Re: How to traverse all the local variables that declared in the current routine?

[Qing Zhao <QING.ZHAO@ORACLE.COM>]

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.guenther@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.