[PATCH] D24616: [ELF] Improve section GC for comdat groups

[Eugene Leviant via llvm-commits]

First of all, I need to identify this section, because, like I said, it's
being added only once to a single object file. When some group is seen next
time, all member sections are discarded. This means that relocations in
some object file may point to discarded sections. When you see this
discarded section in GC, you should (IMHO) find it counterpart and mark it
live.
What I'm doing now is:

a) Get group signature, given object file and some input section index.
This is done using SectionGroupSig hash map.
b) Store all group signatures in hash set
c) Iterate object files and fetch all input sections, which are members of
comdat groups with signatures in this hash set.

Now I'm trying to understand your suggestion. You suggest to keep list of
member sections in each member of the group, correct?
You cannot do so in discarded section, so you still need to find real
section, having some object file and section number in it, right?


2016-09-15 22:45 GMT+03:00 Rui Ueyama <ruiu at google.com>:

> Imagine any section that is in some comdat group have a GroupMembers
> vector, so that starting from any comdat group member section, you can
> reach all siblings in the same group. With that, all you have to do for a
> section S to make all its siblings live is to do `for (InputSectionData
> *Succ : S->GroupMember) Enque({Succ, 0});`. Doesn't it work?
>
> On Thu, Sep 15, 2016 at 12:32 PM, Eugene Leviant <evgeny.leviant at gmail.com
> > wrote:
>
>> evgeny777 added a comment.
>>
>> Let's elaborate the idea. The main problem is that symbol 'D' inside
>> resolveReloc() may point to InputSectionBase<ELFT>::Discarded. This
>> happens because comdat group is added to only one object file and causes
>> crash in GC, because forEachSuccessor implicitly casts Discarded to
>> InputSection<ELFT> and tries to fetch relocs from it. How this
>> 'GroupMembers' vector would help?
>>
>>
>> Repository:
>>   rL LLVM
>>
>> https://reviews.llvm.org/D24616
>>
>>
>>
>>
>
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