[Bug target/86952] Avoid jump table for switch statement with -mindirect-branch=thunk

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86952

--- Comment #6 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to H.J. Lu from comment #5)
> (In reply to Martin Liška from comment #4)
> > (In reply to H.J. Lu from comment #3)
> > > (In reply to Martin Liška from comment #2)
> > > > H.J. I can write a patch for it. Do you expect more expensive costs when
> > > > retpolines are enabled?
> > > 
> > > retpoline is more expensive than 4 branches.
> > 
> > Can you please make a microbenchmark that will expose how exactly is that
> > expensive? Based on that I can tune current costs.
> 
> Is there a testcase where GCC generates a jump table for a five-entry
> switch statement?

$ cat jt.c
int global;

int foo3 (int x)
{
  switch (x) {
    case 0:
      return 11;
    case 1:
      return 123;
    case 2:
      global += 1;
      return 3;
    case 3:
      return 44;
    case 4:
      return 444;
    default:
      return 0;
  }
}

$ gcc jt.c -O2  -S -o/dev/stdout
        .file   "jt.c"
        .text
        .p2align 4,,15
        .globl  foo3
        .type   foo3, @function
foo3:
.LFB0:
        .cfi_startproc
        cmpl    $4, %edi
        ja      .L2
        movl    %edi, %edi
        jmp     *.L4(,%rdi,8)
        .section        .rodata
        .align 8
        .align 4
.L4:
        .quad   .L9
        .quad   .L7
        .quad   .L6
        .quad   .L5
        .quad   .L3
        .text
        .p2align 4,,10
        .p2align 3
...