https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113985
Bug ID: 113985
Summary: redundant copy of return values at O0
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: absoler at smail dot nju.edu.cn
Target Milestone: ---
hi, I found with -O0 flag, there are redundant operations on return values of
structure type.
```
typedef signed char int8_t;
typedef unsigned char uint8_t;
typedef signed short int int16_t;
typedef unsigned short int uint16_t;
typedef signed int int32_t;
typedef unsigned int uint32_t;
typedef signed long int int64_t;
typedef unsigned long int uint64_t;
struct S0 {
int32_t f0;
uint8_t f1;
int32_t f2;
};
struct S0 g_91 = {0x6732C952L,0x6BL,6L};
int64_t g_205[9] =
{0L,0L,0x47DC22DBA5B9078ELL,0L,0L,0x47DC22DBA5B9078ELL,0L,0L,0x47DC22DBA5B9078ELL};
void func_1(void);
struct S0 func_2();
void func_1() {
struct S0 a;
struct S0 *b = &g_91;
*b = func_2(0, a, a.f1, g_205[0]);
}
struct S0 func_2() { return g_91;}
```
the disassembly:
```
0000000000401186 <func_1>:
func_1():
/root/loadtest0/test/output2.c:44
401186: push %rbp
401187: mov %rsp,%rbp
40118a: sub $0x20,%rsp
/root/loadtest0/test/output2.c:46
40118e: movq $0x404080,-0x8(%rbp)
/root/loadtest0/test/output2.c:47
401196: mov 0x2f03(%rip),%rsi # 4040a0 <g_205>
40119d: movzbl -0x10(%rbp),%eax
4011a1: movzbl %al,%ecx
4011a4: mov -0x14(%rbp),%rdx
4011a8: mov -0xc(%rbp),%eax
4011ab: mov %rsi,%r8
4011ae: mov %rdx,%rsi
4011b1: mov %eax,%edx
4011b3: mov $0x0,%edi
4011b8: mov $0x0,%eax
4011bd: callq 4011d7 <func_2>
4011c2: mov -0x8(%rbp),%rcx
4011c6: mov %rax,(%rcx)
4011c9: mov 0x8(%rcx),%eax // redundant
4011cc: and $0x0,%eax
4011cf: or %edx,%eax
4011d1: mov %eax,0x8(%rcx)
/root/loadtest0/test/output2.c:48
4011d4: nop
4011d5: leaveq
4011d6: retq
```
we can see that it's unnecessary to load `0x8(%rcx)` into %eax, because it will
be cleared in the next instruction
