[Bug c++/98649] New: Trivial jump table not eliminated

[Bug c++/98649] New: Trivial jump table not eliminated

[pdimov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98649

            Bug ID: 98649
           Summary: Trivial jump table not eliminated
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pdimov at gmail dot com
  Target Milestone: ---

Trivial jump tables where all entries are the same are sometimes not
eliminated. E.g. the following example

```
struct Base { virtual void run( float f ) = 0; };
struct T0: Base { void run( float f ); };
struct T1: Base { void run( float f ); };
struct T2: Base { void run( float f ); };
struct T3: Base { void run( float f ); };
struct T4: Base { void run( float f ); };

template<int I> struct mp_int {};

struct variant
{
    unsigned index_;

    union
    {
        T0 t0_;
        T1 t1_;
        T2 t2_;
        T3 t3_;
        T4 t4_;
    };

    T0& get( mp_int<0> ) { return t0_; }
    T1& get( mp_int<1> ) { return t1_; }
    T2& get( mp_int<2> ) { return t2_; }
    T3& get( mp_int<3> ) { return t3_; }
    T4& get( mp_int<4> ) { return t4_; }
};

template<int I> decltype(auto) get( variant& v )
{
    return v.get( mp_int<I>() );
}

void f1( variant& v, float f )
{
    switch( v.index_ )
    {
        case 0: get<0>(v).run( f ); break;
        case 1: get<1>(v).run( f ); break;
        case 2: get<2>(v).run( f ); break;
        case 3: get<3>(v).run( f ); break;
        case 4: get<4>(v).run( f ); break;
        default: __builtin_unreachable();
    }
}

```

(https://godbolt.org/z/MxzGh8)

results in

```
f1(variant&, float):
        mov     eax, DWORD PTR [rdi]
        lea     r8, [rdi+8]
        jmp     [QWORD PTR .L4[0+rax*8]]
.L4:
        .quad   .L3
        .quad   .L3
        .quad   .L3
        .quad   .L3
        .quad   .L3
.L3:
        mov     rax, QWORD PTR [rdi+8]
        mov     rdi, r8
        mov     rax, QWORD PTR [rax]
        jmp     rax
```

This case may seem contrived, but it's not that rare in practice, because code
using std::variant or equivalent (such as Boost.Variant2, from which the
example has been reduced) is becoming more and more common nowadays.

[Bug middle-end/98649] Trivial jump table not eliminated

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98649

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end
           Severity|normal                      |enhancement

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This might be because there is type based vtable not lowered until expand time.

[Bug middle-end/98649] Trivial jump table not eliminated

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98649

Martin Liška <marxin at gcc dot gnu.org> changed:

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

--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
Yes, that's the case. Switch lowering pass sees:

  <bb 2> [local count: 1073741809]:
  _1 = v_14(D)->index_;
  switch (_1) <default: <L5> [0.00%], case 0: <L7> [20.00%], case 1: <L8>
[20.00%], case 2: <L9> [20.00%], case 3: <L10> [20.00%], case 4: <L11>
[20.00%]>

  <bb 3> [local count: 214748365]:
<L7>:
  _15 = &v_14(D)->D.2444.t0_;
  _2 = MEM[(struct T0 *)v_14(D) + 8B].D.2369._vptr.Base;
  _3 = *_2;
  OBJ_TYPE_REF(_3;(struct T0)_15->0) (_15, f_16(D));
  goto <bb 9>; [100.00%]

  <bb 4> [local count: 214748365]:
<L8>:
  _18 = &v_14(D)->D.2444.t1_;
  _4 = MEM[(struct T1 *)v_14(D) + 8B].D.2383._vptr.Base;
  _5 = *_4;
  OBJ_TYPE_REF(_5;(struct T1)_18->0) (_18, f_16(D));
  goto <bb 9>; [100.00%]

...

[Bug middle-end/98649] Trivial jump table not eliminated

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98649

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-10-02

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. Interesting LLVM does not do the optimization either ...