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

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

Hi, Richard,

Thanks a lot for your suggestion.

Actually, I like this idea. 

My understanding of your suggestion is:

1. During gimplification phase:

For each auto-variable that does not have an explicit initializer, insert the following initializer for it:

X = DEFERRED_INIT (X, INIT)

In which, DEFERRED_INIT is an internal const function, which can be defined as:

DEF_INTERNAL_FN (DEFERRED_INIT, ECF_CONST | ECF_LEAF | ECF_NOTHROW, NULL)

It’s two arguments are:

1st argument:   this uninitialized auto-variable;
2nd argument:  initialized pattern (zero | pattern);

2.  During tree to SSA phase:  

No change, the current tree to SSA phase should automatically change the above new inserted statement as

X_2 = DEFERRED_INIT (X_1(D), INIT);
And all other uses of X-1(D) being replaced by X_2. 

3. During expanding phase:

Expand each call to “DEFERRED_INIT (X, INIT)” to zero or pattern depends on “INIT”. 

Is the above understanding correct? Do I miss anything? 

More comments and questions are embedded below:


> On Dec 3, 2020, at 11:32 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
> 
> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> On Tue, Nov 24, 2020 at 4:47 PM Qing Zhao <QING.ZHAO@oracle.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.

So, current GCC will delete the UB as dead code when X1 is not needed, with
The new option, we should keep this behavior? 

> 
> * Having a function in the way should stop passes from taking advantage
>  of direct uninitialised uses for optimisation.

This will resolve the issue we raised before with directly adding “artificial” zero-initializer 
during gimplification. 

However, I am wondering whether the new added const internal functions will impact the 
optimization and then change the uninitialized analysis behavior? 
> 
> 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.

Yes, with this approach: 

At gimple level, we will not be able to optimize on the new added init values;
At RTL level, we will optimize on the new added init values;
RTL optimizations will be able to eliminate any redundancy introduced by this new
Initializations to reduce the cost of this options. 



> 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).

Yes, this option is for security purpose, and currently have been used in productions by Microsoft, 
Apple and google, etc. 

Qing
> 
> Thanks,
> Richard