https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102855
Bug ID: 102855
Summary: #pragma GCC unroll n should support n being a template
parameter
Product: gcc
Version: 11.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: bernhardmgruber at gmail dot com
Target Milestone: ---
Using `#pragma GCC unroll n` for a loop fails to compile when `n` is a template
parameter. Given the following code:
```c++
void f(int);
template <int n>
void g() {
#pragma GCC unroll n
for (int i = 0; i < n; i++)
f(i);
}
int main() {
constexpr auto n = 100;
g<n>();
}
```
g++-11.2 produces the following diagnostic:
```
source>: In function 'void g()':
<source>:5:24: error: '#pragma GCC unroll' requires an assignment-expression
that evaluates to a non-negative integral constant less than 65535
5 | #pragma GCC unroll n
| ^
```
If `g` is turned into a normal function and the variable `n` is moved form
`main` to `g`, the example compiles fine and produces the expected behavior.
Allowing `n` inside the pragma to be a template parameter helps temendously in
optimizing hot loops in templated numeric codes. Such a feature is also
supported by clang, icc and nvcc. I want to kindly ask you to provide this
additional functionality.
Thank you!
Example on godbolt:
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