Re: [patch tree-optimization]: Improve handling of conditional-branches on targets with high branch costs

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

On Mon, Oct 10, 2011 at 12:35 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
> 2011/10/7 Kai Tietz <ktietz70@googlemail.com>:
>> Hello,
>>
>> this is the updated version with the suggestion
>>
>> 2011/10/7 Richard Guenther <richard.guenther@gmail.com>:
>>> On Thu, Oct 6, 2011 at 4:25 PM, Kai Tietz <ktietz@redhat.com> wrote:
>>>> + ? ? ?&& ((TREE_CODE_CLASS (TREE_CODE (arg1)) != tcc_comparison
>>>> + ? ? ? ? ? && TREE_CODE (arg1) != TRUTH_NOT_EXPR
>>>> + ? ? ? ? ? && simple_operand_p (arg1))
>>>
>>> As I said previously simple_operand_p already rejects covers
>>> comparisons and TRUTH_NOT_EXPR. ?Also arg1 had better
>>> TREE_SIDE_EFFECTS set if the comparison might trap, as
>>> it might just be hidden in something more complicated - so
>>> the simple check isn't enough anyway (and if simple_operand_p
>>> would cover it, the check would be better placed there).
>>
>> I reworked simple_operand_p so that it does this special-casing and additionally
>> also checks for trapping.
>>
>>>> + ? ? ?if (TREE_CODE (arg0) == code
>>>> + ? ? ? ? ?&& !TREE_SIDE_EFFECTS (TREE_OPERAND (arg0, 1))
>>>> + ? ? ? ? ?&& simple_operand_p (TREE_OPERAND (arg0, 1)))
>>>> + ? ? ? {
>>>> + ? ? ? ? tem = build2_loc (loc,
>>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? (code == TRUTH_ANDIF_EXPR ? TRUTH_AND_EXPR
>>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? : TRUTH_OR_EXPR),
>>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? type, TREE_OPERAND (arg0, 1), arg1);
>>>> + ? ? ? ? return build2_loc (loc, code, type, TREE_OPERAND (arg0, 0),
>>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ?tem);
>>>
>>> All trees should be folded, don't use plain build without a good reason.
>>
>> Ok, done
>>
>>>> + ? ? ? }
>>>> + ? ? ?/* Convert X TRUTH-ANDORIF Y to X TRUTH-ANDOR Y, if X and Y
>>>> + ? ? ? ?are simple operands and have no side-effects. ?*/
>>>> + ? ? ?if (simple_operand_p (arg0)
>>>> + ? ? ? ? ?&& !TREE_SIDE_EFFECTS (arg0))
>>>
>>> Again, the checks you do for arg0 do not match those for arg1. ?OTOH
>>> it doesn't matter whether arg0 is simple or not or has side-effects or
>>> not for this transformation, so why check it at all?
>>
>> It is required. ?For left-hand operand, if it isn't a logical
>> and/or/xor, we need to check for side-effects (and for trapping). ?I
>> see that calling of simple_operand_p is wrong here, as it rejects too
>> much. ?Nevertheless the check for side-effects is necessary for having
>> valid sequence-points. ?Without that checking a simple test
>
> So said, it is even required to use for right-hand and left-hand side
> of arguments, if one of them have side-effects or isn't simple. ?Means
> the check in my patch should use for
>
>> + ? ? else if (TREE_CODE (arg0) != TRUTH_AND_EXPR
>> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_OR_EXPR
>> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_ANDIF_EXPR
>> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_ORIF_EXPR
>> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_XOR_EXPR
>> + ? ? ? ? ? ? /* Needed for sequence points and trappings, or side-effects. ?*/
>> + ? ? ? ? ? ? && !TREE_SIDE_EFFECTS (arg0)
>> + ? ? ? ? ? ? && !tree_could_trap_p (arg0))
>> + ? ? ? return fold_build2_loc (loc, ncode, type, arg0, arg1);
>
> instead if (!TREE_SIDE_EFFECTS (arg0) && simple_operand_p (arg0)) ....
> instead.
>
> The cause for this are the consitancies of sequences in tree. ?I
> noticed that by tests in gcc.dg/tree-ssa about builitin_expect.
>
> for example we have
>
> extern int foo (void); /* foo modifies gbl1 */
> int gbl1 = 0;
>
> int foo (int ns1)
> {
> ?if (ns1 && foo () && gbl1)
> ? ?return 1;
> ?return 0;
> }
>
> so chain of trees has to look like this:
> (ANDIF (ns1 (ANDIF foo () gbl1))
>
> but if we don't check here for side-effects for left-hand chaining
> operand, then we end up with
> (AND ns1 (ANDIF foo () gbl1))

No we don't, as the right-hand (ANDIF foo () glbl1) has side-effects.

> As AND and has associative property, tree says that right-hand and
> left-hand are exchangable, which is obviously wrong.

The poitn is that if the right-hand does not have side-effects it doesn't
matter if we execute it before the left-hand (independent on whether
that has side-effects or not).

Richard.

> Cheers,
> Kai
>