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

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

Hi, Martin,

Thanks a lot for your suggestion.

> On Nov 25, 2020, at 6:08 PM, Martin Sebor <msebor@gmail.com> wrote:
> 
> On 11/24/20 9:54 AM, Qing Zhao via Gcc-patches wrote:
>>> On Nov 24, 2020, at 9:55 AM, Richard Biener <richard.guenther@gmail.com> wrote:
>>> 
>>> On Tue, Nov 24, 2020 at 4:47 PM Qing Zhao <QING.ZHAO@oracle.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Nov 24, 2020, at 1:32 AM, Richard Biener <richard.guenther@gmail.com> wrote:
>>>>> 
>>>>> On Tue, Nov 24, 2020 at 12:05 AM Qing Zhao via Gcc-patches
>>>>> <gcc-patches@gcc.gnu.org> wrote:
>>>>>> 
>>>>>> Hi,
>>>>>> 
>>>>>> Does gcc provide an iterator to traverse all the local variables that are declared in the current routine?
>>>>>> 
>>>>>> If not, what’s the best way to traverse the local variables?
>>>>> 
>>>>> Depends on what for.  There's the source level view you get by walking
>>>>> BLOCK_VARS of the
>>>>> scope tree, theres cfun->local_variables (FOR_EACH_LOCAL_DECL) and
>>>>> there's SSA names
>>>>> (FOR_EACH_SSA_NAME).
>>>> 
>>>> 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?
>>> 
>>> Yes, but do you want to catch variables promoted to register as well
>>> or just variables
>>> on the stack?
>> 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 during gimplification.
>> 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:
>> 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.
>> Do you have suggestion on this?
> 
> Not having thought about it very long or hard I'd be tempted to do
> it the other way around.  For each use of an uninitialized variable
> found, first either issue or queue up a -Wuninitialized for it and
> then initialize it.  Then (if queued) at some later point, issue
> the queued up -Wuninitialized.  The last part would be done in
> tree-ssa-uninit.c where the remaining uses of uninitialized
> variables would trigger warnings and induce their initialization
> (if there were any left).


The major issue with this approach is:

There are two passes for uninitialized variable analysis:
pass_early_warn_uninitialized
pass_late_warn_uninitialized

The early pass is placed at the very beginning of the tree optimizer. But the late pass is placed at the very late stage of the tree optimizer. 
If we add the initializations at the early pass, the result of the late pass will be changed by the new added initializations. This does not meet
the requirement. 

Do I miss anything here? 

Qing


> 
> Martin