[Bug c++/99823] New: -funroll-all-loops bugs when using contexpr variable

[Bug c++/99823] New: -funroll-all-loops bugs when using contexpr variable

[ustcw0ng at mail dot ustc.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99823

            Bug ID: 99823
           Summary: -funroll-all-loops bugs when using contexpr variable
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ustcw0ng at mail dot ustc.edu.cn
  Target Milestone: ---

Created attachment 50483
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50483&action=edit
constexpr.cpp && unroll.cpp

-funroll-all-loops indicates "Unroll all loops, even if their number of
iterations is uncertain when the loop is entered. This usually makes programs
run more slowly.". But in the following example constexpr.cpp can never turn to
unroll.cpp, even if number of iterations is certain due to constexpr.

compiler flag: -O3 -std=c++2a -DNDEBUG -funroll-all-loops

[Bug c++/99823] -funroll-all-loops bugs when using contexpr variable

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99823

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-03-30
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
-funroll-all-loops applies to the RTL loop unroller, the GIMPLE level
concludes:

Estimating sizes for loop 1
 BB: 3, after_exit: 0
  size:   1 _1 = MEM[(int (*<T9564>) (int) &)__for_begin_21];
  size:   5 _13 = _1 (s_20);
  size:   1 __for_begin_14 = __for_begin_21 + 8;
  size:   1 ivtmp_4 = ivtmp_11 - 1;
   Induction variable computation will be folded away.
  size:   2 if (ivtmp_4 != 0)
   Exit condition will be eliminated in peeled copies.
   Exit condition will be eliminated in last copy.
   Constant conditional.
 BB: 5, after_exit: 1
size: 10-3, last_iteration: 10-3
  Loop size: 10
  Estimated size after unrolling: 14
Not unrolling loop 1: contains call and code would grow.

so it concludes unrolling isn't profitable (but it would turn indirect into
direct calls).

[Bug tree-optimization/99823] -funroll-all-loops bugs when using contexpr variable

[ustcw0ng at mail dot ustc.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99823

--- Comment #2 from apple <ustcw0ng at mail dot ustc.edu.cn> ---
(In reply to Richard Biener from comment #1)
> -funroll-all-loops applies to the RTL loop unroller, the GIMPLE level
> concludes:
> 
> Estimating sizes for loop 1
>  BB: 3, after_exit: 0
>   size:   1 _1 = MEM[(int (*<T9564>) (int) &)__for_begin_21];
>   size:   5 _13 = _1 (s_20);
>   size:   1 __for_begin_14 = __for_begin_21 + 8;
>   size:   1 ivtmp_4 = ivtmp_11 - 1;
>    Induction variable computation will be folded away.
>   size:   2 if (ivtmp_4 != 0)
>    Exit condition will be eliminated in peeled copies.
>    Exit condition will be eliminated in last copy.
>    Constant conditional.
>  BB: 5, after_exit: 1
> size: 10-3, last_iteration: 10-3
>   Loop size: 10
>   Estimated size after unrolling: 14
> Not unrolling loop 1: contains call and code would grow.
> 
> so it concludes unrolling isn't profitable (but it would turn indirect into
> direct calls).

Anyway, I should mention here that clang can unroll it without this flag and
turn out constexpr.cpp equal to unroll.cpp. And you can try to insert "#pragma
GCC unroll 2“ the loop finally unrolled, even in this pragma, constexpr.cpp is
not equal to unroll.cpp. But this is technically another issue