[Bug c/80410] New: Improve merging of identical branches

[pascal_cuoq at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80410

            Bug ID: 80410
           Summary: Improve merging of identical branches
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pascal_cuoq at hotmail dot com
  Target Milestone: ---

Consider the following code:

  va_list ap;
  void *result;

  va_start(ap, ptr_type);

  switch (ptr_type) {
    case 0: result = va_arg(ap, int*); break;
    case 1: result = va_arg(ap, char*); break;
    case 2: result = va_arg(ap, short*); break;
    default: result = va_arg(ap, void*); break;
  }

  va_end(ap);

GCC correctly recognizes that the assembly code for the cases 0, 1, 2 is
identical, which is the difficult part (the source code for the branches is
different and cannot be merged by the programmer unless
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80409 is resolved). At the time of
writing, the Compiler Explorer trunk version produces:

        movq    %rax, -56(%rsp)
        je      .L15
        cmpl    $2, %edi
        je      .L15
        testl   %edi, %edi
        je      .L16
.L15:
        movq    -56(%rsp), %rax
.L16:

(Compiler Explorer link: https://godbolt.org/g/0NvnSX )

It would be nice if GCC was able to omit all the conditional branches, since
they all lead to doing the same thing at the assembly level.