Re: [PATCH] Fix libgo breakage (PR tree-optimization/67284)

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

On Fri, Aug 21, 2015 at 2:49 PM, Marek Polacek <polacek@redhat.com> wrote:
> On Fri, Aug 21, 2015 at 01:27:43PM +0200, Richard Biener wrote:
>> On Fri, Aug 21, 2015 at 12:52 PM, Marek Polacek <polacek@redhat.com> wrote:
>> > This fixes the libgo breakage.  Seems I really should have removed the
>> > edge after we split the block with null dereference after __builtin_trap
>> > statement so that it's unreachable.
>> >
>> > Bootstrapped/regtested on x86_64-linux + ppc64-linux + bootstrapped on
>> > aarch64-linux, ok for trunk?
>>
>> Hum.  I don't see why this is needed - CFG cleanup (which of course needs
>> to run!) should do this for you.  In fact stray unreachable blocks are usually
>> more of a problem.
>
> Aha.  It seems cleanup does that if I change the code to generate
> __builtin_unreachable instead of __builtin_trap.  A hint maybe? ;)

Not really...

static bool
cleanup_control_flow_bb (basic_block bb)
{
...
  /* Check for indirect calls that have been turned into
     noreturn calls.  */
  else if (is_gimple_call (stmt)
           && gimple_call_noreturn_p (stmt)
           && remove_fallthru_edge (bb->succs))
    retval = true;

and __builtin_trap is NORETURN.  But there is the hint where to debug.

Richard.

> What's going on here is that we find a potential NULL dereference
> (PHI argument has NULL), we call isolate_path, that duplicates a bb
> and then cuts off the outgoing edges of this duplicated bb.  And in
> some cases it leaves COND_EXPR at the end of the bb -> ICE.
> The we insert __builtin_trap on this isolated path.
>
>         Marek