| Issue |
55486
|
| Summary |
x86: missing jmp [rax] optimization on non-niladic virtual functions
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Alcaro
|
```c++
struct x0 {
virtual void y();
};
void a(x0* b) {
b->y();
}
struct x1 {
virtual void y(int);
};
void a(x1* b) {
b->y(0);
}
void a(x1* b, int t) {
b->y(t);
}
```
with -O2 or -Os. Result:
```asm
a(x0*): # @a(x0*)
mov rax, qword ptr [rdi]
jmp qword ptr [rax] # TAILCALL
a(x1*): # @a(x1*)
mov rax, qword ptr [rdi]
mov rax, qword ptr [rax]
xor esi, esi
jmp rax # TAILCALL
a(x1*, int): # @a(x1*, int)
mov rax, qword ptr [rdi]
mov rax, qword ptr [rax]
jmp rax # TAILCALL
```
The first function is fully optimized, but the latter two are not. I'd rather see
```asm
a(x0*): # @a(x0*)
mov rax, qword ptr [rdi]
jmp qword ptr [rax] # TAILCALL
a(x1*): # @a(x1*)
mov rax, qword ptr [rdi]
xor esi, esi
jmp qword ptr [rax] # TAILCALL
a(x1*, int): # @a(x1*, int)
mov rax, qword ptr [rdi]
jmp qword ptr [rax] # TAILCALL
```
GCC fully optimizes the third function, but not the second. https://godbolt.org/z/5xoxdb6YK
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