[Bug c/123212] [15/16 Regression] GCC 15 no longer compiles switch statements to jump tables by default with Og

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123212

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
            Summary|GCC 15 no longer compiles   |[15/16 Regression] GCC 15
                   |switch statements to jump   |no longer compiles switch
                   |tables by default with Og   |statements to jump tables
                   |                            |by default with Og
                 CC|                            |pheeck at gcc dot gnu.org
   Target Milestone|---                         |15.3
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2025-12-19

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
switch conversion would have chosen a load from a constant initializer, not a
jump table.  Instead it's switch lowering that turns the switch into a binary
tree.

I wonder why we chose that for the dense case.  At least at -O0 expanding
to a jump table feels more natural (it's also smaller by a small margin
at -O0, but larger by a larger margin at -Og, so ...).

int example(int num) {
    switch(num) {
        case 0: return 3420;
        case 1: return 1304123;
        case 2: return 2139;
        case 3: return 13;
        case 4: return 320;
        case 5: return 134123;
        case 6: return 239;
        case 7: return 1;
    }

    return 0;
}