| Issue |
162031
|
| Summary |
[clang] [codegen] unexpected codegen when struct is returned from a function or when struct has specific field order
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
03F001
|
clang 21.1 x86-64 -O3
https://godbolt.org/z/3WY37brEY simplified case where f1 codegen is expected to match f2. Define `FIX1` to make the codegen match, or define `FIX2`.
`FIX1` changes the struct field order so that the bool does not occur as the last field.
`FIX2` changes struct creation is returned from a function rather than constructed in the function f2.
Full code:
```cpp
void act(unsigned);
// baseline
void f1(unsigned stop)
{
auto i = 0;
do {
act(i);
++i;
} while ( i != stop );
}
struct Iter
{
#ifdef FIX1
bool done = false;
unsigned i = 0;
unsigned stop;
#else
unsigned i = 0;
unsigned stop;
bool done = false; // codegen can differ when last field
#endif
explicit constexpr Iter(unsigned stop) noexcept
: stop(stop) {}
constexpr bool more() const noexcept { return !done; }
constexpr unsigned front() const noexcept { return i; }
constexpr void popFront() noexcept
{
++i;
done = i == stop;
}
};
// codegen can differ when Iter is returned from a function
constexpr Iter mkIter(unsigned stop) noexcept
{
return Iter(stop);
}
// does not match f1
// define FIX1 to match f1
// define FIX2 to match f1
void f2(unsigned stop)
{
#ifdef FIX2
auto r = Iter(stop);
#else
auto r = mkIter(stop);
#endif
for ( ; r.more(); r.popFront() ) {
act(r.front());
}
}
```
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