https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99128
Bug ID: 99128
Summary: Stack used for concatenating values when returning
struct by register
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: magiblot at hotmail dot com
Target Milestone: ---
Consider the following C++ example (https://godbolt.org/z/deqf4T):
> struct A { int x, y; };
>
> A makeA(int a, int b)
> {
> return {a, b};
> }
>
> struct B { int x, y, z; };
>
> B makeB(int a, int b)
> {
> return {a, b};
> }
In 'makeA' the whole struct fits into a 64-bit register and 'a' and 'b' are
concatenated with arithmetic operations, as expected:
> makeA(int, int):
> sal rsi, 32
> mov eax, edi
> or rax, rsi
> ret
But in 'makeB' the struct has a third field which gets returned in a different
register. In this case, 'a' and 'b' are written into the stack and then moved
to the return register:
> makeB(int, int):
> mov DWORD PTR [rsp-20], edi
> xor edx, edx
> mov DWORD PTR [rsp-16], esi
> mov rax, QWORD PTR [rsp-20]
> ret
Here is another example:
> struct C {
> union {
> char a;
> char b[3];
> };
> char c;
> };
>
> C makeC(char a)
> {
> return {a, 1};
> }
The assembly looks like this:
> makeC(char):
> xor eax, eax
> mov BYTE PTR [rsp-4], dil
> mov WORD PTR [rsp-3], ax
> mov BYTE PTR [rsp-1], 1
> mov eax, DWORD PTR [rsp-4]
> ret
Which is pretty bad compared to what Clang produces:
> makeC(char):
> movzx eax, dil
> or eax, 16777216
> ret
The 'makeB' example results in suboptimal x86_64 assembly in all versions of
GCC available at Godbolt which can compile the code.
The 'makeC' example, though, is fine in GCC 8.3 and earlier but produces
suboptimal assembly in GCC 9.1 and later.
EXPECTED RESULT
GCC should be able to concatenate primitive types with arithmetic-logic
operations instead of using the stack when returning structs by register.
Thank you.
