Check whether a function is a pure function

Check whether a function is a pure function

[Hanke Zhang]

Hi, I'm trying to write a pass to erase some useless functions or to
put it another way, detect whether a function is pure or not. (Of
course I know some passes can do the clean after inline)

Here is the problem I got, If a function satisfy the following points,
can it be considered safe to delete?

1. return value is ignored or has no return
2. params are pure 32bit Integer Const or has no params
3. No use of global variable
4. No function call, assembly code, goto statement
5. No cast

And in my pass, I try to do these things through these points:

1. check GIMPLE_RETURN statement and check all the function call points
2. check DECL_PARAM(fndecl) and check the input params at all the
function call points
3. check that fndecl is not in the global_var->referring list
4. check GIMPLE_CALL, GIMPLE_ASM, GIMPLE_GOTO are not present
5. check that there is no difference between TREE_TYPE(lhs) and
TREE_TYPE(rhs1) types in the GIMPLE_ASSIGN statement

I would like to ask that if there are any omissions or errors in the
prerequisites and corresponding implementation plans I listed above.

Thanks
Hanke Zhang

Re: Check whether a function is a pure function

[Richard Biener]

On Tue, Oct 17, 2023 at 11:21 AM Hanke Zhang via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi, I'm trying to write a pass to erase some useless functions or to
> put it another way, detect whether a function is pure or not. (Of
> course I know some passes can do the clean after inline)
>
> Here is the problem I got, If a function satisfy the following points,
> can it be considered safe to delete?
>
> 1. return value is ignored or has no return
> 2. params are pure 32bit Integer Const or has no params
> 3. No use of global variable
> 4. No function call, assembly code, goto statement
> 5. No cast
>
> And in my pass, I try to do these things through these points:
>
> 1. check GIMPLE_RETURN statement and check all the function call points
> 2. check DECL_PARAM(fndecl) and check the input params at all the
> function call points
> 3. check that fndecl is not in the global_var->referring list
> 4. check GIMPLE_CALL, GIMPLE_ASM, GIMPLE_GOTO are not present
> 5. check that there is no difference between TREE_TYPE(lhs) and
> TREE_TYPE(rhs1) types in the GIMPLE_ASSIGN statement
>
> I would like to ask that if there are any omissions or errors in the
> prerequisites and corresponding implementation plans I listed above.

You can look into ipa-pure-const.cc which implements pure/const (and more)
discovery.  The DCE pass should, with this info, already be able to
"erase some useless function [calls]"

> Thanks
> Hanke Zhang