Re: [gimple-ssa] result_decl and ssa_name

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Apr 8, 2024 at 10:47 AM Pierrick Philippe
<pierrick.philippe@irisa.fr> wrote:
>
> 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.

There is no DECL_CONTEXT for SSA names, if the name is anonymous (no
SSA_NAME_VAR) then the context is
implicityly the whole function, thus the FUNCTION_DECL.

>
> 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