[Bug tree-optimization/103072] New: Folding common switch code

[Bug tree-optimization/103072] New: Folding common switch code

[llvm at rifkin dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103072

            Bug ID: 103072
           Summary: Folding common switch code
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

In the following code, GCC can optimize foo but not bar:

void foo(int x) {
    switch(x) {
        case 1: func1(); break;
        case 2: func1(); break;
        case 3: func1(); break;
        case 4: func1(); break;
        case 5: func1(); break;
        default: __builtin_unreachable();
    }
}

void bar(int x) {
    switch(x) {
        case 1: func2(x); break;
        case 2: func2(x); break;
        case 3: func2(x); break;
        case 4: func2(x); break;
        case 5: func2(x); break;
        default: __builtin_unreachable();
    }
}

https://godbolt.org/z/3eqnG71f5 (LLVM optimizes both)

[Bug tree-optimization/103072] Folding common switch code

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103072

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
We optimize the second function to:
void baz(int x) {
    switch(x) {
        case 1: func2(1); break;
        case 2: func2(2); break;
        case 3: func2(3); break;
        case 4: func2(4); break;
        case 5: func2(5); break;
        default: __builtin_unreachable();
    }
}
during evrp and cross-jumping isn't able to merge it then.  Users could very
well  write it the above way too.  So maybe the switchconv pass could be
improved not to do just the linear etc. expression handling, but also consider
code sequences that are the same except for the linear expression somewhere in
it and turn it back into use of a temporary.

[Bug tree-optimization/103072] Folding common switch code

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103072

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-11-04
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

[Bug tree-optimization/103072] Folding common switch code

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103072

--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
Or we can alternatively merge all the case blocks into one..

[Bug tree-optimization/103072] Folding common switch code

[llvm at rifkin dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103072

--- Comment #3 from Jeremy R. <llvm at rifkin dot dev> ---
(In reply to Jakub Jelinek from comment #1)
> So maybe the switchconv pass could be
> improved not to do just the linear etc. expression handling, but also
> consider code sequences that are the same except for the linear expression
> somewhere in it and turn it back into use of a temporary.

I think this is how LLVM approaches it. It might be nice to be able to fold
switches where it's not a linear sequence too, such as:

void bar(int x) {
    switch(x) {
        case 1: func(x); break;
        case 4: func(x); break;
        case 7: func(x); break;
        case 3: func(x); break;
        case 9: func(x); break;
        default: __builtin_unreachable();
    }
}

LLVM is able to fold this out of a switch though it also struggles due to the
early constant propagation and doesn't fold this perfectly.

[Bug tree-optimization/103072] Folding common switch code

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103072

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is a dup of bug 96245.

*** This bug has been marked as a duplicate of bug 96245 ***