[llvm-dev] [libLTO] accessing llvm.global_ctors

[Steven Wu via llvm-dev]

> On Jul 26, 2021, at 3:39 PM, Wael Yehia <wyehia at ca.ibm.com> wrote:
> 
> -----"Steven Wu" <stevenwu at apple.com <mailto:stevenwu at apple.com>> wrote: -----
> To: "Wael Yehia" <wyehia at ca.ibm.com <mailto:wyehia at ca.ibm.com>>
> From: "Steven Wu" <stevenwu at apple.com <mailto:stevenwu at apple.com>>
> Date: 07/26/2021 04:03PM
> Cc: "Fangrui Song" <maskray at google.com <mailto:maskray at google.com>>, "LLVM-DEV LIST" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> Subject: [EXTERNAL] Re: [llvm-dev] [libLTO] accessing llvm.global_ctors
> 
> 
>>> On Jul 26, 2021, at 10:59 AM, Wael Yehia <wyehia at ca.ibm.com> wrote:
>>> 
>>> 
>>> 
>>> 
>>> -----"Steven Wu" <stevenwu at apple.com> wrote: -----
>>> To: "Wael Yehia" <wyehia at ca.ibm.com>
>>> From: "Steven Wu" <stevenwu at apple.com>
>>> Date: 07/26/2021 12:22PM
>>> Cc: "Fangrui Song" <maskray at google.com>, llvm-dev at lists.llvm.org
>>> Subject: [EXTERNAL] Re: [llvm-dev] [libLTO] accessing llvm.global_ctors
>>> 
>>>> It also sounds like that you just need to know if the object file has constructor or not, but don't really care anything more than that?
>>> yes
>>> 
>>> 
>>>> Can you read the section name from the IRSymtab to figure out if that is a constructor/destructor or not?
>>> Is this a suggestion of how to implement the query I'm after?
>>> My initial thought was to simply check for the presence of the symbols `llvm.global_ctors` and `llvm.global_dtors` in the ModuleSymbolTable in the LTOModule. 
>> 
>> The reason why I don't want to do that is because they are not real symbols and linker do not know how to resolve them. You could stamp a different attribute on them but the old linker will not be able to understand them.
> 
> Just to be clear, i'm not proposing that these two symbols start being accessible through the `lto_module_get_[num_symbols|symbol_name|symbol_attribute]` functions (which would make them visible to a linker).
> I'm proposing something like:
> 
> lto_bool_t lto_module_has_ctor_dtor(lto_module_t mod) {
>  return unwrap(mod)->hasCtorDtor();
> }
> 
> bool LTOModule::hasCtorDtor() {
>   // either (1) traverse `ModuleSymbolTable SymTab` or 
>   // (2) compute and cache the answer when LTOModule::parseSymbols runs
> }

That sounds fine to me.

Steven

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