Question about finding parameters in function bodies from SSA variables

Question about finding parameters in function bodies from SSA variables

[Erick Ochoa]

Hello Richard,

I'm still working on the points-to analysis and I am happy to say that
after reviewing the ipa-cp code I was able to generate summaries for
local variables, ssa variables, heap variables, global variables and
functions. I am also using the callback hooks to find out if
cgraph_nodes and varpool_nodes are added or deleted between
read_summaries and execute. Even though I don't update the solutions
between execute and function_transform yet, I am reading the points-to
pairs and remapping the constraint variables back to trees during
function_transform and printing the name of pointer-pointee pairs.

This is still very much a work in progress and a very weak points-to
analysis. I have almost finished my Andersen's / field insensitive /
context insensitive / flow-insensitive / intraprocedural analysis with
the LTO framework (without interacting with other transformations
yet). The only thing that I am missing is assigning parameters to be
pointing to NONLOCAL memory upon entry to the function and perhaps
some corner cases where gimple is not exactly how I expect it to be.

I am wondering, none of the variables in
function->gimple_df->ssa_names and function->local_decls are
PARM_DECL. I'm also not entirely sure if I should be looking for
PARM_DECLs since looking at function bodies' gimple representation I
don't see the formal parameters being used inside the function.
Instead, it appears that some SSA variables are automatically
initialized with the parameter value. Is this the case?

For example, for a function:

foo (struct a* $NAME)

The variable $NAME is nowhere used inside the function. I also found
that there is an ssa variable in location X ( in
function->gimple_df->ssa_names[X]) named with a variation like
$NAME_$X(D) and this seems to correspond to the parameter $NAME. How
can one (preferably looking only at
function->gimple_df->ssa_names[$X]) find out that this tree
corresponds to a parameter?

Many thanks!
-Erick

Re: Question about finding parameters in function bodies from SSA variables

[Richard Biener]

On Thu, Aug 5, 2021 at 3:45 PM Erick Ochoa <eochoa@gcc.gnu.org> wrote:
>
> Hello Richard,
>
> I'm still working on the points-to analysis and I am happy to say that
> after reviewing the ipa-cp code I was able to generate summaries for
> local variables, ssa variables, heap variables, global variables and
> functions. I am also using the callback hooks to find out if
> cgraph_nodes and varpool_nodes are added or deleted between
> read_summaries and execute. Even though I don't update the solutions
> between execute and function_transform yet, I am reading the points-to
> pairs and remapping the constraint variables back to trees during
> function_transform and printing the name of pointer-pointee pairs.
>
> This is still very much a work in progress and a very weak points-to
> analysis. I have almost finished my Andersen's / field insensitive /
> context insensitive / flow-insensitive / intraprocedural analysis with
> the LTO framework (without interacting with other transformations
> yet). The only thing that I am missing is assigning parameters to be
> pointing to NONLOCAL memory upon entry to the function and perhaps
> some corner cases where gimple is not exactly how I expect it to be.
>
> I am wondering, none of the variables in
> function->gimple_df->ssa_names and function->local_decls are
> PARM_DECL. I'm also not entirely sure if I should be looking for
> PARM_DECLs since looking at function bodies' gimple representation I
> don't see the formal parameters being used inside the function.
> Instead, it appears that some SSA variables are automatically
> initialized with the parameter value. Is this the case?
>
> For example, for a function:
>
> foo (struct a* $NAME)
>
> The variable $NAME is nowhere used inside the function. I also found
> that there is an ssa variable in location X ( in
> function->gimple_df->ssa_names[X]) named with a variation like
> $NAME_$X(D) and this seems to correspond to the parameter $NAME. How
> can one (preferably looking only at
> function->gimple_df->ssa_names[$X]) find out that this tree
> corresponds to a parameter?

Parameters that are written into SSA form have their default definition
represent the incoming value.  Such parameter SSA names are identified
by SSA_NAME_IS_DEFAULT_DEF (name) &&
SSA_NAME_VAR (name) && TREE_CODE (SSA_NAME_VAR (name)) == PARM_DECL
and the corresponding PARM_DECL is the SSA_NAME_VAR.  Parameters
not written into SSA will appear as PARM_DECL in the IL.  Note similar
things happen to DECL_BY_REFERENCE RESULT_DECLs, the pointer
will be in SSA form and the default def is the address of the return slot so
you might see *result_3(D) = x; in the IL.

See get_constraint_for_ssa_var for example.

>
> Many thanks!
> -Erick