Traps for signed arithmetic overflow

[Helmut Eller]

Hello,

when compiling this example with gcc -O2 -ftrapv:

  long foo (long x, long y) { return x + y; }
  
  long bar (long x, long y) {
    long z;
    if (__builtin_add_overflow (x, y, &z))
      __builtin_trap ();
    return z;
  }

then GCC seems to produce less efficient code for foo than for bar:

  foo:
        subq    $8, %rsp
        call    __addvdi3@PLT
        addq    $8, %rsp
        ret

  bar:
        movq    %rdi, %rax
        addq    %rsi, %rax
        jo      .L9
        rep ret
  .L9:
        ud2

I see several inefficiencies:

1.) __addvdi3 is not inlined.

2.) %rsp is adjusted before calling __addvdi3.  Why is that needed?

3.) Obviously __addvdi3 is not implemented as sibling-call even though
    -O2 should enable that.

Where should I start, if I wanted to teach GCC how to produce the same
code for foo as for bar?  Would it be enough to add a pattern to
i386.md?  There is already a pattern for "addv<mode>4", but apparently
it's not used in this case.

Helmut