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
