[Bug c++/87613] New: Non-reachable default required in switch statement to get optimal code

[chtz at informatik dot uni-bremen.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87613

            Bug ID: 87613
           Summary: Non-reachable default required in switch statement to
                    get optimal code
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chtz at informatik dot uni-bremen.de
  Target Milestone: ---

Created attachment 44841
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44841&action=edit
Sample code with non-reachable default case

Compiling the attached code with -O2 generates a jump-table, which directly
contains the `foo<int>()` function addresses. Removing the (non-reachable)
`default` case generates an indirect jump-table which jumps to code that first
moves the function address into a register and then jumps.

In g++-8.x this always generates optimal code, but without the default-case a
warning is generated:
   warning: control reaches end of non-void function [-Wreturn-type]

Godbolt-Link (try around with commenting in/out the default or return 0;)
  https://godbolt.org/z/itpvL6

Directly returning writing this also leads to an indirect jump-table:
  case X: return foo<X>();