Hi,
On Thu, Jul 08 2021, Qing Zhao wrote:
> (Resend this email since the previous one didn’t quote, I changed one
> setting in my mail client, hopefully that can fix this issue).
>
> Hi, Martin,
>
> Thank you for the review and comment.
>
>> 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.
>
> 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.
> 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.
>
>>
>> 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)
>
>> 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.
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,
I think.
Martin
