[llvm-dev] [RFC] Changing .llvm.call-graph-profile to use relocations
Alexander Yermolovich via llvm-dev
llvm-dev at lists.llvm.org
Thu Jun 10 18:05:31 PDT 2021
Created a new diff with the _NONE implementation:
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Alexander Yermolovich via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Thursday, May 27, 2021 5:44 PM
To: maskray at google.com <maskray at google.com>
Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] Changing .llvm.call-graph-profile to use relocations
From: maskray at google.com <maskray at google.com>
Sent: Thursday, May 27, 2021 5:23 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 2021-05-27, Alexander Yermolovich wrote:
>I was thinking keeping both structures for backward compatibility in case object files with old representation are fed in to new llvm-objdump, and even lld. For example if someone will use older clang release with lld/llvm-objdump after this change.
>For just changing the value and keeping the name. Looks like it will leave a gap in this sequential sequence. If a new flag is added later down the road and in this case latest clang used with older lld/llvm-objdump this will also present a problem as older tools will interpret new flag as SHT_LLVM_CALL_GRAPH_PROFILE.
>Not sure if these are valid concerns, what do you think?
>If we go for clean state, then maybe leave 0x6fff4c02 entry and change it to SHT_LLVM_CALL_GRAPH_PROFILE_DEPRICATED, and emit warning if objects with it are passed in.
> SHT_LLVM_ODRTAB = 0x6fff4c00, // LLVM ODR table.
> SHT_LLVM_LINKER_OPTIONS = 0x6fff4c01, // LLVM Linker Options.
> SHT_LLVM_CALL_GRAPH_PROFILE = 0x6fff4c02, // LLVM Call Graph Profile.
> SHT_LLVM_ADDRSIG = 0x6fff4c03, // List of address-significant symbols
> // for safe ICF.
> SHT_LLVM_DEPENDENT_LIBRARIES =
> 0x6fff4c04, // LLVM Dependent Library Specifiers.
> SHT_LLVM_SYMPART = 0x6fff4c05, // Symbol partition specification.
> SHT_LLVM_PART_EHDR = 0x6fff4c06, // ELF header for loadable partition.
> SHT_LLVM_PART_PHDR = 0x6fff4c07, // Phdrs for loadable partition.
> SHT_LLVM_BB_ADDR_MAP = 0x6fff4c08, // LLVM Basic Block Address Map.
Do you have measurement how well SHT_LLVM_CALL_GRAPH_PROFILE optimizes?
My understanding is that with ThinLTO+PGO it is has very tiny benefit.
[Alex] I do not. Wenlei mentioned he can dig up some numbers.
The value reading old format is low. I don't expect users want to mix
newer object files with old object files and want to have optimization
from the old object files. SHT_LLVM_CALL_GRAPH_PROFILE_DEPRICATED is
just adding maintenance cost.
Changing the value but retaining the name is sufficient for the various
Reid's size concern is valid. Have you measured the size overhead?
[Alex] Good point. I'll measure once I implement new approach with SHT_REL + R_X86_64_NONE.
We can use the SHT_REL format for SHT_LLVM_CALL_GRAPH_PROFILE
relocations, which brings down the per entry cost from 24 bytes to 16
bytes for ELFCLASS64. (It is a bit unfortunate that the Linux kernel
does not support ELFCLASS32 executables on a 64-bit architecture.
For most usage we are using the small code model and don't benefit
from 64-bit addresses/sizes.)
>From: maskray at google.com <maskray at google.com>
>Sent: Thursday, May 27, 2021 11:27 AM
>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 2021-05-27, 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 ?
>Preversing the structure is not needed because the symbol representation
>is changed anyway.
>You just need to change the value in
>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<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.
>>>"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<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
>>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<https://sourceware.org/bugzilla/show_bug.cgi?id=23817> . Proposed solution for that was also to use relocations.
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