isExported, intended to replace exportDynamic, is primarily set in two
locations, (a) after parseSymbolVersion and (b) during demoteSymbols.
In the future, we should try removing exportDynamic. Currently,
merging exportDynamic/isExported would cause
riscv-gp.s to fail:
* The first isExported computation considers the undefined symbol exported
* Defined as a linker-synthesized symbol
* isExported remains true, while it should be false
We've noticed that for large builds executing thin-link can take on the
order of 10s of minutes. We are only using a single thread to write the
sharded indices and import files for each input bitcode file. While we
need to ensure the index file produced lists modules in a deterministic
order, that doesn't prevent us from executing the rest of the work in
parallel.
In this change we use a thread pool to execute as much of the backend's
work as possible in parallel. In local testing on a machine with 80
cores, this change makes a thin-link for ~100,000 input files run in ~2
minutes. Without this change it takes upwards of 10 minutes.
---------
Co-authored-by: Nuri Amari <nuriamari@fb.com>
This reapplies commit 1911a50fae8a441b445eb835b98950710d28fc88 with a
minor fix in lld/ELF/LTO.cpp which sets Options.BBAddrMap when
`--lto-basic-block-sections=labels` is passed.
Remove the global variable `symtab` and add a member variable
(`std::unique_ptr<SymbolTable>`) to `Ctx` instead.
This is one step toward eliminating global states.
Pull Request: https://github.com/llvm/llvm-project/pull/109612
Fix the use-after-free bug and re-apply
https://github.com/llvm/llvm-project/pull/106193
* Without the fix, the string referenced by `objSym.Name` could be
destroyed even if string saver keeps a copy of the referenced string.
This caused use-after-free.
* The fix ([latest
commit](9776ed44cf))
updates `objSym.Name` to reference (via `StringRef`) the string saver's
copy.
Test:
1. For `lld/test/ELF/lto/asmundef.ll`, its test failure is reproducible
with `-DLLVM_USE_SANITIZER=Address` and gone with the fix.
3. Run all tests by following
https://github.com/google/sanitizers/wiki/SanitizerBotReproduceBuild#try-local-changes.
* Without the fix, `ELF/lto/asmundef.ll` aborted the multi-stage test at
`@@@BUILD_STEP stage2/asan_ubsan check@@@`, defined
[here](https://github.com/llvm/llvm-zorg/blob/main/zorg/buildbot/builders/sanitizers/buildbot_fast.sh#L30)
* With the fix, the [multi-stage
test](https://github.com/llvm/llvm-zorg/blob/main/zorg/buildbot/builders/sanitizers/buildbot_fast.sh)
pass stage2 {asan, ubsan, masan}. This is also the test used by
https://lab.llvm.org/buildbot/#/builders/169
**Original commit message**
`StringMap<T>` creates a [copy of the
string](d4c519e7b2/llvm/include/llvm/ADT/StringMapEntry.h (L55-L58))
for entry insertions and intentionally keep copies [since the
implementation optimizes string memory
usage](d4c519e7b2/llvm/include/llvm/ADT/StringMap.h (L124)).
On the other hand, linker keeps copies of symbol names [1] in
`lld:🧝:parseFiles` [2] before invoking `compileBitcodeFiles` [3].
This change proposes to optimize away string copies inside
[LTO::GlobalResolutions](24e791b416/llvm/include/llvm/LTO/LTO.h (L409)),
which will make LTO indexing more memory efficient for ELF. There are
similar opportunities for other (COFF, wasm, MachO) formats.
The optimization takes place for lld (ELF) only. For the rest of use
cases (gold plugin, `llvm-lto2`, etc), LTO owns a string saver to keep
copies and use global resolution key for de-duplication.
Together with @kazutakahirata's work to make `ComputeCrossModuleImport`
more memory efficient, we see a ~20% peak memory usage reduction in a
binary where peak memory usage needs to go down. Thanks to the
optimization in
329ba523cc,
the max (as opposed to the sum) of `ComputeCrossModuleImport` or
`GlobalResolution` shows up in peak memory usage.
* Regarding correctness, the set of
[resolved](80c47ad3ae/llvm/lib/LTO/LTO.cpp (L739))
[per-module
symbols](80c47ad3ae/llvm/include/llvm/LTO/LTO.h (L188-L191))
is a subset of
[llvm::lto::InputFile::Symbols](80c47ad3ae/llvm/include/llvm/LTO/LTO.h (L120)).
And bitcode symbol parsing saves symbol name when iterating
`obj->symbols` in `BitcodeFile::parse` already. This change updates
`BitcodeFile::parseLazy` to keep copies of per-module undefined symbols.
* Presumably the undefined symbols in a LTO unit (copied in this patch
in linker unique saver) is a small set compared with the set of symbols
in global-resolution (copied before this patch), making this a
worthwhile trade-off. Benchmarking this change alone shows measurable
memory savings across various benchmarks.
[1] ELF
1cea5c2138/lld/ELF/InputFiles.cpp (L1748)
[2]
ef7b18a53c/lld/ELF/Driver.cpp (L2863)
[3]
ef7b18a53c/lld/ELF/Driver.cpp (L2995)
`StringMap<T>` creates a [copy of the
string](d4c519e7b2/llvm/include/llvm/ADT/StringMapEntry.h (L55-L58))
for entry insertions and intentionally keep copies [since the
implementation optimizes string memory
usage](d4c519e7b2/llvm/include/llvm/ADT/StringMap.h (L124)).
On the other hand, linker keeps copies of symbol names [1] in
`lld:🧝:parseFiles` [2] before invoking `compileBitcodeFiles` [3].
This change proposes to optimize away string copies inside
[LTO::GlobalResolutions](24e791b416/llvm/include/llvm/LTO/LTO.h (L409)),
which will make LTO indexing more memory efficient for ELF. There are
similar opportunities for other (COFF, wasm, MachO) formats.
The optimization takes place for lld (ELF) only. For the rest of use
cases (gold plugin, `llvm-lto2`, etc), LTO owns a string saver to keep
copies and use global resolution key for de-duplication.
Together with @kazutakahirata's work to make `ComputeCrossModuleImport`
more memory efficient, we see a ~20% peak memory usage reduction in a
binary where peak memory usage needs to go down. Thanks to the
optimization in
329ba523cc,
the max (as opposed to the sum) of `ComputeCrossModuleImport` or
`GlobalResolution` shows up in peak memory usage.
* Regarding correctness, the set of
[resolved](80c47ad3ae/llvm/lib/LTO/LTO.cpp (L739))
[per-module
symbols](80c47ad3ae/llvm/include/llvm/LTO/LTO.h (L188-L191))
is a subset of
[llvm::lto::InputFile::Symbols](80c47ad3ae/llvm/include/llvm/LTO/LTO.h (L120)).
And bitcode symbol parsing saves symbol name when iterating
`obj->symbols` in `BitcodeFile::parse` already. This change updates
`BitcodeFile::parseLazy` to keep copies of per-module undefined symbols.
* Presumably the undefined symbols in a LTO unit (copied in this patch
in linker unique saver) is a small set compared with the set of symbols
in global-resolution (copied before this patch), making this a
worthwhile trade-off. Benchmarking this change alone shows measurable
memory savings across various benchmarks.
[1] ELF
1cea5c2138/lld/ELF/InputFiles.cpp (L1748)
[2]
ef7b18a53c/lld/ELF/Driver.cpp (L2863)
[3]
ef7b18a53c/lld/ELF/Driver.cpp (L2995)
Summary:
Currently the `--lto-emit-llvm` option writes out the
post-internalization bitcode. This is the bitcode before any
optimizations or other pipelines have been run on it. This patch changes
that to use the pre-codegen module, which is the state of the LLVM-IR
after the optimizations have been run.
I believe that this makes sense as the `--lto-emit-llvm` option seems to
imply that we should emit the final output of the LLVM pass as if it
were the desired output. This should include optimizations at the
requested optimization level. My main motivation for this change is to
be able to use this to link several LLVM-IR files into a single one that
I can then pass back to `ld.lld` later (for JIT purposes).
Today `-split-machine-functions` and `-fbasic-block-sections={all,list}`
cannot be combined with `-basic-block-sections=labels` (the labels
option will be ignored).
The inconsistency comes from the way basic block address map -- the
underlying mechanism for basic block labels -- encodes basic block
addresses
(https://lists.llvm.org/pipermail/llvm-dev/2020-July/143512.html).
Specifically, basic block offsets are computed relative to the function
begin symbol. This relies on functions being contiguous which is not the
case for MFS and basic block section binaries. This means Propeller
cannot use binary profiles collected from these binaries, which limits
the applicability of Propeller for iterative optimization.
To make the `SHT_LLVM_BB_ADDR_MAP` feature work with basic block section
binaries, we propose modifying the encoding of this section as follows.
First let us review the current encoding which emits the address of each
function and its number of basic blocks, followed by basic block entries
for each basic block.
| | |
|--|--|
| Address of the function | Function Address |
| Number of basic blocks in this function | NumBlocks |
| BB entry 1
| BB entry 2
| ...
| BB entry #NumBlocks
To make this work for basic block sections, we treat each basic block
section similar to a function, except that basic block sections of the
same function must be encapsulated in the same structure so we can map
all of them to their single function.
We modify the encoding to first emit the number of basic block sections
(BB ranges) in the function. Then we emit the address map of each basic
block section section as before: the base address of the section, its
number of blocks, and BB entries for its basic block. The first section
in the BB address map is always the function entry section.
| | |
|--|--|
| Number of sections for this function | NumBBRanges |
| Section 1 begin address | BaseAddress[1] |
| Number of basic blocks in section 1 | NumBlocks[1] |
| BB entries for Section 1
|..................|
| Section #NumBBRanges begin address | BaseAddress[NumBBRanges] |
| Number of basic blocks in section #NumBBRanges |
NumBlocks[NumBBRanges] |
| BB entries for Section #NumBBRanges
The encoding of basic block entries remains as before with the minor
change that each basic block offset is now computed relative to the
begin symbol of its containing BB section.
This patch adds a new boolean codegen option `-basic-block-address-map`.
Correspondingly, the front-end flag `-fbasic-block-address-map` and LLD
flag `--lto-basic-block-address-map` are introduced.
Analogously, we add a new TargetOption field `BBAddrMap`. This means BB
address maps are either generated for all functions in the compiling
unit, or for none (depending on `TargetOptions::BBAddrMap`).
This patch keeps the functionality of the old
`-fbasic-block-sections=labels` option but does not remove it. A
subsequent patch will remove the obsolete option.
We refactor the `BasicBlockSections` pass by separating the BB address
map and BB sections handing to their own functions (named
`handleBBAddrMap` and `handleBBSections`). `handleBBSections` renumbers
basic blocks and places them in their assigned sections.
`handleBBAddrMap` is invoked after `handleBBSections` (if requested) and
only renumbers the blocks.
- New tests added:
- Two tests basic-block-address-map-with-basic-block-sections.ll and
basic-block-address-map-with-mfs.ll to exercise the combination of
`-basic-block-address-map` with `-basic-block-sections=list` and
'-split-machine-functions`.
- A driver sanity test for the `-fbasic-block-address-map` option
(basic-block-address-map.c).
- An LLD test for testing the `--lto-basic-block-address-map` option.
This reuses the LLVM IR from `lld/test/ELF/lto/basic-block-sections.ll`.
- Renamed and modified the two existing codegen tests for basic block
address map (`basic-block-sections-labels-functions-sections.ll` and
`basic-block-sections-labels.ll`)
- Removed `SHT_LLVM_BB_ADDR_MAP_V0` tests. Full deprecation of
`SHT_LLVM_BB_ADDR_MAP_V0` and `SHT_LLVM_BB_ADDR_MAP` version less than 2
will happen in a separate PR in a few months.
Port COFF's https://reviews.llvm.org/D78221 and
https://reviews.llvm.org/D137217 to ELF. For the in-process ThinLTO
link, `ld.lld --save-temps a.o d/b.o -o out` will create
ELF relocatable files `out.lto.a.o`/`d/out.lto.b.o` instead of
`out1.lto.o`/`out2.lto.o`. Deriving the LTO-generated relocatable file
name from bitcode file names helps debugging.
The relocatable file name from the first regular LTO partition does not
change: `out.lto.o`. The second, if present due to `--lto-partition=`,
changes from `out1.lto.o` to `lto.1.o`.
For an archive member, e.g. `d/a.a(coll.o at 8)`,
the relocatable file is `d/out.lto.a.a(coll.o at 8).o`.
`--lto-emit-asm` file names are changed similarly. `--lto-emit-asm -o
out` now creates `out.lto.s` instead of `out`, therefore the
`--lto-emit-asm -o -` idiom no longer works. However, I think this new
behavior (which matches COFF) is better since keeping or removing
`--lto-emit-asm` will dump different files, instead of overwriting the
`-o` output file from an executable/shared object to an assembly file.
Reviewers: rnk, igorkudrin, xur-llvm, teresajohnson, ZequanWu
Reviewed By: teresajohnson
Pull Request: https://github.com/llvm/llvm-project/pull/78835
Based on https://reviews.llvm.org/D45375 . Introduce a new InputFile
kind `InternalKind`, use it for
* `ctx.internalFile`: for linker-defined symbols and some synthesized
`Undefined`
* `createInternalFile`: for symbol assignments and --defsym
I picked "internal" instead of "synthetic" to avoid confusion with
SyntheticSection.
Currently a symbol's file is one of: nullptr, ObjKind, SharedKind,
BitcodeKind, BinaryKind. Now it's non-null (I plan to add an
`assert(file)` to Symbol::Symbol and change `toString(const InputFile
*)`
separately).
Debugging and error reporting gets improved. The immediate user-facing
difference is more descriptive "File" column in the --cref output. This
patch may unlock further simplification.
Currently each symbol assignment gets its own
`createInternalFile(cmd->location)`. Two symbol assignments in a linker
script do not share the same file. Making the file the same would be
nice, but would require non trivial code.
Discussion about this approach: https://discourse.llvm.org/t/rfc-safer-whole-program-class-hierarchy-analysis/65144/18
When enabling WPD in an environment where native binaries are present, types we want to optimize can be derived from inside these native files and devirtualizing them can lead to correctness issues. RTTI can be used as a way to determine all such types in native files and exclude them from WPD providing a safe checked way to enable WPD.
The approach is:
1. In the linker, identify if RTTI is available for all native types. If not, under `--lto-validate-all-vtables-have-type-infos` `--lto-whole-program-visibility` is automatically disabled. This is done by examining all .symtab symbols in object files and .dynsym symbols in DSOs for vtable (_ZTV) and typeinfo (_ZTI) symbols and ensuring there's always a match for every vtable symbol.
2. During thinlink, if `--lto-validate-all-vtables-have-type-infos` is set and RTTI is available for all native types, identify all typename (_ZTS) symbols via their corresponding typeinfo (_ZTI) symbols that are used natively or outside of our summary and exclude them from WPD.
Testing:
ninja check-all
large Meta service that uses boost, glog and libstdc++.so runs successfully with WPD via --lto-whole-program-visibility. Previously, native types in boost caused incorrect devirtualization that led to crashes.
Reviewed By: MaskRay, tejohnson
Differential Revision: https://reviews.llvm.org/D155659
This will make it easy for callers to see issues with and fix up calls
to createTargetMachine after a future change to the params of
TargetMachine.
This matches other nearby enums.
For downstream users, this should be a fairly straightforward
replacement,
e.g. s/CodeGenOpt::Aggressive/CodeGenOptLevel::Aggressive
or s/CGFT_/CodeGenFileType::
The unified LTO pipeline creates a single LTO bitcode structure that can
be used by Thin or Full LTO. This means that the LTO mode can be chosen
at link time and that all LTO bitcode produced by the pipeline is
compatible, from an optimization perspective. This makes the behavior of
LTO a bit more predictable by normalizing the set of LTO features
supported by each LTO bitcode file.
Example usage:
clang -flto -funified-lto -fuse-ld=lld foo.c
clang -flto=thin -funified-lto -fuse-ld=lld foo.c
clang -c -flto -funified-lto foo.c # -flto={full,thin} are identical in
terms of compilation actions
clang -flto=thin -fuse-ld=lld foo.o # pass --lto=thin to ld.lld
clang -c -flto -funified-lto foo.c clang -flto -fuse-ld=lld foo.o
The RFC discussing the details and rational for this change is here:
https://discourse.llvm.org/t/rfc-a-unified-lto-bitcode-frontend/61774
Differential Revision: https://reviews.llvm.org/D123805
Currently, the --thinlto-prefix-replace="oldpath;newpath" option is used during
distributed ThinLTO thin links to specify the mapping of the input bitcode object
files' directory tree (oldpath) to the directory tree (newpath) used for both:
1) the output files of the thin link itself (the .thinlto.bc index files and the
optional .imports files)
2) the specified object file paths written to the response file given in the
--thinlto-index-only=${response} option, which is used by the final native
link and must match the paths of the native object files that will be
produced by ThinLTO backend compiles.
This patch expands the --thinlto-prefix-replace option to allow a separate directory
tree mapping to be specified for the object file paths written to the response file
(number 2 above). This is important to support builds and build systems where the
same output directory may not be written by multiple build actions (e.g. the thin link
and the ThinLTO backend compiles).
The new format is: --thinlto-prefix-replace="origpath;outpath[;objpath]"
This replaces the origpath directory tree of the thin link input files with
outpath when writing the thin link index and imports outputs (number 1
above). If objpath is specified it replaces origpath of the input files with
objpath when writing the response file (number 2 above), otherwise it
falls back to the old behavior of using outpath for this as well.
Reviewed By: tejohnson, MaskRay
Differential Revision: https://reviews.llvm.org/D144596
Allow controlling the CodeGenOpt::Level independent of the LTO
optimization level in LLD via new options for the COFF, ELF, MachO, and
wasm frontends to lld. Most are spelled as --lto-CGO[0-3], but COFF is
spelled as -opt:lldltocgo=[0-3].
See D57422 for discussion surrounding the issue of how to set the CG opt
level. The ultimate goal is to let each function control its CG opt
level, but until then the current default means it is impossible to
specify a CG opt level lower than 2 while using LTO. This option gives
the user a means to control it for as long as it is not handled on a
per-function basis.
Reviewed By: MaskRay, #lld-macho, int3
Differential Revision: https://reviews.llvm.org/D141970
D138560 was abandonned as the use case can already be covered by `-Xoffload-linker --lto-emit-asm`.
However the output from `--lto-emit-asm` doesn't have
comments like the Clang `-S` output.
This patch adds verbose assembly output to LLD ELF LTO
so that the resulting assembly file more closely matches Clang's.
Having comments is especially important on targets such as AMDGPU because
they contain additional information about the kernel(s) being compiled.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D141268
MC and lld/ELF defaults were flipped in 2016. For Clang: CMake
ENABLE_X86_RELAX_RELOCATIONS defaults to on in 2020. It makes sense for
the TargetOptions default to be true now.
R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX require GNU ld newer than 2015-10
(subsumed by the current requirement of -fbinutils-version=).
This should fix `rustc -Z plt=no` PIC relocatable files with GNU ld.
(See https://github.com/rust-lang/rust/pull/106380)
This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated. The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.
This is part of an effort to migrate from llvm::Optional to
std::optional:
https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716
Breaks build of LLVMgold here:
```
/repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1108:19: error: no matching function for call to 'localCache'
Cache = check(localCache("ThinLTO", "Thin", options::cache_dir, AddBuffer));
^~~~~~~~~~
/repositories/llvm-project/llvm/include/llvm/Support/Caching.h:72:21: note: candidate function not viable: no known conversion from '(lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1102:20)' to 'llvm::AddBufferFn' (aka 'function<void (unsigned int, const llvm::Twine &, std::unique_ptr<MemoryBuffer>)>') for 4th argument
Expected<FileCache> localCache(
^
/repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1110:18: error: no viable conversion from '(lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20)' to 'llvm::AddStreamFn' (aka 'function<Expected<std::unique_ptr<CachedFileStream>> (unsigned int, const llvm::Twine &)>')
check(Lto->run(AddStream, Cache));
^~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/std_function.h:375:7: note: candidate constructor not viable: no known conversion from '(lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20)' to 'std::nullptr_t' for 1st argument
function(nullptr_t) noexcept
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/std_function.h:386:7: note: candidate constructor not viable: no known conversion from '(lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20)' to 'const std::function<llvm::Expected<std::unique_ptr<llvm::CachedFileStream>> (unsigned int, const llvm::Twine &)> &' for 1st argument
function(const function& __x)
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/std_function.h:404:7: note: candidate constructor not viable: no known conversion from '(lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20)' to 'std::function<llvm::Expected<std::unique_ptr<llvm::CachedFileStream>> (unsigned int, const llvm::Twine &)> &&' for 1st argument
function(function&& __x) noexcept
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/std_function.h:435:2: note: candidate template ignored: requirement '_Callable<(lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20) &, (lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20), std::__invoke_result<(lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20) &, unsigned int, const llvm::Twine &>>::value' was not satisfied [with _Functor = (lambda at /repositories/llvm-project/llvm/tools/gold/gold-plugin.cpp:1094:20) &]
function(_Functor&& __f)
^
/repositories/llvm-project/llvm/include/llvm/LTO/LTO.h:278:25: note: passing argument to parameter 'AddStream' here
Error run(AddStreamFn AddStream, FileCache Cache = nullptr);
^
```
This reverts commit 387620aa8cea33174b6c1fb80c1af713fee732ac.
Currently the lto native object files have names like main.exe.lto.1.obj. In
PDB, those names are used as names for each compiland. Microsoft’s tool
SizeBench uses those names to present to users the size of each object files.
So, names like main.exe.lto.1.obj is not user friendly.
This patch makes the lto native object file names more readable by using
the bitcode file names as part of the file names. For example, if the input
bitcode file has path like "path/to/foo.obj", its corresponding lto native
object file path would be "path/to/main.exe.lto.foo.obj". Since the lto native
object file name only bothers PDB, this patch only changes the lld-linker's
behavior.
Reviewed By: tejohnson, MaskRay, #lld-macho
Differential Revision: https://reviews.llvm.org/D137217
A specific case for ThinLTO cache pruning is that the current build is huge, and the cache wasn't big enough to hold the intermediate object files of that build. So in doing that build, a file would be cached, and later in that same build it would be evicted. This was significantly decreasing the effectiveness of the cache. By giving this warning, the user could identify the required cache size/files and improve ThinLTO link speed.
Differential Revision: https://reviews.llvm.org/D135590
Currently when the --thinlto-emit-index-files is used with LLD and a
thin archive is passed containing references to object files to link
against where the object files are in a different folder than the thin
archive and some of the archives aren't linked against (ie stay lazy),
the empty index file writer ends up trying to write to a path that
doesn't exist. This patch changes the behavior of that function to use
the path of the obj member of the BitcodeFile object rather than just
the path of the BitcodeFile object itself, which matches the behavior of
the default (non-lazy) case.
Fixes#57963
Regression test added.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D135014
Add LLVM_LIBRARY_VISIBILITY to remove unneeded GOT and unique_ptr
indirection. We can move other global variables into ctx without
indirection concern. In the long term we may consider passing Ctx
as a parameter to various functions and eliminate global state as
much as possible and then remove `Ctx::reset`.
Symbol::replace intends to overwrite a few fields (mostly Elf{32,64}_Sym
fields), but the implementation copies all fields then restores some old fields.
This is error-prone and wasteful. Add Symbol::overwrite to copy just the
needed fields and add other overwrite member functions to copy the extra
fields.
They may modify thinlto optimization.
This patch only extends support for `-mllvm`. There is another way to
pass llvm flags, `-plugin-opt`, but its processing is different and will
be provided in a subsequent patch.
Differential Revision: https://reviews.llvm.org/D134013
When the same bitcode object file is given multiple times from the Command-line
as lazy object file, empty index is generated which overwrites the one from thinlink.
This could cause undefined symbols during final link.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D133740
