Re: Check whether a function is a pure function

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

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