Currently, if multiple external weak symbols are defined at the same
address in an object file (e.g., by using the .set assembler directive
to alias them to a single weak variable), ld64.lld treats them as a
single unit. When any one of these symbols is overridden by a strong
definition, all of the original weak symbols resolve to the strong
definition.
This patch changes the behavior in `transplantSymbolsAtOffset`. When a
weak symbol is being replaced by a strong one, only non-external (local)
symbols at the same offset are moved to the new symbol's section. Other
*external* symbols are no longer transplanted.
This allows each external weak symbol to be overridden independently.
This behavior is consistent with Apple's ld-classic, but diverges from
ld-prime in one case, as noted on
https://github.com/llvm/llvm-project/issues/167262 (this discrepancy has
recently been reported to Apple).
### Backward Compatibility
This change alters linker behavior for a specific scenario. The creation
of multiple external weak symbols aliased to the same address via
assembler directives is primarily an advanced technique. It's unlikely
that existing builds rely on the current behavior of all aliases being
overridden together.
If there are concerns, this could be put behind a linker option, but the
new default seems more correct, less surprising, and is consistent with
ld-classic.
### Testing
The new lit test `test/MachO/weak-alias-override.s` verifies this
behavior using llvm-nm.
Fixes#167262
The existing comparison does not insert symbols in the intended place.
Closes#120559.
---------
Co-authored-by: Bjorn Pettersson <bjorn.a.pettersson@ericsson.com>
Currently, when moving symbols from one `InputSection` to another (like
in ICF) we directly update the symbol's `isec`, `unwindEntry` and
`size`. By doing this we lose the original information. This information
will be needed in a future change. Since when moving symbols we always
set the symbol's `wasCoalesced` and `isec-> replacement`, we can just
use this info to conditionally get the information we need at access
time.
Previously, `makeSyntheticInputSection` would create a new
`ConcatInputSection` without setting `live` explicitly for it. Without
`-dead_strip` this would be OK since `live` would default to `true`.
However, with `-dead_strip`, `live` would default to false, and it would
remain set to `false`.
This hasn't resulted in any issues so far since no code paths that
exposed this issue were present.
However a recent change - ObjC relative method lists
(https://github.com/llvm/llvm-project/pull/86231) exposes this issue by
creating relocations to the `SyntheticInputSection`.
When these relocations are attempted to be written, this ends up with a
crash(assert), since the `SyntheticInputSection` they refer to is marked
as dead (`live` = `false`).
With this change, we set the correct behavior - `live` will always be
`true`. We add a test case that before this change would trigger an
assert in the linker.
LLD resolves symbols before performing LTO compilation, assuming that the symbols in question are resolved by the resulting object files from LTO. However, there is a scenario where the prevailing symbols might be resolved incorrectly due to specific assembly symbols not appearing in the symbol table of the bitcode. This patch deals with such a scenario by generating an error instead of silently allowing a mis-linkage.
If a prevailing symbol is resolved through post-loaded archives via LC linker options, a warning will now be issued.
Reviewed By: #lld-macho, thevinster
Differential Revision: https://reviews.llvm.org/D158003
We never really supported 32-bit ARM arch entirely, and partial support was added for
very specific features. Regardless, it fails to even link the most basic applications that at
this point, it might be better to move this arch as unsupported. Given that Apple will be
moving towards arm64 long term, I don't see any reason for anyone to invest time in
supporting this either, and for those who still need it should use apple's ld64 linker.
Fixes#62691
Reviewed By: #lld-macho, int3
Differential Revision: https://reviews.llvm.org/D150544
This supersedes {D139069}. In some ways we are now closer to ld64's
behavior: we previously only did this coalescing for private-label
symbols, but now we do it for all locals, just like ld64. However, we no
longer generate weak binds when a local alias to a weak symbol is
referenced. This is merely for implementation simplicity; it's not clear
to me that any real-world programs depend on us emulating this behavior.
The problem with the previous approach is that we ended up with
duplicate references to the same symbol instance in our InputFiles,
which translated into duplicate symbols in our output. While we could
work around that problem by performing a dedup step before emitting the
symbol table, it seems cleaner to not generate duplicate references in
the first place.
Numbers for chromium_framework on my 16 Core Intel Mac Pro:
base diff difference (95% CI)
sys_time 2.243 ± 0.093 2.231 ± 0.066 [ -2.5% .. +1.4%]
user_time 6.529 ± 0.087 6.080 ± 0.050 [ -7.5% .. -6.3%]
wall_time 6.928 ± 0.175 6.474 ± 0.112 [ -7.7% .. -5.4%]
samples 26 31
Yep, that's a massive win... because it turns out that {D140606} and
{D139069} caused a regression (of about the same size.) I just didn't
think to measure them back then. I'm guessing all the extra symbols we
have been emitting did not help perf at all...
Reviewed By: lgrey
Differential Revision: https://reviews.llvm.org/D145455
... instead of mapping them to the intermediate object file.
This matches ld64.
Reviewed By: #lld-macho, Roger
Differential Revision: https://reviews.llvm.org/D136380
{D116279}, in addition to adding support for other demanglers, also
factored out some of the demangling logic. However, I don't think the
abstraction really carries its weight -- after {D135942}, only the ELF
and WASM backends call it with anything other than a non-constant
`shouldDemangle` argument. The COFF and Mach-O backends were already
doing the should-demangle check before calling `demangle()`.
Reviewed By: MaskRay, #lld-macho
Differential Revision: https://reviews.llvm.org/D135943
The only difference in the reland is that the loop at the top of
getAlternativeSpelling() now calls dyn_cast_or_null() instead
of dyn_cast() -- a file's symbols list can contain null entries.
The test for this might be slightly involved, so I'll land it in
a follow-up, to make the reland similar to the original commit.
Originally reviewed at:
Differential Revision: https://reviews.llvm.org/D135038
This reverts commit 317b5582b813c51d1fb6723fd44b227b7f274bc7.
Makes things look more similar to the ELF port, and removes some
slightly deep nesting.
No behavior change.
Differential Revision: https://reviews.llvm.org/D135032
Builds that error out on duplicate symbols can still succeed if the symbols
will be dead stripped. Currently, this is the current behavior in ld64.
https://github.com/apple-oss-distributions/ld64/blob/main/src/ld/Resolver.cpp#L2018.
In order to provide an easier to path for adoption, introduce a new flag that will
retain compatibility with ld64's behavior (similar to `--deduplicate-literals`). This is
turned off by default since we do not encourage this behavior in the linker.
Reviewed By: #lld-macho, thakis, int3
Differential Revision: https://reviews.llvm.org/D134794
This is similar to the `-alias` CLI option, but it gives finer-grained
control in that it allows the aliased symbols to be treated as private
externs.
While working on this, I realized that our `-alias` handling did not
cover the cases where the aliased symbol is a common or dylib symbol,
nor the case where we have an undefined that gets treated specially and
converted to a defined later on. My N_INDR handling neglects this too
for now; I've added checks and TODO messages for these.
`N_INDR` symbols cropped up as part of our attempt to link swift-stdlib.
Reviewed By: #lld-macho, thakis, thevinster
Differential Revision: https://reviews.llvm.org/D133825
This is similar to the `-alias` CLI option, but it gives finer-grained
control in that it allows the aliased symbols to be treated as private
externs.
While working on this, I realized that our `-alias` handling did not
cover the cases where the aliased symbol is a common or dylib symbol,
nor the case where we have an undefined that gets treated specially and
converted to a defined later on. My N_INDR handling neglects this too
for now; I've added checks and TODO messages for these.
`N_INDR` symbols cropped up as part of our attempt to link swift-stdlib.
Reviewed By: #lld-macho, thakis, thevinster
Differential Revision: https://reviews.llvm.org/D133825
This creates a symbol alias similar to --defsym in the elf linker. This
is used by swiftpm for all executables, so it's useful to support. This
doesn't implement -alias_list but that could be done pretty easily as
needed.
Differential Revision: https://reviews.llvm.org/D129938
This fixes https://github.com/llvm/llvm-project/issues/56238. ld64.lld currently does not generate __dof section in Mach-O, and -no_dtrace_dof option is on by default. However when there are user-defined dtrace symbols, ld64.lld will treat them as undefined symbols, which causes the linking to fail because lld cannot find their definitions. This patch allows ld64.lld to rewrite the instructions calling dtrace symbols to instructions like nop as what ld64 does; therefore, when encountered with user-provided dtrace probes, the linking can still succeed.
I'm not sure whether support for dtrace is expected in lld, so for now I didn't add codes to make lld emit __dof section like ld64, and only made it possible to link with dtrace symbols provided. If this feature is needed, I can add that part in Dtrace.cpp & Dtrace.h.
Reviewed By: int3, #lld-macho
Differential Revision: https://reviews.llvm.org/D129062
Similarly to how undefined symbol diagnostics were changed in D128184,
we now show where in the source file duplicate symbols are defined at:
ld64.lld: error: duplicate symbol: _foo
>> defined in bar.c:42
>> /path/to/bar.o
>> defined in baz.c:1
>> /path/to/libbaz.a(baz.o)
For objects that don't contain DWARF data, the format is unchanged.
A slight difference to undefined symbol diagnostics is that we don't
print the name of the symbol on the third line, as it's already
contained on the first line.
Differential Revision: https://reviews.llvm.org/D128425
The error used to look like this:
ld64.lld: error: undefined symbol: _foo
>>> referenced by /path/to/bar.o:(symbol _baz+0x4)
If DWARF line information is available, we now show where in the source
the references are coming from:
ld64.lld: error: unreferenced symbol: _foo
>>> referenced by: bar.cpp:42 (/path/to/bar.cpp:42)
>>> /path/to/bar.o:(symbol _baz+0x4)
The reland is identical to the first time this landed. The fix was in D128294.
This reverts commit 0cc7ad417585b3185c32e395cc5e6cf082a347af.
Differential Revision: https://reviews.llvm.org/D128184
The error used to look like this:
ld64.lld: error: undefined symbol: _foo
>>> referenced by /path/to/bar.o:(symbol _baz+0x4)
If DWARF line information is available, we now show where in the source
the references are coming from:
ld64.lld: error: unreferenced symbol: _foo
>>> referenced by: bar.cpp:42 (/path/to/bar.cpp:42)
>>> /path/to/bar.o:(symbol _baz+0x4)
Differential Revision: https://reviews.llvm.org/D128184
ld64.lld used to print the "undefined symbol" line for each reference to
an undefined symbol previously:
ld64.lld: error: undefined symbol: _foo
>>> referenced by /path/to/bar.o:(symbol _baz+0x0)
ld64.lld: error: undefined symbol: _foo
>>> referenced by /path/to/bar.o:(symbol _quux+0x1)
Now they are deduplicated:
ld64.lld: error: undefined symbol: _foo
>>> referenced by /path/to/bar.o:(symbol _baz+0x0)
>>> referenced by /path/to/bar.o:(symbol _quux+0x1)
As with the other lld ports, only the first 3 references are printed.
Differential Revision: https://reviews.llvm.org/D127753
The error used to look like this:
ld64.lld: error: undefined symbol: _foo
>>> referenced by /path/to/bar.o
Now it displays the name of the function that contains the undefined
reference as well:
ld64.lld: error: undefined symbol: _foo
>>> referenced by /path/to/bar.o:(symbol _baz+0x4)
Differential Revision: https://reviews.llvm.org/D127696
{D123302} got me looking deeper at `includeInSymtab`. I thought it was a
little odd that there were excluded (live) symbols for which
`includeInSymtab` was false; we shouldn't have so many different ways to
exclude a symbol. As such, this diff makes the `L`-prefixed-symbol
exclusion code use `includeInSymtab` too. (Note that as part of our
support for `__eh_frame`, we will also be excluding all `__eh_frame`
symbols from the symtab in a future diff.)
Another thing I noticed is that the `emitStabs` code never has to deal
with excluded symbols because `SymtabSection::finalize()` already
filters them out. As such, I've updated the comments and asserts from
{D123302} to reflect this.
Reviewed By: #lld-macho, thakis
Differential Revision: https://reviews.llvm.org/D123433
I was wondering if SymtabSection::emitStabs() should check
defined->includeInSymtab. Add asserts and comments explaining why that's not
necessary.
No behavior change.
Differential Revision: https://reviews.llvm.org/D123302
All references to interposable symbols can be redirected at runtime to
point to a different symbol definition (with the same name). For
example, if both dylib A and B define symbol _foo, and we load A before
B at runtime, then all references to _foo within dylib B will point to
the definition in dylib A.
ld64 makes all extern symbols interposable when linking with
`-flat_namespace`.
TODO 1: Support `-interposable` and `-interposable_list`, which should
just be a matter of parsing those CLI flags and setting the
`Defined::interposable` bit.
TODO 2: Set Reloc::FinalDefinitionInLinkageUnit correctly with this info
(we are currently not setting it at all, so we're erring on the
conservative side, but we should help the LTO backend generate more
optimal code.)
Reviewed By: modimo, MaskRay
Differential Revision: https://reviews.llvm.org/D119294
Earlier in LLD's evolution, I tried to create the illusion that
subsections were indistinguishable from "top-level" sections. Thus, even
though the subsections shared many common field values, I hid those
common values away in a private Shared struct (see D105305). More
recently, however, @gkm added a public `Section` struct in D113241 that
served as an explicit way to store values that are common to an entire
set of subsections (aka InputSections). Now that we have another "common
value" struct, `Shared` has been rendered redundant. All its fields can
be moved into `Section` instead, and the pointer to `Shared` can be replaced
with a pointer to `Section`.
This `Section` pointer also has the advantage of letting us inspect other
subsections easily, simplifying the implementation of {D118798}.
P.S. I do think that having both `Section` and `InputSection` makes for
a slightly confusing naming scheme. I considered renaming `InputSection`
to `Subsection`, but that would break the symmetry with `OutputSection`.
It would also make us deviate from LLD-ELF's naming scheme.
This change is perf-neutral on my 3.2 GHz 16-Core Intel Xeon W machine:
base diff difference (95% CI)
sys_time 1.258 ± 0.031 1.248 ± 0.023 [ -1.6% .. +0.1%]
user_time 3.659 ± 0.047 3.658 ± 0.041 [ -0.5% .. +0.4%]
wall_time 4.640 ± 0.085 4.625 ± 0.063 [ -1.0% .. +0.3%]
samples 49 61
There's also no stat sig change in RSS (as measured by `time -l`):
base diff difference (95% CI)
time 998038627.097 ± 13567305.958 1003327715.556 ± 15210451.236 [ -0.2% .. +1.2%]
samples 31 36
Reviewed By: #lld-macho, oontvoo
Differential Revision: https://reviews.llvm.org/D118797
In ld.lld, when an ObjFile/BitcodeFile is read in --start-lib state, the file is
given archive semantics. --end-lib closes the previous --start-lib. A build
system can use this feature as an alternative to archives. This patch ports
the feature to lld-macho.
--start-lib and --end-lib are positional, unlike usual ld64 options.
I think the slight drawback does not matter as (a) reusing option names
make build systems convenient (b) `--start-lib a.o b.o --end-lib` conveys more
information than an alternative design: `-objlib a.o -objlib b.o` because
--start-lib makes it clear which objects are in the same conceptual archive.
This provides flexibility (c) `-objlib`/`-filelist` interaction may be weird.
Close https://github.com/llvm/llvm-project/issues/52931
Reviewed By: #lld-macho, Jez Ng, oontvoo
Differential Revision: https://reviews.llvm.org/D116913
D116913 will add LazyObject. Rename LazySymbol to LazyArchive to avoid confusion
and mirror ELF.
Reviewed By: #lld-macho, Jez Ng
Differential Revision: https://reviews.llvm.org/D116914
We were fetching archive symbols too eagerly, bloating binary size as well as
just screwing up binaries that expected to look up certain symbols only at
runtime.
Reviewed By: #lld-macho, oontvoo
Differential Revision: https://reviews.llvm.org/D115092
autohide symbols behaves similarly to private_extern symbols.
However, LD64 allows exporting autohide symbols. LLD currently does not.
This patch allows LLD to export them.
Differential Revision: https://reviews.llvm.org/D113167
PR/52372
Differential Revision: https://reviews.llvm.org/D112977
New changes:
- use llvm-otool instead of `otool` which doesn't in exist on non-OSX platforms
- add llvm-otool to the set of tools used by test so that the bot will use the <build_dir>/bin/llvm-otool instead of the unqualified `llvm-otool` (which may not exist)
- update tests since the latest (TOT) llvm-otool prints a space between two bytes and the old one doesn't.
Compact unwind entries (CUEs) contain pointers to their respective
function symbols. However, during the link process, it's far more useful
to have pointers from the function symbol to the CUE than vice versa.
This diff adds that pointer in the form of `Defined::compactUnwind`.
In particular, when doing dead-stripping, we want to mark CUEs live when
their function symbol is live; and when doing ICF, we want to dedup
sections iff the symbols in that section have identical CUEs. In both
cases, we want to be able to locate the symbols within a given section,
as well as locate the CUEs belonging to those symbols. So this diff also
adds `InputSection::symbols`.
The ultimate goal of this refactor is to have ICF support dedup'ing
functions with unwind info, but that will be handled in subsequent
diffs. This diff focuses on simplifying `-dead_strip` --
`findFunctionsWithUnwindInfo` is no longer necessary, and
`Defined::isLive()` is now a lot simpler. Moreover, UnwindInfoSection no
longer has to check for dead CUEs -- we simply avoid adding them in the
first place.
Additionally, we now support stripping of dead LSDAs, which follows
quite naturally since `markLive()` can now reach them via the CUEs.
Reviewed By: #lld-macho, gkm
Differential Revision: https://reviews.llvm.org/D109944
These symbols are somewhat interesting in that they create non-existing
segments, which as far as I know is the only way to create segments
that don't contain any sections.
Final part of part of PR50760. Like D106629, but for segments instead
of sections. I'm not aware of anything that needs this in practice.
Differential Revision: https://reviews.llvm.org/D106767
With this, libclang_rt.profile_osx.a can be linked, that is coverage
and PGO-instrumented builds should now work with lld.
section$start and section$end symbols can create non-existing sections.
They're also undefined symbols that are only magic if there isn't a
regular symbol with their name, which means the need to be handled
in treatUndefined() instead of just looping over all existing
sections and adding start and end symbols like the ELF port does.
To represent the actual symbols, this uses absolute symbols that
get their value updated once an output section is layed out.
segment$start and segment$end are still missing for now, but they produce a
nicer error message after this patch.
Main part of PR50760.
Differential Revision: https://reviews.llvm.org/D106629
