[v4] rtl-optimization/105231 - distribute_notes and REG_EH_REGION

  I was still unhappy with the previous patch and indeed, re-thinking
all the special casing I put in there I found a hole with respect
to externally throwing stmts which I totally forgot about and which
might end up in must-not-throw regions after the previous patch.

Fortunately all of the complicated situations only arise with
non-call exceptions and when there's a REG_EH_REGION note to
distribute.  So first of all the new patch makes that explicit
and does not affect the not non-call EH path (apart from the
new assert in distribute_notes).  It also does not affect the
non-call EH path when there is no REG_EH_REGION on any of the
insns.

I resisted trying to be clever with lp_nr == 0 or INT_MIN
(the nothrow notes that we could in theory just drop), but I
put in an extra check in case we have a REG_EH_REGION note
on an insn that cannot ever throw (and drop those at
distribute_notes time).

The patch preserves the main part of the very original patch,
that we only ever place the REG_EH_REGION on i3.  The
split precondition should make sure that i2 never throws
(but no assert since the trigger happy may_trap_p might
be confused by some "optimization" done on the split part later).
The hunk at the start of try_combine makes sure that we only
have a single REG_EH_REGION note to distribute which ends up
on i3 which accumulates all possibly throwing side-effects
thanks to the split precondition.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

OK for trunk?

Thanks,
Richard.

---

The following mitigates a problem in combine distribute_notes which
places an original REG_EH_REGION based on only may_trap_p which is
good to test whether a non-call insn can possibly throw but not if
actually it does or we care.  That's something we decided at RTL
expansion time where we possibly still know the insn evaluates
to a constant.

In fact, the REG_EH_REGION note with lp > 0 can only come from the
original i3 and an assert is added to that effect.  That means we only
need to retain the note on i3 or, if that cannot trap, drop it but we
should never move it to i2.

The following places constraints on the insns to combine with
non-call exceptions since we cannot handle the case where we
have more than one EH side-effect in the IL.  The patch also
makes sure we can accumulate that on i3 and do not split
a possible exception raising part of it to i2.  As a special
case we do not place any restriction on all externally
throwing insns when there is no REG_EH_REGION present.

2022-04-22  Richard Biener  <rguenther@suse.de>

	PR rtl-optimization/105231
	* combine.cc (distribute_notes): Assert that a REG_EH_REGION
	with landing pad > 0 is from i3.  Put any REG_EH_REGION note
	on i3 or drop it if the insn can not trap.
	(try_combine): Ensure that we can merge REG_EH_REGION notes
	with non-call exceptions.  Ensure we are not splitting a
	trapping part of an insn with non-call exceptions when there
	is any REG_EH_REGION note to preserve.

	* gcc.dg/torture/pr105231.c: New testcase.
---
 gcc/combine.cc                          | 81 +++++++++++++++++++------
 gcc/testsuite/gcc.dg/torture/pr105231.c | 15 +++++
 2 files changed, 78 insertions(+), 18 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/torture/pr105231.c
  

Hi!

On Fri, Apr 22, 2022 at 10:53:30AM +0200, Richard Biener wrote:
> I was still unhappy with the previous patch and indeed, re-thinking
> all the special casing I put in there I found a hole with respect
> to externally throwing stmts which I totally forgot about and which
> might end up in must-not-throw regions after the previous patch.
> 
> Fortunately all of the complicated situations only arise with
> non-call exceptions and when there's a REG_EH_REGION note to
> distribute.  So first of all the new patch makes that explicit
> and does not affect the not non-call EH path (apart from the
> new assert in distribute_notes).  It also does not affect the
> non-call EH path when there is no REG_EH_REGION on any of the
> insns.
> 
> I resisted trying to be clever with lp_nr == 0 or INT_MIN
> (the nothrow notes that we could in theory just drop), but I
> put in an extra check in case we have a REG_EH_REGION note
> on an insn that cannot ever throw (and drop those at
> distribute_notes time).
> 
> The patch preserves the main part of the very original patch,
> that we only ever place the REG_EH_REGION on i3.  The
> split precondition should make sure that i2 never throws
> (but no assert since the trigger happy may_trap_p might
> be confused by some "optimization" done on the split part later).
> The hunk at the start of try_combine makes sure that we only
> have a single REG_EH_REGION note to distribute which ends up
> on i3 which accumulates all possibly throwing side-effects
> thanks to the split precondition.

Right, that is foolproof :-)

> +  bool nce_any_eh_region_note = false;

Name this "has_non_call_exception" please?

> +  /* With non-call exceptions we can end up trying to combine multiple
> +     stmts with possible EH side effects.  Make sure we can combine
> +     that to a single stmt which means there must be at most one insn
> +     in the combination with an EH side effect.  */

s/stmt/insn/g

Or, what else does "statement" mean here?

> +  if (cfun->can_throw_non_call_exceptions)
> +    {
> +      if (find_reg_note (i3, REG_EH_REGION, NULL_RTX)
> +	  || find_reg_note (i2, REG_EH_REGION, NULL_RTX)
> +	  || (i1 && find_reg_note (i1, REG_EH_REGION, NULL_RTX))
> +	  || (i0 && find_reg_note (i0, REG_EH_REGION, NULL_RTX)))
> +	{
> +	  nce_any_eh_region_note = true;
> +	  if (insn_could_throw_p (i3)
> +	      + insn_could_throw_p (i2)
> +	      + (i1 ? insn_could_throw_p (i1) : 0)
> +	      + (i0 ? insn_could_throw_p (i0) : 0) > 1)
> +	    {
> +	      if (dump_file && (dump_flags & TDF_DETAILS))
> +		fprintf (dump_file, "Can't combine multiple insns with EH "
> +			 "side-effects\n");
> +	      undo_all ();
> +	      return 0;
> +	    }
> +	}
> +    }

Nice :-)

> +	  /* We should not split a possibly trapping part when we
> +	     care about non-call EH and have REG_EH_REGION notes
> +	     to distribute.  */
> +	  && (!cfun->can_throw_non_call_exceptions
> +	      || !nce_any_eh_region_note
> +	      || !may_trap_p (*split)))

I'm still not a fan of manual De Morgan, it is too easy to make a
mistake in it, or in reading it.  It is better as
	  && (!(cfun->can_throw_non_call_exceptions
		&& nce_any_eh_region_note
		&& may_trap_p (*split))))
imo.  But, this kind of thing is not new of course, and combine is one
of the worst "complicated expressions are great!" offenders around ;-)

> +	  {
> +	    /* This handling needs to be kept in sync with the
> +	       prerequisite checking in try_combine.  */
> +	    int lp_nr = INTVAL (XEXP (note, 0));

Please spell out "landing pad", in the comment if not in the code.

> +	    /* A REG_EH_REGION note transfering control can only ever come
> +	       from i3.  */
> +	    if (lp_nr > 0)
> +	      gcc_assert (from_insn == i3);
> +	    /* We are making sure there is a single effective REG_EH_REGION
> +	       note and it's valid to put it on i3.  */
> +	    if (!insn_could_throw_p (from_insn))
> +	      /* Deal with stray notes on insns that can never throw.  */
> +	      ;

"Throw away stray notes" etc.?

> +/* { dg-do compile } */
> +/* { dg-require-effective-target int32plus } */
> +/* { dg-require-effective-target dfp } */
> +/* { dg-additional-options "-fsanitize-coverage=trace-pc -fnon-call-exceptions --param=max-cse-insns=1 -frounding-math" } */
> +/* { dg-additional-options "-mstack-arg-probe" { target x86_64-*-* i?86-*-* } } */
> +
> +void baz (int *);
> +void bar (double, double, _Decimal64);
> +
> +void
> +foo (void)
> +{
> +  int s __attribute__((cleanup (baz)));
> +  bar (0xfffffffffffffffe, 0xebf3fff2fbebaf7f, 0xffffffffffffff);
> +}

Why the int32plus?  It needs 64-bit integers, and the size of "int"
does not matter at all afaics?  Maybe you want lp64?

Okay for trunk with the naming and comment stuff looked at.  Thank you!


Segher