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

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

> On Dec 7, 2020, at 12:05 PM, Richard Sandiford <richard.sandiford@arm.com> wrote:
> 
> Qing Zhao <QING.ZHAO@ORACLE.COM <mailto:QING.ZHAO@ORACLE.COM>> writes:
>>> On Dec 7, 2020, at 11:10 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
>>>>>> 
>>>>>> 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.
>>>>>> 
>>>>>> Do you have any comment and suggestions?
>>>>>> 
>>>>>> 
>>>>>> As said above - do you want to cover registers as well as locals?  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)
>>>>>> 
>>>>>> 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.
>>>>>> 
>>>>>> 
>>>>>> Haven't thought about this much, so it might be a daft idea, but would a
>>>>>> compromise be to use a const internal function:
>>>>>> 
>>>>>> X1 = .DEFERRED_INIT (X0, INIT)
>>>>>> 
>>>>>> where the X0 argument is an uninitialised value and the INIT argument
>>>>>> describes the initialisation pattern?  So for a decl we'd have:
>>>>>> 
>>>>>> X = .DEFERRED_INIT (X, INIT)
>>>>>> 
>>>>>> and for an SSA name we'd have:
>>>>>> 
>>>>>> X_2 = .DEFERRED_INIT (X_1(D), INIT)
>>>>>> 
>>>>>> with all other uses of X_1(D) being replaced by X_2.  The idea is that:
>>>>>> 
>>>>>> * Having the X0 argument would keep the uninitialised use of the
>>>>>> variable around for the later warning passes.
>>>>>> 
>>>>>> * Using a const function should still allow the UB to be deleted as dead
>>>>>> if X1 isn't needed.
>>>>>> 
>>>>>> * Having a function in the way should stop passes from taking advantage
>>>>>> of direct uninitialised uses for optimisation.
>>>>>> 
>>>>>> This means we won't be able to optimise based on the actual init
>>>>>> value at the gimple level, but that seems like a fair trade-off.
>>>>>> AIUI this is really a security feature or anti-UB hardening feature
>>>>>> (in the sense that users are more likely to see predictable behaviour
>>>>>> “in the field” even if the program has UB).
>>>>>> 
>>>>>> 
>>>>>> The question is whether it's in line of peoples expectation that
>>>>>> explicitely zero-initialized code behaves differently from
>>>>>> implicitely zero-initialized code with respect to optimization
>>>>>> and secondary side-effects (late diagnostics, latent bugs, etc.).
>>>>>> 
>>>>>> Introducing a new concept like .DEFERRED_INIT is much more
>>>>>> heavy-weight than an explicit zero initializer.
>>>>>> 
>>>>>> 
>>>>>> What exactly you mean by “heavy-weight”? More difficult to implement or much more run-time overhead or both? Or something else?
>>>>>> 
>>>>>> The major benefit of the approach of “.DEFERRED_INIT”  is to enable us keep the current -Wuninitialized analysis untouched and also pass
>>>>>> the “uninitialized” info from source code level to “pass_expand”.
>>>>> 
>>>>> Well, "untouched" is a bit oversimplified.  You do need to handle
>>>>> .DEFERRED_INIT as not
>>>>> being an initialization which will definitely get interesting.
>>>> 
>>>> Yes, during uninitialized variable analysis pass, we should specially handle the defs with “.DEFERRED_INIT”, to treat them as uninitializations.
>>> 
>>> Are you sure we need to do that?  The point of having the first argument
>>> to .DEFERRED_INIT was that that argument would still provide an
>>> uninitialised use of the variable.  And the values are passed and
>>> returned by value, so the lack of initialisation is explicit in
>>> the gcall itself, without knowing what the target function does.
>>> 
>>> The idea is that we can essentially treat .DEFERRED_INIT as a normal
>>> (const) function call.  I'd be surprised if many passes needed to
>>> handle it specially.
>>> 
>> 
>> Just checked with a small testing case (to emulate the .DEFERRED_INIT approach):
>> 
>> qinzhao@gcc10:~/Bugs/auto-init$ cat t.c
>> extern int DEFFERED_INIT (int, int) __attribute__ ((const));
>> 
>> int foo (int n, int r)
>> {
>>  int v;
>> 
>>  v = DEFFERED_INIT (v, 0);
>>  if (n < 10) 
>>    v = r;
>> 
>>  return v;
>> }
>> qinzhao@gcc10:~/Bugs/auto-init$ sh t
>> /home/qinzhao/Install/latest_write/bin/gcc -O -Wuninitialized -fdump-tree-all -S t.c
>> t.c: In function ‘foo’:
>> t.c:7:7: warning: ‘v’ is used uninitialized [-Wuninitialized]
>>    7 |   v = DEFFERED_INIT (v, 0);
>>      |       ^~~~~~~~~~~~~~~~~~~~
>> 
>> We can see that the current uninitialized variable analysis treats the new added artificial initialization as the first use of the uninialized variable.  Therefore report the warning there.
>> However, we should report warning at “return v”. 
> 
> Ah, OK, so this is about the quality of the warning, rather than about
> whether we report a warning or not?
> 
>> So, I think that we still need to specifically handle the new added artificial initialization during uninitialized analysis phase.
> 
> Yeah, that sounds like one approach.  But if we're adding .DEFERRED_INIT
> in response to known uninitialised uses, two other approaches might be:
> 
> (1) Give the call the same source location as one of the uninitialised uses.
> 
> (2) Pass the locations of all uninitialised uses as additional arguments.

If we add .DEFERRED_INIT during gimplification phase, is the “uninitialized uses” information available at that time? 

Qing
> 
> The uninit pass would then be picking the source location differently
> from normal, but I don't know what effect it would have on the quality
> of diagnostics.  One obvious problem is that if there are multiple
> uninitialised uses, some of them might get optimised away later.
> On the other hand, using early source locations might give better
> results in some cases.  I guess it will depend.
> 
> Thanks,
> Richard