https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102855
康桓瑋 <hewillk at gmail dot com> changed:
What |Removed |Added
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CC| |hewillk at gmail dot com
--- Comment #2 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Bernhard Manfred Gruber from comment #0)
> 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:
> https://godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(filename:'1',fontScale:
> 14,fontUsePx:'0',j:1,lang:c%2B%2B,selection:(endColumn:2,endLineNumber:8,
> positionColumn:2,positionLineNumber:8,selectionStartColumn:2,
> selectionStartLineNumber:8,startColumn:2,startLineNumber:8),source:
> 'void+f(int)%3B%0A%0Atemplate+%3Cint+n%3E%0Avoid+g()+%7B%0A++++%23pragma+GCC+
> unroll+n%0A++++for+(int+i+%3D+0%3B+i+%3C+n%3B+i%2B%2B)%0A++++++++f(i)%3B%0A%7
> D%0A%0Aint+main()+%7B%0A++++constexpr+auto+n+%3D+100%3B%0A++++g%3Cn%3E()%3B%0
> A%7D'),l:'5',n:'0',o:'C%2B%2B+source+%231',t:'0')),k:54.105263157894754,l:
> '4',n:'0',o:'',s:0,t:'0'),(g:!((g:!((h:compiler,i:(compiler:g112,filters:(b:
> '0',binary:'1',commentOnly:'0',demangle:'0',directives:'0',execute:'1',intel:
> '0',libraryCode:'0',trim:'1'),flagsViewOpen:'1',fontScale:14,fontUsePx:'0',j:
> 4,lang:c%2B%2B,libs:!(),options:'-O3+-Wall+-Wextra',selection:(endColumn:1,
> endLineNumber:1,positionColumn:1,positionLineNumber:1,selectionStartColumn:1,
> selectionStartLineNumber:1,startColumn:1,startLineNumber:1),source:1,tree:
> '1'),l:'5',n:'0',o:'x86-64+gcc+11.2+(C%2B%2B,+Editor+%231,+Compiler+%234)',t:
> '0')),k:20.000000000000004,l:'4',m:50,n:'0',o:'',s:0,t:'0'),(g:!((h:output,i:
> (compiler:4,editor:1,fontScale:14,fontUsePx:'0',tree:'1',wrap:'1'),l:'5',n:
> '0',o:'Output+of+x86-64+gcc+11.2+(Compiler+%234)',t:'0')),header:(),l:'4',m:
> 50,n:'0',o:'',s:0,t:'0')),k:45.894736842105274,l:'3',n:'0',o:'',t:'0')),l:
> '2',n:'0',o:'',t:'0')),version:4
Why your godbolt link is so Dope.
