Re: [PATCH] check_cfg assert fix

[Maxim Kuvyrkov <maxim at codesourcery dot com>]

On 7/09/2011, at 3:13 AM, Steven Bosscher wrote:

> On Tue, Sep 6, 2011 at 11:14 AM, Tom de Vries <vries@codesourcery.com> wrote:
>> Hi,
>> 
>> During testing the approved-for-commit middle-end patch for bug 43864 on ARM, I
>> ran into a gcc.dg/torture/pr46068.c ICE.
>> 
>> The following assert in haifa-sched.c:check_cfg triggered:
>> ...
>>                  else if (any_condjump_p (head))
>>                    gcc_assert (/* Usual case.  */
>>                                (EDGE_COUNT (bb->succs) > 1
>>                                 && !BARRIER_P (NEXT_INSN (head)))
>>                                /* Or jump to the next instruction.  */
>>                                || (EDGE_COUNT (bb->succs) == 1
>>                                    && (BB_HEAD (EDGE_I (bb->succs, 0)->dest)
>>                                        == JUMP_LABEL (head))));
>> 
...
>> Bootstrapped and reg-tested on x86_64 and build and reg-tested on arm.
>> 
>> OK for trunk?
> 
> No. If the conditional return is not taken, you should not fall into a
> barrier. It looks like the CFG somehow got corrupted, and if that's
> indeed the case then your patch just papers over the problem. That
> follows after the barrier?

Initially, I thought so too, that is why I wrote the above assert in the first place.  However, __builtin_unreachable() adds a whole new spin on where a BARRIER can occur; after all, this built-in expands directly to BARRIER.

Before Tom's optimization the fall-through path of conditional jump was followed by an unconditional jump to a label directly followed by a barrier.  Tom's patch removed the middle-man in the face of unconditional jump so that the barrier now follows a conditional jump.  This is very odd, but, given the initial test case, is a valid case.

For reference, the test case in question:

/* { dg-do compile } */

void
foo ()
{
  asm goto (""::::l1);
  __builtin_unreachable ();
l1:;
}

void
bar ()
{
  foo ();
  foo ();
}

--
Maxim Kuvyrkov
CodeSourcery / Mentor Graphics

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