[llvm-dev] [RFC] Changing .llvm.call-graph-profile to use relocations

[maskray@google.com via llvm-dev]

On 2021-05-27, Alexander Yermolovich wrote:
>Thank you for feedback, let me try to use R_X86_64_NONE, and introduce another type. . I thought R_X86_64_32 would be less impactful as structure of is preserved, but it is space wasteful.
>For type name: ELF::SHT_LLVM_CALL_GRAPH_PROFILE_RELA ?
>
>Alex

Preversing the structure is not needed because the symbol representation
is changed anyway.

You just need to change the value in
llvm/llvm/include/llvm/BinaryFormat/ELF.h
The name doesn't need to change.

>From: maskray at google.com <maskray at google.com>
>Sent: Wednesday, May 26, 2021 5:03 PM
>To: Alexander Yermolovich <ayermolo at fb.com>
>Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>; Wenlei He <wenlei at fb.com>; Modi Mo <modimo at fb.com>; dblaikie at gmail.com <dblaikie at gmail.com>
>Subject: Re: [RFC] Changing .llvm.call-graph-profile to use relocations
>
>On 2021-05-26, Alexander Yermolovich wrote:
>>Currently when .llvm.call-graph-profile is created by llvm it explicitly encodes the symbol indices. This section is basically a black box for post processing tools. For example, if we run strip -s on the object files the symbol table changes, but indices in that section do not.
>>
>>We propose to change this section to use relocations. The Frequency will still be in the .llvm.call-graph-profile, but symbol information will be in relocation section. In LLD information from both sections is used to reconstruct call graph profile. Relocations themselves will never be applied.
>
>Yes, a relocation based approach will be more robust and can fix the
>.llvm_addrsig issue https://sourceware.org/bugzilla/show_bug.cgi?id=23817
>
>The relocation type doesn't matter. Your implementation uses R_X86_64_32 and from/to/value/from/to/value/from/to/value
>An alternative design is R_X86_64_NONE + value/value/value, i.e. from/to do not occupy space in the content.
>We will get a 3x space saving.
>
>We need to change the section type since changing representation is incompatible
>and the sections from old object files should be ignored.
>The new section type will be ignored by old LLVM tools as well.
>
>>With this approach post processing tools that handle relocations correctly work for this section also.
>>
>>One thing is section is marked with SHF_EXCLUDE.
>>From spec
>>"This section is excluded from input to the link-edit of an executable or shared object. This flag is ignored if the SHF_ALLOC flag is also set, or if relocations exist against the section."
>>
>>So technically speaking it needs to be kept, and presumably relocations applied, but LLD follows gold and ld and discards sections marked as SHF_EXCLUDE even with relocations. So, I think this approach should be fine. https://reviews.llvm.org/D24966
>
>This is Solaris's spec (since 1996), not standard ELF's.  It can be advisory but
>our behavior (mostly GNU ABI+Linux ABI+LLVM extensions) does not necessarily
>follow it.  I cannot find a definition in the x86-64 psABI or a GNU ABI
>document.
>
>I think the behavior as implemented in gold and LLD is more useful.
>
>>Finally, this bug seems similar to https://sourceware.org/bugzilla/show_bug.cgi?id=23817 . Proposed solution for that was also to use relocations.
>>
>>Implementation: https://reviews.llvm.org/D103212