[ELF] Move InputFiles global variables (memoryBuffers, objectFiles, etc) into Ctx. NFC

[ELF] Simplify MergeInputSection::getParentOffset. NFC

and remove overly verbose comments.

[ELF] Don't rely on Symbols.h's transitive inclusion of InputFiles.h. NFC

[ELF] Remove SharedSymbol::getFile. NFC

Symbol.h depends on InputFiles.h. This change moves us toward dropping the
weird dependency.

The call sites will become slightly uglier (`cast<SharedFile>(s-

[ELF] Remove SharedSymbol::getFile. NFC

Symbol.h depends on InputFiles.h. This change moves us toward dropping the
weird dependency.

The call sites will become slightly uglier (`cast<SharedFile>(s->file)`), but
the compromise is acceptable.

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[ELF] Replace uncompressed InputSectionBase::data() with rawData. NFC

In many call sites we know uncompression cannot happen (non-SHF_ALLOC, or the
data (even if compressed) must have been uncompres

[ELF] Replace uncompressed InputSectionBase::data() with rawData. NFC

In many call sites we know uncompression cannot happen (non-SHF_ALLOC, or the
data (even if compressed) must have been uncompressed by a previous pass).
Prefer rawData in these cases. data() increases code size and prevents
optimization on rawData.

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[ELF] Clean up headers. NFC

[ELF] Simplify SectionBase::partition handling and make it live by default. NFC

Previously an InputSectionBase is dead (`partition==0`) by default.
SyntheticSection calls markLive and BssSection ove

[ELF] Simplify SectionBase::partition handling and make it live by default. NFC

Previously an InputSectionBase is dead (`partition==0`) by default.
SyntheticSection calls markLive and BssSection overrides that with markDead.

It is more natural to make InputSectionBase live by default and let
--gc-sections mark InputSectionBase dead.

When linking a Release build of clang:

* --no-gc-sections:, the removed `inputSections` loop decreases markLive time from 4ms to 1ms.
* --gc-sections: the extra `inputSections` loop increases markLive time from 0.181296s to 0.188526s.
This is as of we lose the removing one `inputSections` loop optimization (4374824ccf6e7ae68264d996a9ae5bb5e3be7fc5).
I believe the loss can be mitigated if we refactor markLive.

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Re-land [LLD] Remove global state in lldCommon

Move all variables at file-scope or function-static-scope into a hosting structure (lld::CommonLinkerContext) that lives at lldMain()-scope. Drivers wi

Re-land [LLD] Remove global state in lldCommon

Move all variables at file-scope or function-static-scope into a hosting structure (lld::CommonLinkerContext) that lives at lldMain()-scope. Drivers will inherit from this structure and add their own global state, in the same way as for the existing COFFLinkerContext.

See discussion in https://lists.llvm.org/pipermail/llvm-dev/2021-June/151184.html

The previous land f860fe362282ed69b9d4503a20e5d20b9a041189 caused issues in https://lab.llvm.org/buildbot/#/builders/123/builds/8383, fixed by 22ee510dac9440a74b2e5b3fe3ff13ccdbf55af3.

Differential Revision: https://reviews.llvm.org/D108850

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Revert [LLD] Remove global state in lldCommon

It seems to be causing issues on https://lab.llvm.org/buildbot/#/builders/123/builds/8383

[LLD] Remove global state in lldCommon

Move all variables at file-scope or function-static-scope into a hosting structure (lld::CommonLinkerContext) that lives at lldMain()-scope. Drivers will inher

[LLD] Remove global state in lldCommon

Move all variables at file-scope or function-static-scope into a hosting structure (lld::CommonLinkerContext) that lives at lldMain()-scope. Drivers will inherit from this structure and add their own global state, in the same way as for the existing COFFLinkerContext.

See discussion in https://lists.llvm.org/pipermail/llvm-dev/2021-June/151184.html

Differential Revision: https://reviews.llvm.org/D108850

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[ELF] Switch cNamedSections to SmallVector. NFC

Make it smaller

[ELF] --gc-sections: Work around SHT_PROGBITS .init_array.N for Rust

See https://github.com/rust-lang/rust/issues/92181

[ELF] Remove stale comment which was duplicated in MarkLive<ELFT>::run

Pointed out by thakis

[ELF] --gc-sections: combine two iterations over inputSections

There is a slight speed-up.

[ELF] --gc-sections: Work around SHT_PROGBITS .init_array

Older Go cmd/link used SHT_PROGBITS for .init_array .
Work around the lack of https://golang.org/cl/373734 for a while.
It does not generate

[ELF] --gc-sections: Work around SHT_PROGBITS .init_array

Older Go cmd/link used SHT_PROGBITS for .init_array .
Work around the lack of https://golang.org/cl/373734 for a while.
It does not generate .fini_array or .preinit_array

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[ELF] --gc-sections: Change startwith(".jcr") to exact match

GNU ld's internal linker script keeps `.jcr`, but not other sections
starting with `.jcr`.

[ELF] --gc-sections: Change startwith(".init") (and ".fini") to exact match

GNU ld's internal linker script keeps `.init`, but not other sections starting
with `.init`. .fini is similar.

[ELF] Change objectFiles to ELFFileBase *

This can sometimes avoid `cast<ObjFile<...>>`.

I intentionally do not touch postScanRelocations to wait for its stabilization.

[ELF] Replace InputBaseSection::{areRelocsRela,firstRelocation,numRelocation} with relSecIdx

For `InputSection` `.foo`, its `InputBaseSection::{areRelocsRela,firstRelocation,numRelocation}` basicall

[ELF] Replace InputBaseSection::{areRelocsRela,firstRelocation,numRelocation} with relSecIdx

For `InputSection` `.foo`, its `InputBaseSection::{areRelocsRela,firstRelocation,numRelocation}` basically
encode the information of `.rel[a].foo`. However, one uint32_t (the relocation section index)
suffices. See the implementation of `relsOrRelas`.

This change decreases sizeof(InputSection) from 184 to 176 on 64-bit Linux.

The maximum resident set size linking a large application (1.2G output) decreases by 0.39%.

Differential Revision: https://reviews.llvm.org/D112513

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[ELF] Update comments/diagnostics for some long options to use the canonical two-dash form

Rewrite some comments as appropriate.

Revert "[ELF] --gc-sections: allow GC on reserved sections in a group"

clang may place dynamic initializations for explicitly specialized class
template static data members in comdat.
Such in-comdat

Revert "[ELF] --gc-sections: allow GC on reserved sections in a group"

clang may place dynamic initializations for explicitly specialized class
template static data members in comdat.
Such in-comdat SHT_INIT_ARRAY was an abuse but we have to work around it for a while.

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[ELF] --gc-sections: allow GC on reserved sections in a group

This generalizes D70146 (SHT_NOTE) to more reserved sections and makes our rules
more consistent. Now SHF_GROUP is more similar to SHF_L

[ELF] --gc-sections: allow GC on reserved sections in a group

This generalizes D70146 (SHT_NOTE) to more reserved sections and makes our rules
more consistent. Now SHF_GROUP is more similar to SHF_LINK_ORDER.

For SHT_INIT_ARRAY/SHT_FINI_ARRAY, the rule will be closer to PE/COFF link.exe.

Previously sanitizers use llvm.global_ctors to make module_ctor a GC
root, which is considered an abuse.
https://groups.google.com/g/generic-abi/c/TpleUEkNoQI

We can squeak through on compatibility issues because compilers otherwise don't
use SHF_GROUP special sections.

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[ELF] Default to -z start-stop-gc with a glibc "__libc_" special case

Change the default to facilitate GC for metadata section usage, so that they
don't need SHF_LINK_ORDER or SHF_GROUP just to drop

[ELF] Default to -z start-stop-gc with a glibc "__libc_" special case

Change the default to facilitate GC for metadata section usage, so that they
don't need SHF_LINK_ORDER or SHF_GROUP just to drop the unhelpful rule (if they
want to be unconditionally retained, use SHF_GNU_RETAIN
(`__attribute__((retain))`) or linker script `KEEP`).

The dropped SHF_GROUP special case makes the behavior of -z start-stop-gc and -z
nostart-stop-gc closer to GNU ld>=2.37 (https://sourceware.org/PR27451).

However, we default to -z start-stop-gc (which actually matches more closely to
GNU ld before 2015-10 https://sourceware.org/PR19167), which is different from
modern GNU ld (which has the unhelpful rule to work around glibc). As a
compensation, we special case `__libc_` sections as a workaround for glibc<2.34
(https://sourceware.org/PR27492).

Since -z start-stop-gc as the default actually matches the traditional GNU ld
behavior, there isn't much to be aware of. There was a systemd usage which has
been fixed by https://github.com/systemd/systemd/pull/19144

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[ELF] Add -z start-stop-gc to let __start_/__stop_ not retain C identifier name sections

For one metadata section usage, each text section references a metadata section.
The metadata sections have a

[ELF] Add -z start-stop-gc to let __start_/__stop_ not retain C identifier name sections

For one metadata section usage, each text section references a metadata section.
The metadata sections have a C identifier name to allow the runtime to collect them via `__start_/__stop_` symbols.

Since `__start_`/`__stop_` references are always present from live sections, the
C identifier name sections appear like GC roots, which means they cannot be
discarded by `ld --gc-sections`.

To make such sections GCable, either SHF_LINK_ORDER or a section group is needed.

SHF_LINK_ORDER is not suitable for the references can be inlined into other functions
(See D97430:
Function A (in the section .text.A) references its `__sancov_guard` section.
Function B inlines A (so now .text.B references `__sancov_guard` - this is invalid with the semantics of SHF_LINK_ORDER).

In the linking stage,
if `.text.A` gets discarded, and `__sancov_guard` is retained via the reference from `.text.B`,
the output will be invalid because `__sancov_guard` references the discarded `.text.A`.
LLD errors "sh_link points to discarded section".
)

A section group have size overhead, and is cumbersome when there is just one metadata section.

Add `-z start-stop-gc` to drop the "__start_/__stop_ references retain
non-SHF_LINK_ORDER non-SHF_GROUP C identifier name sections" rule.
We reserve the rights to switch the default in the future.

Reviewed By: phosek, jrtc27

Differential Revision: https://reviews.llvm.org/D96914

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[lld][ELF] __start_/__stop_ refs don't retain C-ident named group sections

The special root semantics for identifier-named sections is meant
specifically for the metadata sections. In the context of

[lld][ELF] __start_/__stop_ refs don't retain C-ident named group sections

The special root semantics for identifier-named sections is meant
specifically for the metadata sections. In the context of group
semantics, where group members are always retained or discarded as a
unit, it's natural not to have this semantics apply to a section in a
group, otherwise we would never discard the group defeating the purpose
of using the group in the first place.

This change modifies the GC behavior so that __start_/__stop_ references
don't retain C identifier named sections in section groups which allows
for these groups to be collected. This matches the behavior of BFD ld.

The only kind of existing case that might break is interdependent
metadata sections that are all in a group together, but that group
doesn't contain any other sections referenced by anything except
implicit inclusion in a `__start_` and/or `__stop_`-referenced
identifier-named section, but such cases should be unlikely.

Differential Revision: https://reviews.llvm.org/D96753

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