llvm dev - Jul 2021 - [libLTO] accessing llvm.global_ctors

-----"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.

 
>> Certain linker flag forces it to include all archive members that have
a constructor or destructor. I agree it's strange.
>
> Is it possible to disclose what linker and what platform that is? You
don't have to but it would be easier to maintain if you can document this
behavior that is unique to the platform you are looking at.
The linker is the system linker on AIX, and the option is -bcdtors:all
(documented here: https://www.ibm.com/docs/en/aix/7.2?topic=l-ld-command)

> 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.

If the only thing you care about is checking the existence of ctors and dtors,
and all you need is using fullLTO with libLTO, reading intrinsics is probably
fine. ThinLTO or using the new C++ LTO interface doesn't really use
ModuleSymbolTable. Also trying to read what the constructors are will be quite
expensive from those functions because they needs to be materialized so you
don't want to do that when you just querying the symbol names
> 
> 
>>> Certain linker flag forces it to include all archive members that
have a constructor or destructor. I agree it's strange.
>> 
>> Is it possible to disclose what linker and what platform that is? You
don't have to but it would be easier to maintain if you can document this
behavior that is unique to the platform you are looking at.
> The linker is the system linker on AIX, and the option is -bcdtors:all
(documented here: https://www.ibm.com/docs/en/aix/7.2?topic=l-ld-command)
> 
Looks reasonable. Just remember to mention that in your patch.

Steven

-----"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 04:03PM
Cc: "Fangrui Song" <maskray at google.com>, "LLVM-DEV
LIST" <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
}