| Issue |
150120
|
| Summary |
Missed optimization: Lowering struct materialization into cold branches
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
jeremy-rifkin
|
Clang optimizes sub-optimally for the following code:
```cpp
struct S {
int& x;
int& y;
bool check() {
return x < y;
}
};
[[noreturn]] [[gnu::cold]] void bar(const S& s);
void foo(int a, int b) {
S s{a, b};
if(s.check()) [[unlikely]] { // very unlikely and cold
bar(s);
}
}
```
```asm
foo(int, int):
sub rsp, 24
mov dword ptr [rsp + 4], edi
mov dword ptr [rsp], esi
lea rax, [rsp + 4]
mov qword ptr [rsp + 8], rax
mov rax, rsp
mov qword ptr [rsp + 16], rax
cmp edi, esi
jl .LBB0_2
add rsp, 24
ret
.LBB0_2:
lea rdi, [rsp + 8]
call bar(S const&)@PLT
```
Struct `S` must be on the stack in order to call `bar()`, however, that's only needed in the unlikely case that the condition fails. Ideally the codegen should be the following:
```asm
foo(int, int):
cmp edi, esi
jl .LBB0_2
ret
.LBB0_2:
sub rsp, 24
... copy edi/esi to the stack and make struct S ...
call bar(S const&)@PLT
```
MSVC generates something along these lines, gcc and clang do not: https://godbolt.org/z/4axKfoe8x
I can't simply write `if(a < b)` or delay the construction of `S` until inside the branch. My specific use case that results in code like this is [libassert](https://github.com/jeremy-rifkin/libassert), where an _expression_ template is built from the user's condition and that is evaluated and inspected during assertion failure.
Even if the code is written as follows, clang still generates sub-ideal code:
```cpp
void foo(int a, int b) {
if(a < b) [[unlikely]] { // very unlikely and cold
S s{a, b};
bar(s);
}
}
```
```asm
foo(int, int):
sub rsp, 24
mov dword ptr [rsp + 4], edi
mov dword ptr [rsp], esi
cmp edi, esi
jl .LBB0_2
add rsp, 24
ret
.LBB0_2:
lea rax, [rsp + 4]
mov qword ptr [rsp + 8], rax
mov rax, rsp
mov qword ptr [rsp + 16], rax
lea rdi, [rsp + 8]
call bar(S const&)@PLT
```
This may be a tricky optimization to perform, however, due to the above I expect it would benefit a large amount of code.
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