[Bug middle-end/114086] New: Boolean switches could have a lot better codegen, possibly utilizing bit-vectors

[janschultke at googlemail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114086

            Bug ID: 114086
           Summary: Boolean switches could have a lot better codegen,
                    possibly utilizing bit-vectors
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janschultke at googlemail dot com
  Target Milestone: ---

https://godbolt.org/z/3acqbbn3E

enum struct E { a, b, c, d, e, f, g, h };

bool test_switch(E e) {
    switch (e) {
    case E::a:
    case E::c:
    case E::e:
    case E::g: return true;
    default: return false;
    }
}


Expected output
===============

test_switch(E):
  mov eax, edi
  and eax, 1
  ret



Actual output (-O3)
===================

test_switch(E):
  xor eax, eax
  cmp edi, 6
  ja .L1
  mov eax, 85
  bt rax, rdi
  setc al
.L1:
  ret


Explanation
===========

Boolean switches in general can be optimized a lot better than what GCC
currently does. Clang does find the optimization to a bitwise AND, although
this may be a big ask.

Generally, contiguous boolean switches (that is, switch statements where all
cases yield a boolean value and the labels are contiguous) can be optimized to
accessing a bit vector.

That switch could have been transformed into:

> return 0b01010101 >> int(e);