[Bug tree-optimization/94836] New: Failure to optimize condition based on known value of static variable

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94836

            Bug ID: 94836
           Summary: Failure to optimize condition based on known value of
                    static variable
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f(int x)
{
    static int s;

    if (s)
        s = x;

    return s;
}

This can be optimized to `return 0`. This transformation is done by LLVM, but
not by GCC

LLVM -O3 outputs :

f(int): # @f(int)
  xor eax, eax
  ret

GCC -O3 outputs :

f(int):
  mov eax, DWORD PTR f(int)::s[rip]
  test eax, eax
  je .L1
  mov DWORD PTR f(int)::s[rip], edi
  mov eax, edi
.L1:
  ret

(see also https://godbolt.org/z/FzpjCb)