Hi, Richard,
Could you please comment on the following approach:
Instead of adding the zero-initializer quite late at the pass “pass_expand”, we
can add it as early as during gimplification.
However, we will mark these new added zero-initializers as “artificial”. And
passing this “artificial” information to
“pass_early_warn_uninitialized” and “pass_late_warn_uninitialized”, in these
two uninitialized variable analysis passes,
(i.e., in tree-sea-uninit.c) We will update the checking on
“ssa_undefined_value_p” to consider “artificial” zero-initializers.
(i.e, if the def_stmt is marked with “artificial”, then it’s a undefined
value).
With such approach, we should be able to address all those conflicts.
Do you see any obvious issue with this approach?
Thanks a lot for your help.
Qing
> On Nov 25, 2020, at 3:11 AM, Richard Biener <richard.guent...@gmail.com>
> wrote:
>>
>>
>> I am planing to add a new phase immediately after
>> “pass_late_warn_uninitialized” to initialize all auto-variables that are
>> not explicitly initialized in the declaration, the basic idea is following:
>>
>> ** The proposal:
>>
>> A. add a new GCC option: (same name and meaning as CLANG)
>> -ftrivial-auto-var-init=[pattern|zero], similar pattern init as CLANG;
>>
>> B. add a new attribute for variable:
>> __attribute((uninitialized)
>> the marked variable is uninitialized intentionaly for performance purpose.
>>
>> C. The implementation needs to keep the current static warning on
>> uninitialized
>> variables untouched in order to avoid "forking the language".
>>
>>
>> ** The implementation:
>>
>> There are two major requirements for the implementation:
>>
>> 1. all auto-variables that do not have an explicit initializer should be
>> initialized to
>> zero by this option. (Same behavior as CLANG)
>>
>> 2. keep the current static warning on uninitialized variables untouched.
>>
>> In order to satisfy 1, we should check whether an auto-variable has
>> initializer
>> or not;
>> In order to satisfy 2, we should add this new transformation after
>> "pass_late_warn_uninitialized".
>>
>> So, we should be able to check whether an auto-variable has initializer or
>> not after “pass_late_warn_uninitialized”,
>> If Not, then insert an initialization for it.
>>
>> For this purpose, I guess that “FOR_EACH_LOCAL_DECL” might be better?
>>
>>
>> I think both as long as they are source-level auto-variables. Then which one
>> is better?
>>
>>
>> Another issue is, in order to check whether an auto-variable has
>> initializer, I plan to add a new bit in “decl_common” as:
>> /* In a VAR_DECL, this is DECL_IS_INITIALIZED. */
>> unsigned decl_is_initialized :1;
>>
>> /* IN VAR_DECL, set when the decl is initialized at the declaration. */
>> #define DECL_IS_INITIALIZED(NODE) \
>> (DECL_COMMON_CHECK (NODE)->decl_common.decl_is_initialized)
>>
>> set this bit when setting DECL_INITIAL for the variables in FE. then keep it
>> even though DECL_INITIAL might be NULLed.
>>
>>
>> For locals it would be more reliable to set this flag-Wmaybe-uninitialized.
>>
>>
>> You mean I can set the flag “DECL_IS_INITIALIZED (decl)” inside the routine
>> “gimpley_decl_expr” (gimplify.c) as following:
>>
>> if (VAR_P (decl) && !DECL_EXTERNAL (decl))
>> {
>> tree init = DECL_INITIAL (decl);
>> ...
>> if (init && init != error_mark_node)
>> {
>> if (!TREE_STATIC (decl))
>> {
>> DECL_IS_INITIALIZED(decl) = 1;
>> }
>>
>> Is this enough for all Frontends? Are there other places that I need to
>> maintain this bit?
>>
>>
>>
>> Do you have any comment and suggestions?
>>
>>
>> As said above - do you want to cover registers as well as locals?
>>
>>
>> All the locals from the source-code point of view should be covered. (From
>> my study so far, looks like that Clang adds that phase in FE).
>> If GCC adds this phase in FE, then the following design requirement
>>
>> C. The implementation needs to keep the current static warning on
>> uninitialized
>> variables untouched in order to avoid "forking the language”.
>>
>> cannot be satisfied. Since gcc’s uninitialized variables analysis is
>> applied quite late.
>>
>> So, we have to add this new phase after “pass_late_warn_uninitialized”.
>>
>> I'd do
>> the actual zeroing during RTL expansion instead since otherwise you
>> have to figure youself whether a local is actually used (see
>> expand_stack_vars)
>>
>>
>> Adding this new transformation during RTL expansion is okay. I will check
>> on this in more details to see how to add it to RTL expansion phase.
>>
>>
>> Note that optimization will already made have use of "uninitialized" state
>> of locals so depending on what the actual goal is here "late" may be too
>> late.
>>
>>
>> This is a really good point…
>>
>> In order to avoid optimization to use the “uninitialized” state of locals,
>> we should add the zeroing phase as early as possible (adding it in FE might
>> be best
>> for this issue). However, if we have to met the following requirement:
>
> So is optimization supposed to pick up zero or is it supposed to act
> as if the initializer
> is unknown?
>
>> C. The implementation needs to keep the current static warning on
>> uninitialized
>> variables untouched in order to avoid "forking the language”.
>>
>> We have to move the new phase after all the uninitialized analysis is done
>> in order to avoid “forking the language”.
>>
>> So, this is a problem that is not easy to resolve.
>
> Indeed, those are conflicting goals.
>
>> Do you have suggestion on this?
>
> No, not any easy ones. Doing more of the uninit analysis early (there
> is already an early
> uninit pass) which would mean doing IPA analysis turing GCC into more
> of a static analysis
> tool. Theres the analyzer now, not sure if that can employ an early
> LTO phase for example.
>
> Richard.
