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.
The MachO format supports relative offsets for ObjC method lists. This
support is present already in ld64. With this change we implement this
support in lld also.
Relative method lists can be identified by a specific flag (0x80000000)
in the method list header. When this flag is present, the method list
will contain 32-bit relative offsets to the current Program Counter
(PC), instead of absolute pointers.
Additionally, when relative method lists are used, the offset to the
selector name will now be relative and point to the selector reference
(selref) instead of the name itself.
In a previous PR: https://github.com/llvm/llvm-project/pull/83878, the
intent was to make no functional changes, just refactor out the code for
reuse.
However, by creating `ObjCSelRefsSection` as a `SyntheticSection` - this
slightly changed the functionality of the application as the
`SyntheticSection` constructor registers the `SyntheticSection` as a
functional one - with an associated `SyntheticInputSection`.
With this change we remove this unintended consequence by making the
code not use a `SyntheticSection` as base, but just by having it be a
static helper.
Before this change, after `InputSection` objects are created, they need
to be added to the appropriate container for tracking.
The logic for selecting the appropriate container lives in `Driver.cpp`
/ `gatherInputSections`, where the `InputSection` is added to the
matching container depending on the input config and the type of
`InputSection`.
Also, multiple other locations also insert directly into `inputSections`
array - assuming that that is the appropriate container for the
`InputSection`'s they create. Currently this is the correct assumption,
however an upcoming feature will change this.
For an upcoming feature (relative method lists), we need to route
`InputSection`'s either to `inputSections` array or to a synthetic
section, depending on weather the relative method list optimization is
enabled or not.
We can achieve the above either by duplicating some of the logic or
refactoring the routing and `InputSection`'s and reusing that.
The refactoring & code sharing approach seems the correct way to go - as
such this diff performs the refactoring while not introducing any
functional changes. Later on we can just call `addInputSection` and not
have to worry about routing logic.
---------
Currently ObjCStubsSection is handling both the logic for the
"__objc_stubs" section, as well as the logic for the "__objc_selrefs"
section.
While this is OK for now, it will be an issue for other features that
want to interact with the "__objc_selrefs" section, such as upcoming
relative method lists feature - which will also want to create /
reference entries in the "__objc_selrefs" section.
In this PR we split the logic relating to handling the "__objc_selrefs"
section into a new SyntheticSection (ObjCSelRefsSection). Non-functional
change - neither the behavior nor implementation changes, the interface
is just made more friendly to not have "__objc_selrefs" so bound to
"__objc_stubs".
---------
Co-authored-by: Alex B <alexborcan@meta.com>
When the data-in-code entries are in separate sections, they are not
guaranteed to be sorted. In particular, 68b1cc36f3df marked some libc++
string functions as noinline, which leads to global ctors involving
strings now producing data-in-code sections in __TEXT,__StaticInit,
which is why this now happens in practice.
Since data-in-code entries are relatively rare and small, just sort
them. No observed performance impact.
See also crbug.com/41487860
This patch implements `-objc_stubs_small` targeting arm64, aiming to
align with ld64's behavior.
1. `-objc_stubs_fast`: As previously implemented, this always uses the
Global Offset Table (GOT) to invoke `objc_msgSend`. The alignment of the
objc stub is 32 bytes.
2. `-objc_stubs_small`: This behavior depends on whether `objc_msgSend`
is defined. If it is, it directly jumps to `objc_msgSend`. If not, it
creates another stub to indirectly jump to `objc_msgSend`, minimizing
the size. The alignment of the objc stub in this case is 4 bytes.
Detail:
LD64 uses the name provided via -[dylib]install_name as "Identifier", when available.
For compatiblity, LLD should do that too.
Differential Revision: https://reviews.llvm.org/D155508
xxh3 is substantially faster than xxh64.
For lld/ELF, there is substantial speedup in `.debug_str` duplicate
elimination (D154813). Use xxh3 for lld-macho as well.
Reviewed By: #lld-macho, oontvoo
Differential Revision: https://reviews.llvm.org/D155677
Apple deprecated bitcode in the deployment process in Xcode 14.0. Last
month Apple started requiring Xcode 14.1+ to submit apps to the App
Store. Since there isn't a use for bundling bitcode outside of
submitting to the App Store we should be safe to delete this handling
entirely from LLD.
Differential Revision: https://reviews.llvm.org/D150697
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
Specifically, we support this:
ld64.lld -dylib foo.o libbar.dylib -exported_symbol _bar -o libfoo.dylib
Where `_bar` is defined in libbar.dylib.
Reviewed By: #lld-macho, smeenai
Differential Revision: https://reviews.llvm.org/D144153
At some point PlatformInfo's Target changed types to a type that also
has minimum deployment target info. This caused ambiguity if you tried
to get the target triple from the Target, as the actual minimum version
info was being stored separately. This bulk of this change is changing
the parsing of these values to support this.
Differential Revision: https://reviews.llvm.org/D145263
I love C++17!
chromium_framework_less_dwarf on my 16-core Mac Pro shows no stat sig change in wall time but a slight decrease in user time:
```
base diff difference (95% CI)
sys_time 1.759 ± 0.037 1.761 ± 0.033 [ -0.9% .. +1.1%]
user_time 4.920 ± 0.043 4.886 ± 0.051 [ -1.2% .. -0.2%]
wall_time 5.950 ± 0.117 5.900 ± 0.116 [ -1.8% .. +0.2%]
samples 26 37
```
Reviewed By: #lld-macho, thakis
Differential Revision: https://reviews.llvm.org/D136518
... instead of mapping them to the intermediate object file.
This matches ld64.
Reviewed By: #lld-macho, Roger
Differential Revision: https://reviews.llvm.org/D136380
This commit adds support for chained fixups, which were introduced in
Apple's late 2020 OS releases. This format replaces the dyld opcodes
used for supplying rebase and binding information, and encodes most of
that data directly in the memory location that will have the fixup
applied.
This reduces binary size and is a requirement for page-in linking, which
will be available starting with macOS 13.
A high-level overview of the format and my implementation can be found
in SyntheticSections.h.
This feature is currently gated behind the `-fixup_chains` flag, and
will be enabled by default for supported targets in a later commit.
Like in ld64, lazy binding is disabled when chained fixups are in use,
and the `-init_offsets` transformation is performed by default.
Differential Revision: https://reviews.llvm.org/D132560
So @keith observed
[here](https://reviews.llvm.org/D128108#inline-1263900) that the
StringRefs we were returning from `CStringInputSection::getStringRef()`
included the null terminator in their total length, but regular
StringRefs do not. Let's fix that so these StringRefs are less confusing
to use.
Reviewed By: #lld-macho, keith, Roger
Differential Revision: https://reviews.llvm.org/D133728
Previously, we would add entries to DataInCodeSection in the order they
appeared in input files. Because of this, entries would not be sorted if
sections were reordered due to e.g. `-order_file` or call graph profile
sorting. ld64 always keeps data-in-code information sorted.
This commit also fixes an incorrect assertion. The original assertion
from D103006 used to check that data-in-code entries are sorted in the
input objects -- likely because we use binary search on that data. In
D115556, the assertion was moved into `collectDataInCodeEntries`, but
the checked variable's name was not changed, so it ended up checking the
final contents of the DataInCodeSection.
We no longer crash when building LLVM with PGO using an asserts build of
LLD as the linker.
Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=1265937
Numbers for linking the Chromium Framework reproducer from #48001, which
has 6829 data-in-code entries:
x before
+ after
N Min Max Median Avg Stddev
x 20 2.1076453 2.3059683 2.1132485 2.1350302 0.049905767
+ 20 2.1069031 2.3915262 2.14465 2.1728429 0.084065898
No difference proven at 95.0% confidence
Differential Revision: https://reviews.llvm.org/D133581
This section stores 32-bit `__TEXT` segment offsets of initializer
functions, and is used instead of `__mod_init_func` when chained fixups
are enabled.
Storing the offsets lets us avoid emitting fixups for the initializers.
Differential Revision: https://reviews.llvm.org/D132947
We now re-use the existing needsBinding() helper to determine if a
branch has to go through a stub. The logic for determining which type of
binding is needed is moved inside StubsSection::addEntry().
This is an NFC refactor that simplifies my diff that adds support for
chained fixups.
Differential Revision: https://reviews.llvm.org/D132476
Apple Clang in Xcode 14 introduced a new feature for reducing the
overhead of objc_msgSend calls by deduplicating the setup calls for each
individual selector. This works by clang adding undefined symbols for
each selector called in a translation unit, such as `_objc_msgSend$foo`
for calling the `foo` method on any `NSObject`. There are 2
different modes for this behavior, the default directly does the setup
for `_objc_msgSend` and calls it, and the smaller option does the
selector setup, and then calls the standard `_objc_msgSend` stub
function.
The general overview of how this works is:
- Undefined symbols with the given prefix are collected
- The suffix of each matching undefined symbol is added as a string to
`__objc_methname`
- A pointer is added for every method name in the `__objc_selrefs`
section
- A `got` entry is emitted for `_objc_msgSend`
- Stubs are emitting pointing to the synthesized locations
Notes:
- Both `__objc_methname` and `__objc_selrefs` can also exist from object
files, so their contents are merged with our synthesized contents
- The compiler emits method names for defined methods, but not for
undefined symbols you call, but stubs are used for both
- This only implements the default "fast" mode currently just to reduce
the diff, I also doubt many folks will care to swap modes
- This only implements this for arm64 and x86_64, we don't need to
implement this for 32 bit iOS archs, but we should implement it for
watchOS archs in a later diff
Differential Revision: https://reviews.llvm.org/D128108
Previously, we treated it as a regular ConcatInputSection. However, ld64
actually parses its contents and uses that to synthesize a single image
info struct, generating one 8-byte section instead of `8 * number of
object files with ObjC code`.
I'm not entirely sure what impact this section has on the runtime, so I
just tried to follow ld64's semantics as closely as possible in this
diff. My main motivation though was to reduce binary size.
No significant perf change on chromium_framework on my 16-core Mac Pro:
base diff difference (95% CI)
sys_time 1.764 ± 0.062 1.748 ± 0.032 [ -2.4% .. +0.5%]
user_time 5.112 ± 0.104 5.106 ± 0.046 [ -0.9% .. +0.7%]
wall_time 6.111 ± 0.184 6.085 ± 0.076 [ -1.6% .. +0.8%]
samples 30 32
Reviewed By: #lld-macho, thakis
Differential Revision: https://reviews.llvm.org/D130125
This commit reduces the size of the emitted rebase sections by
generating the REBASE_OPCODE_DO_REBASE_ADD_ADDR_ULEB and
REBASE_OPCODE_DO_REBASE_ULEB_TIMES_SKIPPING_ULEB opcodes.
With this change, chromium_framework's rebase section is a 40% smaller
197 kilobytes, down from the previous 320 kB. That is 6 kB smaller than
what ld64 produces for the same input.
Performance figures from my M1 Mac mini:
x before
+ after
N Min Max Median Avg Stddev
x 10 4.2269349 4.3300061 4.2689675 4.2690016 0.031151669
+ 10 4.219331 4.2914009 4.2398136 4.2448277 0.023817308
No difference proven at 95.0% confidence
Differential Revision: https://reviews.llvm.org/D130180
An ADD_ADDR rebase opcode's argument can be encoded as an immediate if
the offset is less than 15 * word size. This change reduces the size of
chromium_framework by 100+ KiB.
Differential Revision: https://reviews.llvm.org/D128798
According to ministat, this is a small but measurable speedup
(using the repro in PR56121):
N Min Max Median Avg Stddev
x 10 3.7439518 3.7783802 3.7730219 3.7655502 0.012375226
+ 10 3.6149218 3.692198 3.6519327 3.6502951 0.025905601
Difference at 95.0% confidence
-0.115255 +/- 0.0190746
-3.06078% +/- 0.506554%
(Student's t, pooled s = 0.0203008)
(Without 858e8b17f7365, this change here to use parallelFor is an 18% speedup,
and doing 858e8b17f7365 on top of this change is just a 2.55% +/- 0.58% win.
Doing both results in a total speedup of 20.85% +/- 0.44%.)
Differential Revision: https://reviews.llvm.org/D128298
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
It's in libSystem, so it doesn't bring in any new deps, and it's
currently much faster than LLVM's current SHA256 implementation.
Makes linking (arm64) Chromium Framework with ld64.lld 17% faster.
See also PR56121.
No behavior change.
Differential Revision: https://reviews.llvm.org/D128290
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
