| Issue |
84277
|
| Summary |
[missed optimization] repeat load operation when returning a struct
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Absoler
|
I have found a test case (reduced from https://github.com/llvm/llvm-project/issues/83812) which showes a missed optimization of clang-17.0.6 -O2
```
struct S0 {
int f0;
int f1;
long long f2;
};
int copy = 0;
struct S0 global_s = {1, 1, 1};
struct S0 value = {2, 2, 2};
int other = 1;
struct S0 get_struct() { return value; }
void func_bad(struct S0 d) {
struct S0 local_s;
global_s = local_s = get_struct();
copy = local_s.f1;
if (copy)
--other;
}
```
clang-17~15 fail to reuse the value loaded from `global_s` while clang-14 could. here's the comparison of generated binaries:
clang-17: ( https://godbolt.org/z/Gx98vzoYP )
```
0000000000001140 <func_bad>:
1140: movaps 0x2ef9(%rip),%xmm0 # 4040 <value>
1147: mov 0x2ef2(%rip),%rax # 4040 <value>
func_bad():
114e: shr $0x20,%rax
1152: movaps %xmm0,0x2ed7(%rip) # 4030 <global_s>
1159: mov %eax,0x2ef9(%rip) # 4058 <copy>
115f: je 1167 <func_bad+0x27>
1161: decl 0x2ee9(%rip) # 4050 <other>
```
clang-14:
```
0000000000401120 <func_bad>:
401120: movdqa 0x2f18(%rip),%xmm0 # 404040 <value>
func_bad():
401128: movq %xmm0,%rax
40112d: shr $0x20,%rax
401131: movdqa %xmm0,0x2ef7(%rip) # 404030 <global_s>
401139: mov %eax,0x2f19(%rip) # 404058 <copy>
40113f: je 401148 <func_bad+0x28>
401141: addl $0xffffffff,0x2f08(%rip) # 404050 <other>
```
we can see that in clang-17's output the load operation at 0x1147 could be removed, and it's better to reuse the value in %xmm0 just like clang-14's output dose at 0x401128.
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