Re: Traps for signed arithmetic overflow

[Helmut Eller <eller.helmut@xxxxxxxxx>]

On Fri, Nov 23 2018, Marc Glisse wrote:

> On Fri, 23 Nov 2018, Helmut Eller wrote:
>
>> when compiling this example with gcc -O2 -ftrapv:
>> [...]
> Try with "-fsanitize=undefined
> -fsanitize-undefined-trap-on-error". -ftrapv is quite broken and not
> really maintained. I think fixing it would mean making it an alias for
> the sanitizer version.

Thanks for the quick reply.  That does indeed produce exactly the code I
want.

It doesn't seem so detect overflow for short or char types, though:

  typedef signed short si;
  
  si foo (si x, si y) { return x + y; }
  
  si bar (si x, si y) {
    si z;
    if (__builtin_add_overflow (x, y, &z))
      __builtin_trap ();
    return z;
  }

compiled with either -O2 -fsanitize=signed-integer-overflow
-fsanitize-undefined-trap-on-error or -ftrapv produces:

  foo:
  	leal	(%rsi,%rdi), %eax
  	ret
  
  bar:
  	movswl	%di, %eax
  	movswl	%si, %edi
  	addw	%di, %ax
  	jo	.L8
  	rep ret
  .L8:
  	ud2

For __int128 it seems to be detected again, but the code seems very
long-winded.

Helmut