Currently for thin-lto, the imported static global values (functions,
variables, etc) will be promoted/renamed from e.g., foo() to
foo.llvm.(). Such a renaming caused difficulties in live patching
since function name is changed ([1]).
It is possible that some global value names have to be promoted to avoid
name collision and linker failure. But in practice, majority of name
promotions can be avoided.
In [2], the suggestion is that thin-lto pre-link decides whether
a particular global value needs name promotion or not. If yes, later on
in thinBackend() the name will be promoted.
I compiled a particular linux kernel version (latest bpf-next tree)
and found 1216 global values with suffix .llvm.. With this patch,
the number of promoted functions is 2, 98% reduction from the
original kernel build.
If some native objects are not participating with LTO, name promotions
have to be done to avoid potential linker issues. So the current
implementation cannot be on by default. But in certain cases, e.g., linux kernel
build, people can enable lld flag --lto-whole-program-visibility to reduce the
number of functions like foo.llvm.().
For ThinLTOCodeGenerator.cpp which is used by llvm-lto tool and a
few other rare cases, reducing the number of renaming due to promotion,
is not implemented as lld flag '-lto-whole-program-visibility' is not
supported in ThinLTOCodeGenerator.cpp for now. In summary, this pull
request only supports llvm-lto2 style workflow.
The feature is off by default. To enable the future, lld flag
'-lto-whole-program-visibility' and llvm flag
'-always-rename-promoted-locals=false' are needed.
The link [3] has more context for the pull request discussions.
[1] https://lpc.events/event/19/contributions/2212
[2] https://discourse.llvm.org/t/rfc-avoid-functions-like-foo-llvm-for-kernel-live-patch/89400
[3] https://github.com/llvm/llvm-project/pull/178587
Currently for thin-lto, the imported static global values (functions,
variables, etc) will be promoted/renamed from e.g., foo() to
foo.llvm.<hash>(). Such a renaming caused difficulties in live patching
since function name is changed ([1]).
It is possible that some global value names have to be promoted to avoid
name collision and linker failure. But in practice, majority of name
promotions can be avoided.
In [2], the suggestion is that thin-lto pre-link decides whether
a particular global value needs name promotion or not. If yes, later on
in thinBackend() the name will be promoted.
I compiled a particular linux kernel version (latest bpf-next tree)
and found 1216 global values with suffix .llvm.<hash>. With this patch,
the number of promoted functions is 2, 98% reduction from the
original kernel build.
If some native objects are not participating with LTO, name promotions
have to be done to avoid potential linker issues. So the current
implementation cannot be on by default. But in certain cases, e.g., linux kernel
build, people can enable lld flag --lto-whole-program-visibility to reduce the
number of functions like foo.llvm.<hash>().
For ThinLTOCodeGenerator.cpp which is used by llvm-lto tool and a
few other rare cases, reducing the number of renaming due to promotion,
is not implemented as lld flag '-lto-whole-program-visibility' is not supported
in ThinLTOCodeGenerator.cpp for now. In summary, this pull request
only supports llvm-lto2 style workflow.
[1] https://lpc.events/event/19/contributions/2212
[2] https://discourse.llvm.org/t/rfc-avoid-functions-like-foo-llvm-for-kernel-live-patch/89400
Enable optimization remark emission during the ThinLTO thin link phase.
This is useful for global analysis passes like MemProf context
disambiguation which operate on the summary index and may need to
report diagnostics before any IR modules are available.
Key changes:
- Create a dummy function ("thinlto_remark_dummy") in a private Module
within the LTO class to provide the necessary Function context for
OptimizationRemarkEmitter.
- Update MemProfContextDisambiguation to use a callback for remark
emission, allowing it to report hinted sizes and other diagnostics
during the thin link.
- Ensure the dummy module and function are safely cleaned up at the end
of the LTO session via the LTO::cleanup mechanism.
Centralize the setup and finalization of optimization remarks for LTO
link actions
outside of the pass pipeline. This ensures that remarks are correctly
captured across
all exit paths from the LTO pipeline.
The motivation for this refactoring is to provide a cleaner and more
robust
interface for managing diagnostics, and to enable easier remark emission
during
the thin link phase in a follow-on PR.
Key changes:
- Added a setupOptimizationRemarks on the LTO class. Call it from
LTO::run
and remove the existing setup from runRegularLTO.
- Added a unified diagnostic API to the LTO class (emitRemark) and use
that
in linkRegularLTO.
- Centralize the finalizeOptimizationRemarks call in LTO::cleanup, which
is already
invoked by LTO::run's scope_exit handler.
Thinlink may decide some symbols with internal linkage should get promoted to external. Such a symbol, when being imported, would have its name changed by appending a suffix (`.llvm.<a hash>`) to avoid collisions - since internal linkage symbols have non-unique names.
Later, still during Thinlink, in `thinLTOResolvePrevailingGUID`, the fact that this symbol was promoted is not considered and we set its linkage to `AvailableExternally`(when reading `thinLTOResolvePrevailingGUID`, note that "prevailing-ness" is not a concept that the original symbol would have participated in)
This should result in a final (native) link error, because the symbol's definition may be elided. But we get lucky: in the post-thinlink backend, during import, in `llvm::thinLTOFinalizeInModule`, after this symbol's name was changed and its linkage also changed to `External` (see `FunctionImportGlobalProcessing::processGlobalForThinLTO`), we try to find it in the `DefinedGlobals`, fail (because its guid is computed from its changed name)) and leave its linkage as-is. Which happens to be correct.
This patch makes this outcome intentional rather than accidental. It becomes critical once we land [this RFC](https://discourse.llvm.org/t/rfc-keep-globalvalue-guids-stable/84801).
As a side-benefit, the extra attribute propagations that weren't happening in `llvm::thinLTOFinalizeInModule` now do.
This change performs a small set of NFC refactors to improve clarity. In
particular, we make it clear that the responsibilities of this class now
extend beyond its original archive-handling role.
Returns uint64_t to simplify callers. The goal is eventually replace
getValueType with this query, which should return the known minimum
reference-able size, as provided (instead of a Type) during create.
Additionally the common isSized query would be replaced with an
isExactKnownSize query to test if that size is an exact definition.
This resolves the FIXME in IRSymtab and cleans up the semantics of the
IRSymtab. The list of preserved symbols really shouldn't be seen as a
property of the IR symbol table, since it's an LTO-specific concern, and
it's very tenuous to claim that this information is actually present in
the bitcode file to be exposed through its symbol table.
Instead, this PR moves this logic into LTO's view of the symbol, which
allows consumers to determine preserved-ness themselves. This was broken
out of #164916; this prevents that PR from introducing a circular
dependency, but it still seems like an independently good idea by virtue
of the above.
Add time-trace scopes to the DTLTO-specific input-handling code to
improve observability and debugging.
These scopes are tested via LLD, as the primary purpose of this code is
to support member files of non-thin archives as DTLTO inputs.
`llvm-lto2` does not currently support archives. Adding archive support
to `llvm-lto2` solely for testing these scopes does not appear to be
worthwhile.
As part of this change, the deletion of temporary DTLTO input files has
been moved. Cleanup now occurs after LTO has completed, rather than
during destruction of the LTO object. This is required since by the time
the LTO object is destroyed, time-traces have already been finalized, so
no additional trace data can be recorded.
Recording time-trace data for temporary file deletion is important, as
this has been a source of performance issues in the past and an area
where we expect to make further performance improvements if supported by
the data.
SIE internal tracker: TOOLCHAIN-21021
This change fixes two issues when processing multi-module bitcode files
in DTLTO:
1. The DTLTO archive handling code incorrectly uses
getSingleBitcodeModule(), which asserts when the bitcode file contains
more than one module.
2. The temporary file containing the contents of an input archive member
was not emitted for multi-module bitcode files. This was due to
incorrect logic for recording whether a bitcode input contains any
ThinLTO modules. In a typical multi-module bitcode file, the first
module is a ThinLTO module while a subsequent auxiliary module is
non-ThinLTO. When modules are processed in order, the auxiliary module
causes the entire bitcode file to be classified as non-ThinLTO, and the
archive-member emission logic then incorrectly skips it.
In addition, this patch adds a test that verifies that multi-module
bitcode files can be successfully linked with DTLTO. The test reproduces
both issues as they existed prior to this change.
SIE Tracker: TOOLCHAIN-21008
This patch implements support for handling archive members in DTLTO.
Unlike ThinLTO, where archive members are passed as in-memory buffers,
DTLTO requires archive members to be materialized as individual files on
the filesystem.
This is necessary because DTLTO invokes clang externally, which expects
file-based inputs.
To support this, this implementation identifies archive members among
the input files,
saves them to the filesystem, and updates their module_id to match their
file paths.
The primary goal of this patch is to add a comprehensive set of DTLTO time traces.
I also have implemented support for time traces in llvm-lto2, to allow for adding a test via Lit.
This patch implements DTLTO cache. DTLTO cache is implemented the same
way as ThinLTO cache. In fact the same class Cache is used for both of
them.
Because parameters for codegen are different for DTLTO and ThinLTO
(DTLTO codegen is done by invoking clang and its codegen parameters are
not fully synchronized with codegen parameters used by LTO backend).
The object files generated by DTLTO and ThinLTO might be different and
shouldn't be mixed. If ThinLTO and DTLTO share the same cache
directory, the cache file won't interfere with each other.
I added a couple of test files in cross-project-test/dtlto directory,
but if more tests are required for initial implementation, I could add
them.
Introduce a new class for the TargetLowering usage. This tracks the
subtarget specific lowering decisions for which libcall to use.
RuntimeLibcallsInfo is a module level property, which may have multiple
implementations of a particular libcall available. This attempts to be
a minimum boilerplate patch to introduce the new concept.
In the future we should have a tablegen way of selecting which
implementations should be used for a subtarget. Currently we
do have some conflicting implementations added, it just happens
to work out that the default cases to prefer is alphabetically
first (plus some of these still are using manual overrides
in TargetLowering constructors).
Avoid the construction of `GUID` when not required. This improves the
performance of a LLD `--thinlto-index-only` link of `clang` by ~4-5% on
both Windows and Linux.
Replace a loop over all summary copies with a simple check for a single
externally available copy of a symbol. The usage of this result has
changed since it was added and we now only need to know if there is a
single one.
We could inadvertently create new entries in the PrevailingModuleForGUID
map during lookup, which was always using operator[]. In most cases we
will have one for external symbols, but not in cases where the
prevailing copy is in a native object. Or if this happened to be looked
up for a local.
Make the map private and create and use accessors.
Add DTLTO linker option `--thinlto-remote-compiler-prepend-arg` to
enable support for the multi-call LLVM driver that requires an
additional option to specify the subcommand, e.g. "llvm clang ...".
Fixes https://github.com/llvm/llvm-project/issues/159125.
Add a flag to the GlobalValueSummaryInfo indicating whether the
associated SummaryList (all summaries with the same GUID) contains any
summaries with local linkage. This flag is set when building the index,
so it is associated with the original linkage type before
internalization and promotion. Consumers should check the
withInternalizeAndPromote() flag on the index before using it.
In most cases we expect a 1-1 mapping between a GUID and a summary with
local linkage, because for locals the GUID is computed from the hash of
"modulepath;name". However, there can be multiple locals with the same
GUID if translation units are not compiled with enough path. And in rare
but theoretically possible cases, there can be hash collisions on the
underlying MD5 computation. So to be safe when looking for local
summaries, analyses currently look through all summaries in the list.
These lists can be extremely long in the case of large binaries with
template function defs in widely used headers (i.e. linkonce_odr).
A follow on change will use this flag to reduce ThinLTO analysis time in
WPD by 5-6% for a large target (details in PR164046 which will be
reworked to use this flag).
Note that in the past we have tried to keep bits related to the GUID in
the ValueInfo (which has a pointer to the associated
GlobalValueSummaryInfo), via its PointerIntPair. However, we are out of
bits there. This change does add a byte to every GlobalValueSummaryInfo
instance, which I measured as a little under 0.90% overhead in a large
target. However, it enables adding 7 bits of other per-GUID flags in the
future without adding more overhead. Note that it was lower overhead to
add this to the GlobalValueSummaryInfo than the ValueInfo, which tends
to be copied into other maps.
In preparation for a follow on change that will require checking every
time a new summary is added to the SummaryList for a GUID, make the
SummaryList private and require all accesses to go through one of two
new interfaces. Most changes are to access the list via the read only
getSummaryList() method, and the few that add new summaries (e.g. while
building the combined summary) use the new addSummary() method.
In preparation for a follow on fix that removes these attributes and
metadata in non-LTO pipelines, convert updateMemProfAttributes to a new
MemProfRemoveInfo pass that executes at the start of the LTO backend
pass pipelines when we don't have an index indicating that we linked
with a library support hot cold operator new.
This is largely NFC from an end user perspective but changes where the
removal can be observed, hence the test updates.
A follow on change will use the new pass for non-LTO pipelines (for
cases when the bitcode is initially matched with memprof data but we
decide to complete the compile without LTO).
There's a pattern throughout LLVM of cl::opts being exported. That in
itself is probably a bit unfortunate, but what's especially bad about it
is that a lot of those symbols are in the global namespace. Move them
into the llvm namespace.
While doing this, I noticed some other variables in the global namespace
and moved them as well.
Currently there are two serialization modes for bitstream Remarks:
standalone and separate. The separate mode splits remark metadata (e.g.
the string table) from actual remark data. The metadata is written into
the object file by the AsmPrinter, while the remark data is stored in a
separate remarks file. This means we can't use bitstream remarks with
tools like opt that don't generate an object file. Also, it is confusing
to post-process bitstream remarks files, because only the standalone
files can be read by llvm-remarkutil. We always need to use dsymutil
to convert the separate files to standalone files, which only works for
MachO. It is not possible for clang/opt to directly emit bitstream
remark files in standalone mode, because the string table can only be
serialized after all remarks were emitted.
Therefore, this change completely removes the separate serialization
mode. Instead, the remark string table is now always written to the end
of the remarks file. This requires us to tell the serializer when to
finalize remark serialization. This automatically happens when the
serializer goes out of scope. However, often the remark file goes out of
scope before the serializer is destroyed. To diagnose this, I have added
an assert to alert users that they need to explicitly call
finalizeLLVMOptimizationRemarks.
This change paves the way for further improvements to the remark
infrastructure, including more tooling (e.g. #159784), size optimizations
for bitstream remarks, and more.
Pull Request: https://github.com/llvm/llvm-project/pull/156715
In order to better see what's going on during ThinLTO linking, this PR
adds more profile tags when using `--time-trace` on a `lld-link.exe`
invocation.
After PR, linking `clang.exe`:
<img width="3839" height="2026" alt="Capture d’écran 2025-09-02 082021"
src="https://github.com/user-attachments/assets/bf0c85ba-2f85-4bbf-a5c1-800039b56910"
/>
Linking a custom (Unreal Engine game) binary gives a completly
different picture, probably because of using Unity files, and the sheer
amount of input files (here, providing over 60 GB of .OBJs/.LIBs).
<img width="1940" height="1008" alt="Capture d’écran 2025-09-02 102048"
src="https://github.com/user-attachments/assets/60b28630-7995-45ce-9e8c-13f3cb5312e0"
/>
…210)"
This reverts commit 9a14b1d254a43dc0d4445c3ffa3d393bca007ba3.
Revert "RuntimeLibcalls: Return StringRef for libcall names (#153209)"
This reverts commit cb1228fbd535b8f9fe78505a15292b0ba23b17de.
Revert "TableGen: Emit statically generated hash table for runtime
libcalls (#150192)"
This reverts commit 769a9058c8d04fc920994f6a5bbb03c8a4fbcd05.
Reverted three changes because of a CMake error while building llvm-nm
as reported in the following PR:
https://github.com/llvm/llvm-project/pull/150192#issuecomment-3192223073
With the current aliases metadata we lose information about which groups
of aliases survive symbol resolution. This causes various problems such
as #150075 where symbol resolution breaks the link between alias groups.
In this redesign of the aliases metadata, we stop representing the
individual aliases in !aliases. Instead, the individual aliases are
represented in !cfi.functions in the same way as functions, and the
alias groups (i.e. groups of symbols with the same address) are stored
in !aliases. At symbol resolution time, we filter out all non-prevailing
members of !aliases; the resulting set is used by LowerTypeTests to
recreate the aliases.
With this change it is now possible for a jump table entry to refer
to an alias in one of the ThinLTO object files (e.g. if a function is
non-prevailing but its alias is prevailing), so instead of deleting them,
rename them with the ".cfi" suffix.
Fixes#150070.
Fixes#150075.
Reviewers: teresajohnson, vitalybuka
Reviewed By: vitalybuka
Pull Request: https://github.com/llvm/llvm-project/pull/150690
Work towards separating the ABI existence of libcalls vs. the
lowering selection. Set libcall selection through enums, rather
than through raw string names.
This flag was used to let us incrementally introduce debug records
into LLVM, however everything is now using records. It serves no
purpose now, so delete it.
Start removing debug intrinsics support -- starting with the flag that
controls production of their replacement, debug records. This patch
removes the command-line-flag and with it the ability to switch back to
intrinsics. The module / function / block level "IsNewDbgInfoFormat"
flags get hardcoded to true, I'll to incrementally remove things that
depend on those flags.
## Purpose
This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch annotates the `llvm/LTO` library. These
annotations currently have no meaningful impact on the LLVM build;
however, they are a prerequisite to support an LLVM Windows DLL (shared
library) build.
## Background
This effort is tracked in #109483. Additional context is provided in
[this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and documentation for `LLVM_ABI` and related annotations is found in the
LLVM repo
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).
The bulk of these changes were generated automatically using the
[Interface Definition Scanner (IDS)](https://github.com/compnerd/ids)
tool, followed formatting with `git clang-format`.
The following manual adjustments were also applied after running IDS on
Linux:
- Add `LLVM_ABI` to a small number of symbols that require export but
are not declared in headers
## Validation
Local builds and tests to validate cross-platform compatibility. This
included llvm, clang, and lldb on the following configurations:
- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang
These are identified by misc-include-cleaner. I've filtered out those
that break builds. Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.
This patch adds initial support for Integrated Distributed ThinLTO
(DTLTO) in LLVM, which manages distribution internally during the
traditional link step. This enables compatibility with any build
system that supports in-process ThinLTO. In contrast, existing
approaches to distributed ThinLTO, which split the thin-link
(--thinlto-index-only), backend compilation, and final link into
separate steps, require build system support, e.g. Bazel.
This patch implements the core DTLTO mechanism, which enables
delegation of ThinLTO backend jobs to an external process (the
distributor). The distributor can then manage job distribution through
systems like Incredibuild. A generic JSON interface is used to
communicate with the distributor, allowing for the creation of new
distributors (and thus integration with different distribution
systems) without modifying LLVM.
Please see llvm/docs/dtlto.rst for more details.
RFC: https://discourse.llvm.org/t/rfc-integrated-distributed-thinlto/69641
Design Review: https://github.com/llvm/llvm-project/pull/126654
See https://discourse.llvm.org/t/rfc-keep-globalvalue-guids-stable/84801
for context.
This is a non-functional change which just changes the interface of
GlobalValue, in preparation for future functional changes. This part
touches a fair few users, so is split out for ease of review. Future
changes to the GlobalValue implementation can then be focused purely on
that class.
This does the following:
* Rename GlobalValue::getGUID(StringRef) to
getGUIDAssumingExternalLinkage. This is simply making explicit at the
callsite what is currently implicit.
* Where possible, migrate users to directly calling getGUID on a
GlobalValue instance.
* Otherwise, where possible, have them call the newly renamed
getGUIDAssumingExternalLinkage, to make the assumption explicit.
There are a few cases where neither of the above are possible, as the
caller saves and reconstructs the necessary information to compute the
GUID themselves. We want to migrate these callers eventually, but for
this first step we leave them be.
On one hand, we intend to force import all functions when the option is
enabled.
On the other hand, we currently drop definitions of some functions and
convert
them to declarations, which contradicts this intent.
With this PR, functions will no longer be converted to declarations when
`force-import-all` is enabled.
DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range. This patch uses insert_range with
iterator ranges. For each case, I've verified that foos is defined as
make_range(foo_begin(), foo_end()) or in a similar manner.
Before this patch, whole program devirtualization is suppressed on a
class if any superclass is visible to regular object files, by recording
the class GUID in `VisibleToRegularObjSymbols`.
This patch suppresses whole program devirtualization on a class if the
LTO unit doesn't have the prevailing definition of this class (e.g., the
prevailing definition is in a shared library)
Implementation summaries:
1. In llvm/lib/LTO/LTO.cpp, `IsVisibleToRegularObj` is updated to look
at the global resolution's `IsPrevailing` bit for ThinLTO and
regularLTO.
2. In llvm/tools/llvm-lto2/llvm-lto2.cpp,
- three command line options are added so `llvm-lto2` can override
`Conf.HasWholeProgramVisibility`, `Conf.ValidateAllVtablesHaveTypeInfos`
and `Conf.AllVtablesHaveTypeInfos`.
The test case is reduced from a small C++ program (main.cc, lib.cc/h
pasted below in [1]). To reproduce the program failure without this
patch, compile lib.cc into a shared library, and provide it to a ThinLTO
build of main.cc (commands are pasted in [2]).
[1]
* lib.h
```
#include <cstdio>
class Derived {
public:
void dispatch();
virtual void print();
virtual void sum();
};
void Derived::dispatch() {
static_cast<Derived*>(this)->print();
static_cast<Derived*>(this)->sum();
}
void Derived::sum() {
printf("Derived::sum\n");
}
__attribute__((noinline)) void* create(int i);
__attribute__((noinline)) void* getPtr(int i);
```
* lib.cc
```
#include "lib.h"
#include <cstdio>
#include <iostream>
class Derived2 : public Derived {
public:
void print() override {
printf("DerivedSharedLib\n");
}
void sum() override {
printf("DerivedSharedLib::sum\n");
}
};
void Derived::print() {
printf("Derived\n");
}
__attribute__((noinline)) void* create(int i) {
if (i & 1)
return new Derived2();
return new Derived();
}
```
* main.cc
```
cat main.cc
#include "lib.h"
class DerivedN : public Derived {
public:
};
__attribute__((noinline)) void* getPtr(int x) {
return new DerivedN();
}
int main() {
Derived*b = static_cast<Derived*>(create(201));
b->dispatch();
delete b;
Derived* a = static_cast<Derived*>(getPtr(202));
a->dispatch();
delete a;
return 0;
}
```
[2]
```
# compile lib.o in a shared library.
$ ./bin/clang++ -O2 -fPIC -c lib.cc -o lib.o
$ ./bin/clang++ -shared -o libdata.so lib.o
# Provide the shared library in `-ldata`
$ ./bin/clang++ -v -g -ldata --save-temps -fno-discard-value-names -Wl,-mllvm,-print-before=wholeprogramdevirt -Wl,-mllvm,-wholeprogramdevirt-check=trap -Rpass=wholeprogramdevirt -Wl,--lto-whole-program-visibility -Wl,--lto-validate-all-vtables-have-type-infos -mllvm -disable-icp=true -Wl,-mllvm,-disable-icp=false -flto=thin -fwhole-program-vtables -fno-split-lto-unit -fuse-ld=lld main.cc -L . -o main >/tmp/wholeprogramdevirt.ir 2>&1
# Run the program hits a segmentation fault with `-Wl,-mllvm,-wholeprogramdevirt-check=trap`
$ LD_LIBRARY_PATH=. ./main
DerivedSharedLib
Trace/breakpoint trap (core dumped)
```
