Also adds support for live_support sections, no_dead_strip sections,
.no_dead_strip symbols.
Chromium Framework 345MB unstripped -> 250MB stripped
(vs 290MB unstripped -> 236M stripped with ld64).
Doing dead stripping is a bit faster than not, because so much less
data needs to be processed:
% ministat lld_*
x lld_nostrip.txt
+ lld_strip.txt
N Min Max Median Avg Stddev
x 10 3.929414 4.07692 4.0269079 4.0089678 0.044214794
+ 10 3.8129408 3.9025559 3.8670411 3.8642573 0.024779651
Difference at 95.0% confidence
-0.144711 +/- 0.0336749
-3.60967% +/- 0.839989%
(Student's t, pooled s = 0.0358398)
This interacts with many parts of the linker. I tried to add test coverage
for all added `isLive()` checks, so that some test will fail if any of them
is removed. I checked that the test expectations for the most part match
ld64's behavior (except for live-support-iterations.s, see the comment
in the test). Interacts with:
- debug info
- export tries
- import opcodes
- flags like -exported_symbol(s_list)
- -U / dynamic_lookup
- mod_init_funcs, mod_term_funcs
- weak symbol handling
- unwind info
- stubs
- map files
- -sectcreate
- undefined, dylib, common, defined (both absolute and normal) symbols
It's possible it interacts with more features I didn't think of,
of course.
I also did some manual testing:
- check-llvm check-clang check-lld work with lld with this patch
as host linker and -dead_strip enabled
- Chromium still starts
- Chromium's base_unittests still pass, including unwind tests
Implemenation-wise, this is InputSection-based, so it'll work for
object files with .subsections_via_symbols (which includes all
object files generated by clang). I first based this on the COFF
implementation, but later realized that things are more similar to ELF.
I think it'd be good to refactor MarkLive.cpp to look more like the ELF
part at some point, but I'd like to get a working state checked in first.
Mechanical parts:
- Rename canOmitFromOutput to wasCoalesced (no behavior change)
since it really is for weak coalesced symbols
- Add noDeadStrip to Defined, corresponding to N_NO_DEAD_STRIP
(`.no_dead_strip` in asm)
Fixes PR49276.
Differential Revision: https://reviews.llvm.org/D103324
Has the effect that `__mh_execute_header` stays in the symbol table of
outputs even after running `strip` on the output. I don't know if that's
important for anything -- my motivation for the patch is just is to make
the output more similar to ld64.
(Corresponds to symbolTableInAndNeverStrip in ld64.)
Differential Revision: https://reviews.llvm.org/D102619
In particular, we should apply the `-undefined` behavior to all
such symbols, include those that are specified via the command line
(i.e. `-e`, `-u`, and `-exported_symbol`). ld64 supports this too.
Reviewed By: #lld-macho, thakis
Differential Revision: https://reviews.llvm.org/D102143
This allows LLVM's LTO to internalize symbols that are not referenced
directly by regular objects. Naturally, this means we need to track
which symbols are referenced by regular objects. The approach taken here
is similar to LLD-COFF's: like the COFF port, we extend
`SymbolTable::insert()` to set the isVisibleToRegularObj bit. (LLD-ELF
relies on the Symbol constructor and `Symbol::mergeProperties()`, but
the Mach-O port does not have a `mergeProperties()` equivalent.)
From what I can tell, ld64 (which uses libLTO) doesn't do this
optimization at all. I'm not even sure libLTO provides a way to do this.
Not having ld64's behavior as a reference implementation is unfortunate;
instead, I am relying on LLD-ELF/COFF's behavior as references while
erring on the conservative side. In particular, LLD-MachO will only do
this optimization for executables right now.
We also don't attempt it when `-flat_namespace` is used -- otherwise
we'd need scan the symbol table to find matches for every un-namespaced
symbol reference, which is expensive.
internalize.ll is based off the LLD-ELF tests `internalize-basic.ll` and
`internalize-undef.ll`. Looks like @davide added some of LLD-ELF's internalize
tests, so adding him as a reviewer...
Reviewed By: #lld-macho, gkm
Differential Revision: https://reviews.llvm.org/D99105
This diff addresses FIXME in SyntheticSections.cpp and removes
the dependency of emitEndFunStab on .subsections_via_symbols.
Test plan: make check-lld-macho
Differential revision: https://reviews.llvm.org/D99054
This fixes defects in D98223 [lld-macho] implement options -(un)exported_symbol(s_list):
* disallow export of hidden symbols
* verify that whitelisted literal names are defined in the symbol table
* reflect export-status overrides in `nlist` attribute of `N_EXT` or `N_PEXT`
Thanks to @thakis for raising these issues
Differential Revision: https://reviews.llvm.org/D98381
Previously, it was difficult to write code that handled both synthetic
and regular sections generically. We solve this problem by creating a
fake InputSection at the start of every SyntheticSection.
This refactor allows us to handle DSOHandle like a regular Defined
symbol (since Defined symbols must be attached to an InputSection), and
paves the way for supporting `__mh_*header` symbols. Additionally, it
simplifies our binding/rebase code.
I did have to extend Defined a little -- it now has a `linkerInternal`
flag, to indicate that `___dso_handle` should not be in the final symbol
table.
I've also added some additional testing for `___dso_handle`.
Reviewed By: #lld-macho, oontvoo
Differential Revision: https://reviews.llvm.org/D98545
Dynamic lookup symbols are symbols that work like dynamic symbols
in ELF: They're not bound to a dylib like normal Mach-O twolevel lookup
symbols, but they live in a global pool and dyld resolves them against
exported symbols from all loaded dylibs.
This adds support for dynamical lookup symbols to lld/mac. They are
represented as DylibSymbols with file set to nullptr.
This also uses this support to implement the -U flag, which makes
a specific symbol that's undefined at the end of the link a
dynamic lookup symbol.
For -U, it'd be sufficient to just to a pass over remaining undefined symbols
at the end of the link and to replace them with dynamic lookup symbols then.
But I'd like to use this code to implement flat_namespace too, and that will
require real support for resolving dynamic lookup symbols in SymbolTable. So
this patch adds this now already.
While writing tests for this, I noticed that we didn't set N_WEAK_DEF in the
symbol table for DylibSymbols, so this fixes that too.
Differential Revision: https://reviews.llvm.org/D97521
This makes our error messages more informative. But the bigger motivation is for
LTO symbol resolution, which will be in an upcoming diff. The changes in this
one are largely mechanical.
Reviewed By: #lld-macho, smeenai
Differential Revision: https://reviews.llvm.org/D94316
Private extern symbols are used for things scoped to the linkage unit.
They cause duplicate symbol errors (so they're in the symbol table,
unlike TU-scoped truly local symbols), but they don't make it into the
export trie. They are created e.g. by compiling with
-fvisibility=hidden.
If two weak symbols have differing privateness, the combined symbol is
non-private external. (Example: inline functions and some TUs that
include the header defining it were built with
-fvisibility-inlines-hidden and some weren't).
A weak private external symbol implicitly has its "weak" dropped and
behaves like a regular strong private external symbol: Weak is an export
trie concept, and private symbols are not in the export trie.
If a weak and a strong symbol have different privateness, the strong
symbol wins.
If two common symbols have differing privateness, the larger symbol
wins. If they have the same size, the privateness of the symbol seen
later during the link wins (!) -- this is a bit lame, but it matches
ld64 and this behavior takes 2 lines less to implement than the less
surprising "result is non-private external), so match ld64.
(Example: `int a` in two .c files, both built with -fcommon,
one built with -fvisibility=hidden and one without.)
This also makes `__dyld_private` a true TU-local symbol, matching ld64.
To make this work, make the `const char*` StringRefZ ctor to correctly
set `size` (without this, writing the string table crashed when calling
getName() on the __dyld_private symbol).
Mention in CommonSymbol's comment that common symbols are now disabled
by default in clang.
Mention in -keep_private_externs's HelpText that the flag only has an
effect with `-r` (which we don't implement yet -- so this patch here
doesn't regress any behavior around -r + -keep_private_externs)). ld64
doesn't explicitly document it, but the commit text of
http://reviews.llvm.org/rL216146 does, and ld64's
OutputFile::buildSymbolTable() checks `_options.outputKind() ==
Options::kObjectFile` before calling `_options.keepPrivateExterns()`
(the only reference to that function).
Fixes PR48536.
Differential Revision: https://reviews.llvm.org/D93609
TREATMENT can be `error`, `warning`, `suppress`, or `dynamic_lookup`
The `dymanic_lookup` remains unimplemented for now.
Differential Revision: https://reviews.llvm.org/D93263
Weak references need not necessarily be satisfied at runtime (but they must
still be satisfied at link time). So symbol resolution still works as per usual,
but we now pass around a flag -- ultimately emitting it in the bind table -- to
indicate if a given dylib symbol is a weak reference.
ld64's behavior for symbols that have both weak and strong references is
a bit bizarre. For non-function symbols, it will emit a weak import. For
function symbols (those referenced by BRANCH relocs), it will emit a
regular import. I'm not sure what value there is in that behavior, and
since emulating it will make our implementation more complex, I've
decided to treat regular weakrefs like function symbol ones for now.
Fixes PR48511.
Reviewed By: #lld-macho, thakis
Differential Revision: https://reviews.llvm.org/D93369
On Unix, it is traditionally allowed to write variable definitions without
initialization expressions (such as "int foo;") to header files. These are
called tentative definitions.
The compiler creates common symbols when it sees tentative definitions. When
linking the final binary, if there are remaining common symbols after name
resolution is complete, the linker converts them to regular defined symbols in
a `__common` section.
This diff implements most of that functionality, though we do not yet handle
the case where there are both common and non-common definitions of the same
symbol.
Reviewed By: #lld-macho, gkm
Differential Revision: https://reviews.llvm.org/D86909
References to symbols in dylibs work very similarly regardless of
whether the symbol is a TLV. The main difference is that we have a
separate `__thread_ptrs` section that acts as the GOT for these
thread-locals.
We can identify thread-locals in dylibs by a flag in their export trie
entries, and we cross-check it with the relocations that refer to them
to ensure that we are not using a GOT relocation to reference a
thread-local (or vice versa).
Reviewed By: #lld-macho, smeenai
Differential Revision: https://reviews.llvm.org/D85081
The C++ ABI requires dylibs to pass a pointer to __cxa_atexit which does
e.g. cleanup of static global variables. The C++ spec says that the pointer
can point to any address in one of the dylib's segments, but in practice
ld64 seems to set it to point to the header, so that's what's implemented
here.
Reviewed By: #lld-macho, smeenai
Differential Revision: https://reviews.llvm.org/D83603
This diff adds support for weak definitions, though it doesn't handle weak
symbols in dylibs quite correctly -- we need to emit binding opcodes for them
in the weak binding section rather than the lazy binding section.
What *is* covered in this diff:
1. Reading the weak flag from symbol table / export trie, and writing it to the
export trie
2. Refining the symbol table's rules for choosing one symbol definition over
another. Wrote a few dozen test cases to make sure we were matching ld64's
behavior.
We can now link basic C++ programs.
Reviewed By: #lld-macho, compnerd
Differential Revision: https://reviews.llvm.org/D83532
With this change, basic archive files can be linked together. Input
section discovery has been refactored into a function since archive
files lazily resolve their symbols / the object files containing those
symbols.
Reviewed By: int3, smeenai
Differential Revision: https://reviews.llvm.org/D78342
This diff implements:
* dylib loading (much of which is being restored from @pcc and @ruiu's
original work)
* The GOT_LOAD relocation, which allows us to load non-lazy dylib
symbols
* Basic bind opcode emission, which tells `dyld` how to populate the GOT
Differential Revision: https://reviews.llvm.org/D76252
Summary:
This is the first commit for the new Mach-O backend, designed to roughly
follow the architecture of the existing ELF and COFF backends, and
building off work that @ruiu and @pcc did in a branch a while back. Note
that this is a very stripped-down commit with the bare minimum of
functionality for ease of review. We'll be following up with more diffs
soon.
Currently, we're able to generate a simple "Hello World!" executable
that runs on OS X Catalina (and possibly on earlier OS X versions; I
haven't tested them). (This executable can be obtained by compiling
`test/MachO/relocations.s`.) We're mocking out a few load commands to
achieve this -- for example, we can't load dynamic libraries, but
Catalina requires binaries to be linked against `dyld`, so we hardcode
the emission of a `LC_LOAD_DYLIB` command. Other mocked out load
commands include LC_SYMTAB and LC_DYSYMTAB.
Differential Revision: https://reviews.llvm.org/D75382
Nico Weber