Re: [gimple-ssa] result_decl and ssa_name

[Pierrick Philippe <pierrick.philippe@irisa.fr>]

[-- Attachment #1: Type: text/plain, Size: 2982 bytes --]

On 06/04/2024 14:53, Richard Biener wrote:
> On Fri, Apr 5, 2024 at 3:44 PM Pierrick Philippe
> <pierrick.philippe@irisa.fr> wrote:
>> On 05/04/2024 14:46, Richard Biener wrote:
>>
>> On Fri, Apr 5, 2024 at 1:59 PM Pierrick Philippe
>> <pierrick.philippe@irisa.fr> wrote:
>>
>> Hi all,
>>
>> I do have a question regarding ssa_name and result_decl.
>>
>> For example on the following gimple function:
>>
>> int f ()
>> {
>>   int x;
>>   int D.2747;
>>   int _2;
>>
>>   <bb 2> :
>>   x_1 = 42;
>>   _2 = x_1;
>>
>>   <bb 3> :
>> <L0>:
>>   return _2;
>>
>> }
>>
>> On the above example, using the macro SSA_NAME_VAR() on _2 does not
>> yield anything usable.
>> Neither to call ssa_default_def() on the result of the result_decl
>> obtain through macro DECL_RESULT().
>>
>> Is there a way to get the ssa_name corresponding to the result_decl of a
>> function obtained through the use of macro DECL_RESULT() on a fn_decl?
>> And/or the other way around? I.e., from the returned ssa_name of a
>> function to the result_decl of that function?
>>
>> I totally might be missing something here, but I cannot figure out what.
>>
>> DECL_RESULT isn't always used (as in your example).  Not all SSA names
>> have corresponding declarations, we have "anonymous" SSA names which
>> have a NULL_TREE SSA_NAME_VAR (such as the _2 in your example).
>>
>> I see, that makes so much more sense to me now.
>>
>> What do you try to find in the end?  If you want to find all returns you can
>> walk predecessors of EXIT_BLOCK and look at their last stmt whether they
>> are greturn statements.
>>
>> I am implementing a state_machine within the analyzer, and I am trying to understand where would be the best place to propagate the state of the return value.
>> I intuitively thought it would be best to do so in the state_machine::on_pop_frame() method, which is called by the analyzer between the two frames of the caller and the callee. What I do have access to is the struct function of the callee/caller, the gcall instruction in the caller and the callee have been processed by my analysis.
> It might make sense to record the analysis of the return value in
> meta-data that the analyzer keeps and access it that way.
> Other than that you'd have to do it the way I said with finding the
> greturn stmts again and look at what is returned there.

That is what I had in mind, thanks for answering me.

I do have another question though, how do you obtain the decl_context of
such an ssa_name? The DECL_CONTEXT macro is failing during the tree
check and I ha ve no idea how to know where a given ssa_name is declared
without accessing its inner var (through SSA_NAME_VAR). And this
operation is failing on _i ssa_name trees.

>> And to illustrate, here I do have the _2 ssa_name and its state which I know in that case should be propagate to the lhs of the caller gcall instruction.
>>
>> Again I might be taking this in a wrong way.
>>
>> Richard.
>>
>> Thanks for your time,
>>
>> Pierrick