[Bug tree-optimization/96658] New: Unhelpful -Wstrict-overflow warning from std::push_heap

[Bug tree-optimization/96658] New: Unhelpful -Wstrict-overflow warning from std::push_heap

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96658

            Bug ID: 96658
           Summary: Unhelpful -Wstrict-overflow warning from
                    std::push_heap
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Reduced from https://stackoverflow.com/q/63405519/981959

This code produces a warning that is of zero value to users:

#include <algorithm>
void f(unsigned long* v) { std::push_heap(v, v+3); }

$ g++ -c -O3 -Wstrict-overflow=3 p.C 
p.C: In function 'void std::push_heap(_RAIter, _RAIter) [with _RAIter = long
unsigned int*]':
p.C:2:52: warning: assuming signed overflow does not occur when changing X +-
C1 cmp C2 to X cmp C2 -+ C1 [-Wstrict-overflow]
    2 | void f(unsigned long* v) { std::push_heap(v, v+3); }
      |                                                    ^

The user doesn't have any signed integers, and the C1 cmp C2 expression doesn't
appear in the user's code (it doesn't even appear in the libstdc++ headers).
This warning seems totally useless to users. The library code is correct, no
useful location is shown to the user, and the user can't change the library
anyway.

The code can be simplified to:

template<typename T>
    inline void
    push_heap(T* first, T* last)
    {    
        T value = (*(last - 1));
        long holeIndex = last - first - 1;
        long topIndex = 0;
        long parent = (holeIndex - 1) / 2;
        while (holeIndex > topIndex && *(first + parent) < value)
        {
            *(first + holeIndex) = *(first + parent);
            holeIndex = parent;
            parent = (holeIndex - 1) / 2;
        }
        *(first + holeIndex) = value;
    }

void f(unsigned long* v) { push_heap(v, v+3); }

Which still warns and still shows no useful information:

p.C: In function 'void push_heap(T*, T*) [with T = long unsigned int]':
p.C:18:47: warning: assuming signed overflow does not occur when changing X +-
C1 cmp C2 to X cmp C2 -+ C1 [-Wstrict-overflow]
   18 | void f(unsigned long* v) { push_heap(v, v+3); }
      |                                               ^


I know the documentation says that -Wstrict-overflow=3 easily gives false
positives, but this case seems especially unhelpful.

[Bug tree-optimization/96658] Unhelpful -Wstrict-overflow warning from std::push_heap

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96658

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |msebor at gcc dot gnu.org
      Known to fail|                            |10.3.0, 11.2.0, 12.0, 9.3.0
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-12-08

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed on trunk plus GCC 9, 10, 11 but not 8, so strictly speaking it's a
regression:

pr96658.C: In function ‘void push_heap(T*, T*) [with T = long unsigned int]’:
pr96658.C:19:47: warning: assuming signed overflow does not occur when changing
X +- C1 cmp C2 to X cmp C2 -+ C1 [-Wstrict-overflow]
   19 | void f(unsigned long* v) { push_heap(v, v+3); }
      |                                               ^

The warning first appeared with r263875.