmitAggregateCopy function in clang/lib/CodeGen/CGExprAgg.cpp. The function only checks if the record is empty using Record->isEmpty(). Either isEmpty is wrong in that case or the calculated data layout of the record is a better choice to skip memcpy emission.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[strimo378]

Just another test case with a combination of fields and inheritance:

struct a {};
struct b : a {};
struct c {
  b d;
  void e();
} f;
void g() {
  c h = f;
  h.e();
} 

In that test case, h = f emits a load/store that is unnecessary (and gcc does not emit one).

[strimo378]

Pure C is also affected:

struct S {
    char : 8;
};

struct S a,b;

void test() {
    a = b;
}