Re: Regression with recent change

[Michael Matz <matz@suse.de>]

Hello,

On Mon, 13 Sep 2021, Jeff Law via Gcc-patches wrote:

> > So it looks like there's some undefined behavior going on, even before
> > my patch.  I'd like to get some feedback, because this is usually the
> > type of problems I see in the presence of a smarter threader... things
> > get shuffled around, problematic code gets isolated, and warning
> > passes have an easier time (or sometimes harder time) diagnosing
> > things.
> The original issue was PRE hanging, so I'd lean towards keeping the test as-is
> and instead twiddling any warning flags we can to make the diagnostics go
> away.

Or use this changed test avoiding the issues that I see with -W -Wall on 
this testcase.  I've verified that it still hangs before r194358 and is 
fixed by that revision.

Generally I think, our testsuite, even for ICEs or these kinds of hangs, 
should make an effort to try to write conforming code; if at all possible.  
Here it is possible.

(I don't know if the new threader causes additional warnings, of course, 
but at least the problems with sequence points and uninitialized use of 
'j' aren't necessary to reproduce the bug)


Ciao,
Michael.

/* { dg-do compile } */
/* { dg-additional-options "-fno-split-loops" } */

typedef unsigned short uint16_t;

uint16_t a, b;

int *j_global;
uint16_t f(void)
{
  int c, **p;
  short d = 2, e = 4;

  for (;; b++)
    {
      int *j = j_global, k = 0;

      for (; *j; j++)
	{
	  for(; c; c++)
	    for(; k < 1; k++)
	      {
		short *f = &d;

		if(b)
		  return *f;
	      }
	}

      if(!c)
	d *= e;

      a = d;
      if ((a ? b = 0 : (**p ? : 1) != (d != 1 ? 1 : (b = 0))) != ((k ? a : 0)
	  < (a * (c = k))))
	**p = 0;
    }
}