Hi,
> On Jul 9, 2021, at 11:18 AM, Martin Jambor <mjam...@suse.cz> wrote:
>>
>>> On Jul 8, 2021, at 8:29 AM, Martin Jambor <mjam...@suse.cz> wrote:
>>>> diff --git a/gcc/tree-sra.c b/gcc/tree-sra.c
>>>> index c05d22f3e8f1..35051d7c6b96 100644
>>>> --- a/gcc/tree-sra.c
>>>> +++ b/gcc/tree-sra.c
>>>> @@ -384,6 +384,13 @@ static struct
>>>>
>>>> /* Numbber of components created when splitting aggregate parameters. */
>>>> int param_reductions_created;
>>>> +
>>>> + /* Number of deferred_init calls that are modified. */
>>>> + int deferred_init;
>>>> +
>>>> + /* Number of deferred_init calls that are created by
>>>> + generate_subtree_deferred_init. */
>>>> + int subtree_deferred_init;
>>>> } sra_stats;
>>>>
>>>> static void
>>>> @@ -4096,6 +4103,110 @@ get_repl_default_def_ssa_name (struct access
>>>> *racc, tree reg_type)
>>>> return get_or_create_ssa_default_def (cfun, racc->replacement_decl);
>>>> }
>>>>
>>>> +
>>>> +/* Generate statements to call .DEFERRED_INIT to initialize scalar
>>>> replacements
>>>> + of accesses within a subtree ACCESS; all its children, siblings and
>>>> their
>>>> + children are to be processed.
>>>> + GSI is a statement iterator used to place the new statements. */
>>>> +static void
>>>> +generate_subtree_deferred_init (struct access *access,
>>>> + tree init_type,
>>>> + tree is_vla,
>>>> + gimple_stmt_iterator *gsi,
>>>> + location_t loc)
>>>> +{
>>>> + do
>>>> + {
>>>> + if (access->grp_to_be_replaced)
>>>> + {
>>>> + tree repl = get_access_replacement (access);
>>>> + gimple *call
>>>> + = gimple_build_call_internal (IFN_DEFERRED_INIT, 3,
>>>> + TYPE_SIZE_UNIT (TREE_TYPE (repl)),
>>>> + init_type, is_vla);
>>>> + gimple_call_set_lhs (call, repl);
>>>> + gsi_insert_before (gsi, call, GSI_SAME_STMT);
>>>> + update_stmt (call);
>>>> + gimple_set_location (call, loc);
>>>> + sra_stats.subtree_deferred_init++;
>>>> + }
>>>> + else if (access->grp_to_be_debug_replaced)
>>>> + {
>>>> + tree drepl = get_access_replacement (access);
>>>> + tree call = build_call_expr_internal_loc
>>>> + (UNKNOWN_LOCATION, IFN_DEFERRED_INIT,
>>>> + TREE_TYPE (drepl), 3,
>>>> + TYPE_SIZE_UNIT (TREE_TYPE (drepl)),
>>>> + init_type, is_vla);
>>>> + gdebug *ds = gimple_build_debug_bind (drepl, call,
>>>> + gsi_stmt (*gsi));
>>>> + gsi_insert_before (gsi, ds, GSI_SAME_STMT);
>>>
>>> Is handling of grp_to_be_debug_replaced accesses necessary here? If so,
>>> why? grp_to_be_debug_replaced accesses are there only to facilitate
>>> debug information about a part of an aggregate decl is that is likely
>>> going to be entirely removed - so that debuggers can sometimes show to
>>> users information about what they would contain had they not removed.
>>> It seems strange you need to mark them as uninitialized because they
>>> should not have any consumers. (But perhaps it is also harmless.)
>>
>> This part has been discussed during the 2nd version of the patch, but
>> I think that more discussion might be necessary.
>>
>> In the previous discussion, Richard Sandiford mentioned:
>> (https://gcc.gnu.org/pipermail/gcc-patches/2021-April/568620.html):
>>
>> =====
>>
>> I guess the thing we need to decide here is whether -ftrivial-auto-var-init
>> should affect debug-only constructs too. If it doesn't, exmaining removed
>> components in a debugger might show uninitialised values in cases where
>> the user was expecting initialised ones. There would be no security
>> concern, but it might be surprising.
>>
>> I think in principle the DRHS can contain a call to DEFERRED_INIT.
>> Doing that would probably require further handling elsewhere though.
>>
>> =====
>>
>> I am still not very confident now for this part of the change.
>
> I see. I still tend to think that with or without the generation of
> gimple_build_debug_binds, the debugger would still not display any value
> for the component in question. Without it there would be no information
> about the component at a any place in code affected by this, with it the
> component would be explicitely uninitialized. But OK.
So, my current change for access->grp_to_be_debug_replaced is good?
Do I need to modify any other code in addition to this in order to let debugger
work correctly?
Or deleting this part of code might be simple and better?
>>
>> My questions:
>>
>> 1. If we don’t handle grp_to_be_debug_replaced at all, what will
>> happen? ( the user of the debugger will see uninitialized values in
>> the removed part of the aggregate? Or something else?)
>
> Well, can you try? :-) I think the debugger would not have anything to
> display.
I will try to come up with a small example on this.
>
>> 2. On the other hand, if we handle grp_to_be_debug_replaced as the
>> current patch, what will the user of the debugger see?
>
> I don't know. It would be interesting to know if the generated DWARF is
> different at all.
Okay, I will study this a little bit.
>
>>
>>>
>>> On a related note, if the intent of the feature is for optimizers to
>>> behave (almost?) as if it was not taking place,
>>
>> What’s you mean by “it” here?
>
> -ftrivial-auto-var-init (the feature)
Okay.
>
>>
>>> I believe you need to
>>> handle specially, and probably just ignore, calls to IFN_DEFERRED_INIT
>>> in scan_function in tree-sra.c.
>>
>>
>> Do you mean to let tree-sra phase ignore IFN_DEFERRED_INIT calls completely?
>>
>
> I have thought about it a bit more and actually looked at the decision
> making code and I realized I was wrong because initializations of
> aggregates from a call, which IIUC DEFERRED_INIT is, would not result in
> setting neither grp_assignment_write nor grp_scalar_write being set and
> only these two affect the transformation decisions.
>
> Moreover, ignoring DEFERRED_INIT calls in scan_function might cause
> a need for special-casing elsewhere and so it is not worth it. Sorry.
Okay.
>
> But presence of calls to DEFERRED_INIT still might cause total
> scalarization to not happen. I believe you do not want to set a bit in
> cannot_scalarize_away_bitmap in build_access_from_expr function if the
> statement in question is a call to DEFERRED_INIT. You should be able to
> modify gcc/testsuite/gcc.dg/tree-ssa/sra-12.c to get a simple testcase,
Okay, will study this a little bit more.
Might have more questions on this.
Thanks.
Qing
> I think.
>
> Martin
