Re: Bootstrap failure in stage 2 on i386.c

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Nov 23, 2016 at 1:12 AM, Serge Belyshev
<belyshev@depni.sinp.msu.ru> wrote:
>> My builds for the last couple of days have all been failing in stage 2
>> like so:
>>
>> /home/arth/src/gcc/gcc/config/i386/i386.c: In function ‘rtx_def* ix86_expand_bui
>> ltin(tree, rtx, rtx, machine_mode, int)’:
>> /home/arth/src/gcc/gcc/config/i386/i386.c:38407:18: error: ‘fcn’ may be used uni
>> nitialized in this function [-Werror=maybe-uninitialized]
>>         emit_insn (fcn (target, accum, wide_reg, mem));
>>         ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Anyone else seeing this?
>
> I'm seeing it too. The code is new, but the 'may be used unitialized'
> warning itself is, probably, an instance of the old PR36550.
>
> I have reduced this testcase to:
>
> //------------------------------------------------------------------------
> /* { dg-do compile } */
> /* { dg-options "-O2 -Wuninitialized" } */
>
> int force_reg (int, int);
> int expand_expr_real (int, int);
> void f (int);
>
> void ix86_expand_builtin (int fcode, int pmode)
> {
>   int fcn;
>   int i, addr, masked = 1;
>
>   switch (fcode)
>     {
>     case 1:
>       fcn = 1;
>       masked = 0;
>       goto s4fma_expand;
>
>     case 2:
>       fcn = 2;
>       masked = 0;
>       goto s4fma_expand;
>
>     case 4:
>       {
>       s4fma_expand:
>         for (i = 0; i < 4; i++)
>           expand_expr_real (0, 0);
>
>         addr = expand_expr_real (0, 0);
>         force_reg ((pmode ? 0 : 2), addr);
>
>         if (! masked)
>           f (fcn);
>       }
>     }
> }
> //------------------------------------------------------------------------
>
> It fails with every gcc version down to 3.2 (the oldest one I could
> compile on my amd64 box).  Note that this testcase is particularly
> flaky: recent gccs will not issue a warning if one, for example, changes
> the '2' to '1' in the force_reg() call.

It's an interesting case where the uninit pass has to do sth else than
looking for a guard on the incoming path to the uninit PHI value
(there's none).  But it has to simplify the guard on the use with
values on the edge of the uninit var:

  # fcn_1 = PHI <fcn_9(D)(3), 1(14), 2(4)>
  # masked_3 = PHI <1(3), 0(14), 0(4)>
s4fma_expand:
...
  if (masked_3 == 0)
    goto <bb 10>;
  else
    goto <bb 16>;

  <bb 16>:
  goto <bb 11> (<L11>);

  <bb 10>:
  f (fcn_1); [tail call]

that's not something it even tries to do (and this case is particularly
simple even)

Richard.