[Bug middle-end/91883] New: Division by a constant could be optimized for known variables value range

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91883

            Bug ID: 91883
           Summary: Division by a constant could be optimized for known
                    variables value range
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the example:

unsigned long long kBorder = (1ull<<62);

unsigned long long sample(unsigned long long m) {
    if (m >= kBorder) __builtin_unreachable();
    return m / 10;
}

It produces the following assembly:

sample(unsigned long long):
  movabs rdx, -3689348814741910323
  mov rax, rdi
  mul rdx
  mov rax, rdx
  shr rax, 3
  ret

However, knowing that the higher bits are always 0, the constant could be
adjusted to avoid the `shr rax, 3`:

sample(unsigned long long):
  movabs rax, 1844674407370955162
  mul rdi
  mov rax, rdx
  ret

Godbolt playground: https://godbolt.org/z/YU2yAC

This issue is probably related to PR 91881

P.S.: that optimization is important for std::to_chars(..., double) like
functions, where a significant of a double is extracted into an unsigned long
long variable, so its upper bits are always zero.