[Bug tree-optimization/112687] New: missed-optimization: switch statement does not simplify to it's expression

[Bug tree-optimization/112687] New: missed-optimization: switch statement does not simplify to it's expression

[goon.pri.low at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112687

            Bug ID: 112687
           Summary: missed-optimization: switch statement does not
                    simplify to it's expression
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goon.pri.low at gmail dot com
  Target Milestone: ---

This following code:
int unopt(int v) {
    switch (v & 3) {
        case 0: return 0;
        case 1: return 1;
        case 2: return 2;
        case 3: return 3;
        default: __builtin_unreachable();
    }
}

unopt:
        mov     eax, edi
        and     eax, 3
        lea     edx, [rax-1]
        cmp     edx, 3
        mov     edx, 0
        cmovnb  eax, edx
        ret

Ought to be optimized to:
int opt(int v) {
    return v & 3;
}

opt:
        mov     eax, edi
        and     eax, 3
        ret

Note: I decided to separate this problem from
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112645 since it was unrelated to
the main bug, though Richard offers some initial insight into the problem.

[Bug tree-optimization/112687] missed-optimization: switch statement does not simplify to it's expression

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112687

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-11-24
             Status|UNCONFIRMED                 |NEW
                 CC|                            |fkastl at suse dot cz

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

[Bug tree-optimization/112687] missed-optimization: switch statement does not simplify to it's expression

[fkastl at suse dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112687

--- Comment #2 from Filip Kastl <fkastl at suse dot cz> ---
Created attachment 57222
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57222&action=edit
WIP patch to fix the missed optimization, version 0

I'm working on a patch. The problem (as Richard stated in pr112645) is that the
testcase gets optimized into

switch (v & 3) {
    default:
        return 0;
    case 1:
        return 1;
    case 2:
        return 2;
    case 3:
        return 3;
    }

so the information that there are only 4 possible values of the expression 'v &
3' is lost.

I add capability for switch conversion to recover from this. With my patch,
before processing the switch, switch conversion finds out if the default case
only corresponds to one possible value of the index expression. If so, it
creates a case for this value and marks default as unreachable.

With the patch applied, functions opt() and unopt() compile into the same
sequence of assembly instructions. I've attached the patch. Do you think this
is a good approach? I'll also appreciate any suggestions/comments for the
patch.