[patch][version 4]add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc
Qing Zhao
qing.zhao@oracle.com
Mon Jul 12 15:31:56 GMT 2021
> On Jul 12, 2021, at 2:51 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
>
> Martin Jambor <mjambor@suse.cz> writes:
>> 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 <mjambor@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.
>
> FTR, I don't have a strong opinion here. You know the code better
> than I do, so if you think not generating debug binds is better then
> let's do that.
I am okay with not generating debug binds here.
Then I will just delete the part of code that guarded with if (access->grp_to_be_debug_replaced)?
Martin, please confirm on this.
Thanks.
Qing
> Thanks,
> Richard
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