Summary:
For SECTION relocations against absolute symbols, MSVC emits the largest
output section index plus one. I've implemented that by threading a
global variable through DefinedAbsolute that is filled in by the Writer.
A more library-oriented approach would be to thread the Writer through
Chunk::writeTo and SectionChunk::applyRel*, but Rui seems to prefer
doing it this way.
MSVC rejects SECREL relocations against absolute symbols, but only when
the relocation is in a real output section. When the relocation is in a
CodeView debug info section destined for the PDB, it seems that this
relocation error is suppressed, and absolute symbols become zeros in the
object file. This is easily implemented by checking the input section
from which we're applying relocations.
This should fix errors about __safe_se_handler_table and
__guard_fids_table when linking the CRT and generating a PDB.
Reviewers: ruiu
Subscribers: aprantl, llvm-commits
Differential Revision: https://reviews.llvm.org/D34541
llvm-svn: 306071
This is a different implementation than r303225 (which was reverted
in r303270, re-submitted in r303304 and then re-reverted in r303527).
In the previous patch, I tried to add Live bit to each dllimported
symbol. It turned out that it didn't work with "oldnames.lib" which
contains a lot of weak aliases to dllimported symbols.
The way we handle weak aliases is to check if undefined symbols
can be resolved using weak aliases, and if so, memcpy the Defined
symbols to weak Undefined symbols, so that any references to weak
aliases automatically see defined symbols instead of undefined ones.
This memcpy happens before MarkLive kicks in.
That means we may have multiple copies of dllimported symbols. So
turning on one instance's Live bit is not enough.
This patch moves the Live bit to dllimport file. Since multiple
copies of dllsymbols still point to the same file, we can use it as the
central repository to keep track of liveness.
Differential Revision: https://reviews.llvm.org/D33520
llvm-svn: 303814
This reverts commit r303304 because it looks like the change
introduced a crash bug. At least after that change, LLD with thinlto
crashes when linking Chromium.
llvm-svn: 303527
This reverts re-submits r303225 which was reverted in r303270 because it
broke the sanitizer-windows bot.
The reason of the failure is that we were writing dead symbols to the
symbol table. I fixed the issue.
llvm-svn: 303304
and follow-up r303226 "Fix Windows buildbots."
This broke the sanitizer-windows buildbot.
> Previously, the garbage collector (enabled by default or by explicitly
> passing /opt:ref) did not kill dllimported symbols. As a result,
> dllimported symbols could be added to resulting executables' dllimport
> list even if no one was actually using them.
>
> This patch implements dllexported symbol garbage collection. Just like
> COMDAT sections, dllimported symbols now have Live bits to manage their
> liveness, and MarkLive marks reachable dllimported symbols.
>
> Fixes https://bugs.llvm.org/show_bug.cgi?id=32950
>
> Reviewers: pcc
>
> Subscribers: llvm-commits
>
> Differential Revision: https://reviews.llvm.org/D33264
llvm-svn: 303270
Summary:
Previously, the garbage collector (enabled by default or by explicitly
passing /opt:ref) did not kill dllimported symbols. As a result,
dllimported symbols could be added to resulting executables' dllimport
list even if no one was actually using them.
This patch implements dllexported symbol garbage collection. Just like
COMDAT sections, dllimported symbols now have Live bits to manage their
liveness, and MarkLive marks reachable dllimported symbols.
Fixes https://bugs.llvm.org/show_bug.cgi?id=32950
Reviewers: pcc
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D33264
llvm-svn: 303225
Summary: The COFF linker previously implemented link-time optimization using an API which has now been marked as legacy. This change refactors the COFF linker to use the new LTO API, which is also used by the ELF linker.
Reviewers: pcc, ruiu
Reviewed By: pcc
Subscribers: mgorny, mehdi_amini
Differential Revision: https://reviews.llvm.org/D29059
llvm-svn: 293967
I thought for a while about how to remove it, but it looks like we
can just copy the file for now. Of course I'm not happy about that,
but it's just less than 50 lines of code, and we already have
duplicate code in Error.h and some other places. I want to solve
them all at once later.
Differential Revision: https://reviews.llvm.org/D27819
llvm-svn: 290062
Profiling revealed that the majority of lld's execution time on Windows was
spent opening and mapping input files. We can reduce this cost significantly
by performing these operations asynchronously.
This change introduces a queue for all operations on input file data. When
we discover that we need to load a file (for example, when we find a lazy
archive for an undefined symbol, or when we read a linker directive to
load a file from disk), the file operation is launched using a future and
the symbol resolution operation is enqueued. This implies another change
to symbol resolution semantics, but it seems to be harmless ("ninja All"
in Chromium still succeeds).
To measure the perf impact of this change I linked Chromium's chrome_child.dll
with both thin and fat archives.
Thin archives:
Before (median of 5 runs): 19.50s
After: 10.93s
Fat archives:
Before: 12.00s
After: 9.90s
On Linux I found that doing this asynchronously had a negative effect on
performance, probably because the cost of mapping a file is small enough that
it becomes outweighed by the cost of managing the futures. So on non-Windows
platforms I use the deferred execution strategy.
Differential Revision: https://reviews.llvm.org/D27768
llvm-svn: 289760
Using a set here caused us to take about 1 second longer to write the symbol
table when linking chrome_child.dll. With this I consistently get better
performance on Windows with the new symbol table.
Before r289280 and with r289183 reverted (median of 5 runs): 17.65s
After this change: 17.33s
On Linux things look even better:
Before: 10.700480444s
After: 5.735681610s
Differential Revision: https://reviews.llvm.org/D27648
llvm-svn: 289408
This ports the ELF linker's symbol table design, introduced in r268178,
to the COFF linker.
Differential Revision: http://reviews.llvm.org/D21166
llvm-svn: 289280
Previously, we had different way to stringize SymbolBody and InputFile
to construct error messages. This patch defines overloaded function
toString() so that we don't need to memorize all these different
function names.
With that change, it is now easy to include demangled names in error
messages. Now, if there is a symbol name conflict, we'll print out
both mangled and demangled names.
llvm-svn: 288992
I made the field an atomic pointer in hope that we would be able to
parallelize the symbol resolver soon, but that's not going to happen
soon. This patch reverts that change for the sake of readability.
llvm-svn: 248104
Basically the concept of "liveness" is for sections (or chunks in LLD
terminology) and not for symbols. Symbols are always available or live,
or otherwise it indicates a link failure.
Previously, we had isLive() and markLive() methods for DefinedSymbol.
They are confusing methods. What they actually did is to act as a proxy
to backing section chunks. We can simplify eliminate these methods
and call section chunk's methods directly.
llvm-svn: 247869
This is more convenient than the offset from the start of the file as we
don't have to worry about it changing when we move the output section.
This is a port of r245008 from ELF.
llvm-svn: 245018
This has a few advantages
* Less C++ code (about 300 lines less).
* Less machine code (about 14 KB of text on a linux x86_64 build).
* It is more debugger friendly. Just set a breakpoint on the exit function and
you get the complete lld stack trace of when the error was found.
* It is a more robust API. The errors are handled early and we don't get a
std::error_code hot potato being passed around.
* In most cases the error function in a better position to print diagnostics
(it has more context).
llvm-svn: 244215
Windows ARM is the thumb ARM environment, and pointers to thumb code
needs to have its LSB set. When we apply relocations, we need to
adjust the LSB if it points to an executable section.
llvm-svn: 243560
SECREL should sets the 32-bit offset of the target from the beginning
of *target's* output section. Previously, the offset from the beginning
of source's output section was used instead.
SECTION means the target section's index, and not the source section's
index. This patch fixes that issue too.
llvm-svn: 243535
An object file compatible with Safe SEH contains a .sxdata section.
The section contains a list of symbol table indices, each of which
is an exception handler function. A safe SEH-enabled executable
contains a list of exception handler RVAs. So, what the linker has
to do to support Safe SEH is basically to read the .sxdata section,
interpret the contents as a list of symbol indices, unique-fy and
sort their RVAs, and then emit that list to .rdata. This patch
implements that feature.
llvm-svn: 243182
__ImageBase is a special symbol whose value is the image base address.
Previously, we handled __ImageBase symbol as an absolute symbol.
Absolute symbols point to specific locations in memory and the locations
never change even if an image is base-relocated. That means that we
don't have base relocation entries for absolute symbols.
This is not a case for __ImageBase. If an image is base-relocated, its
base address changes, and __ImageBase needs to be shifted as well.
So we have to have base relocations for __ImageBase. That means that
__ImageBase is not really an absolute symbol but a different kind of
symbol.
In this patch, I introduced a new type of symbol -- DefinedRelative.
DefinedRelative is similar to DefinedAbsolute, but it has not a VA but RVA
and is a subject of base relocation. Currently only __ImageBase is of
the new symbol type.
llvm-svn: 243176
In the new design, mutation of Symbol pointers is the name resolution
operation. This patch makes them atomic pointers so that they can
be mutated by multiple threads safely. I'm going to use atomic
compare-exchange on these pointers.
dyn_cast<> doesn't recognize atomic pointers as pointers,
so we need to call load(). This is unfortunate, but in other places
automatic type conversion works fine.
llvm-svn: 241416
We were previously hitting assertion failures in the writer in cases where
a regular object file defined a weak external symbol that was defined by
a bitcode file. Because /export and /entry name mangling were implemented
using weak externals, the same problem affected mangled symbol names in
bitcode files.
The underlying cause of the problem was that weak external symbols were
being resolved before doing LTO, so the symbol table may have contained stale
references to bitcode symbols. The fix here is to defer weak external symbol
resolution until after LTO.
Also implement support for weak external symbols in bitcode files
by modelling them as replaceable DefinedBitcode symbols.
Differential Revision: http://reviews.llvm.org/D10940
llvm-svn: 241391
This worked before, but only by accident, and only with assertions disabled.
We ended up storing a DefinedRegular symbol in the WeakAlias field,
and never using it as an Undefined.
Differential Revision: http://reviews.llvm.org/D10934
llvm-svn: 241376
Previously, SymbolBody::compare(A, B) didn't satisfy weak ordering.
There was a case that A < B and B < A could have been true.
This is because we just pick LHS if A and B are consisdered equivalent.
This patch is to make symbols being weakly ordered. If A and B are
not tie, one of A < B && B > A or A > B && B < A is true.
This is not an improtant property for a single-threaded environment
because everything is deterministic anyways. However, in a multi-
threaded environment, this property becomes important.
If a symbol is defined or lazy, ties are resolved by its file index.
For simple types that we don't really care about their identities,
symbols are compared by their addresses.
llvm-svn: 241294
Previously, we use SymbolTable::rename to resolve AlternateName symbols.
This patch is to merge that mechanism with weak aliases, so that we
remove that function.
llvm-svn: 241230
Previously, the order of adding symbols to the symbol table was simple.
We have a list of all input files. We read each file from beginning of
the list and add all symbols in it to the symbol table.
This patch changes that order. Now all archive files are added to the
symbol table first, and then all the other object files are added.
This shouldn't change the behavior in single-threading, and make room
to parallelize in multi-threading.
In the first step, only lazy symbols are added to the symbol table
because archives contain only Lazy symbols. Member object files
found to be necessary are queued. In the second step, defined and
undefined symbols are added from object files. Adding an undefined
symbol to the symbol table may cause more member files to be added
to the queue. We simply continue reading all object files until the
queue is empty.
Finally, new archive or object files may be added to the queues by
object files' directive sections (which contain new command line
options).
The above process is repeated until we get no new files.
Symbols defined both in object files and in archives can make results
undeterministic. If an archive is read before an object, a new member
file gets linked, while in the other way, no new file would be added.
That is the most popular cause of an undeterministic result or linking
failure as I observed. Separating phases of adding lazy symbols and
undefined symbols makes that deterministic. Adding symbols in each
phase should be parallelizable.
llvm-svn: 241107
The size of this class actually matters because this is the
most popular class among all classes. We create a Defined symbol
for each defined symbol in a symbol table. That can be millions
for a large program. For example, linking LLD instantiates this
class millions times.
llvm-svn: 241025
its members into the base class.
First, to help motivate this kind of change, understand that in
a self-link, LLD creates 5.5 million defined regular symbol bodies (and
6 million symbol bodies total). A significant portion of its time is
spent allocating the memory for these symbols, and befor ethis patch
the defined regular symbol body objects alone consumed some 420mb of
memory during the self link.
As a consequence, I think it is worth expending considerable effort to
make these objects as memory efficient as possible. This is the first of
several components of that. This change starts with the goal of removing
the virtual functins from SymbolBody so that it can avoid having a vptr
embedded in it when it already contains a "kind" member, and that member
can be much more compact than a vptr.
The primary way of doing this is to sink as much of the logic that we
would have to dispatch for into data in the base class. As part of this,
I made the various flags bits that will pack into a bitfield with the
kind tag. I also sank the Name down to eliminate the dispatch for that,
and used LLVM's RTTI-style dispatch for everything else (most of which
is cold and so doesn't matter terribly if we get minutely worse lowering
than a vtable dispatch).
As I was doing this, I wanted to make the RTTI-dispatch (which would
become much hotter than before) as efficient as possible, so I've
re-organized the tags somewhat. Notably, the common case (regular
defined symbols) is now zero which we can test for faster.
I also needed to rewrite the comparison routine used during resolving
symbols. This proved to be quite complex as the semantics of the
existing one were very subtle due to the back-and-forth virtual dispatch
caused by re-dispatching with reversed operands. I've consolidated it to
a single function and tried to comment it quite a bit more to help
explain what is going on. However, this may need more comments or other
explanations. It at least passes all the regression tests. I'm not
working on Windows, so I can't fully test it.
With all of these changes, the size of a DefinedRegular symbol on
a 64-bit build goes from 80 bytes to 64 bytes, and we save approximately
84mb or 20% of the memory consumed by these symbol bodies during the
link.
The link time appears marginally faster as well, and the profile hotness
of the memory allocation subsystem got a bit better, but there is still
a lot of allocation traffic.
Differential Revision: http://reviews.llvm.org/D10792
llvm-svn: 241001
This flattens the entire liveness walk from a recursive mark approach to
a worklist approach. It also sinks the worklist management completely
out of the SectionChunk and into the Writer by exposing the ability to
iterato over children of a chunk and over the symbol bodies of relocated
symbols. I'm not 100% happy with the API names, so suggestions welcome
there.
This allows us to use a single worklist for the entire recursive walk
and would also be a natural place to take advantage of parallelism at
some future point.
With this, we completely inline away the GC walk into the
Writer::markLive function and it makes it very easy to profile what is
slow. Currently, time is being wasted checking whether a Chunk isa
SectionChunk (it essentially always is), finding (or skipping)
a replacement for a symbol, and chasing pointers between symbols and
their chunks. There are a bunch of things we can do to fix this, and its
easier to do them after this change IMO.
This change alone saves 1-2% of the time for my self-link of lld.exe
(which I'm running and benchmarking on Linux ironically).
Perhaps more notably, we'll no longer blow out the stack for large
links. =]
Just as an FYI, at this point, I/O is starting to really dominate the
profile. Well over 10% of the time appears to be inside the kernel doing
page table silliness. I think a decent chunk of this can be nuked as
well, but it's a little odd as cross-linking in this way isn't really
the primary goal here.
Differential Revision: http://reviews.llvm.org/D10790
llvm-svn: 240995
When comparing two COMDAT sections, we need to take section values
and associative sections into account. This patch fixes that bug.
It fixes a crash bug of llvm-tblgen when linked with /opt:lldicf.
One thing I don't understand yet is that this logic seems to be
too strict. MSVC linker is able to create more compact executables
(which of course work correctly). With this ICF algorithm, LLD is
able to make executable smaller, but the outputs are larger than
MSVC's. There must be something I'm missing here.
llvm-svn: 240897
This flag can be used to produce a map file, which is essentially a list
of objects linked into the final output file together with the RVAs of
their symbols. Because our format differs from MSVC's we expose it as a
separate flag.
Differential Revision: http://reviews.llvm.org/D10773
llvm-svn: 240812
Absolute symbols were always handled as external symbols, so if two
or more object files define the same absolute symbol, they would
conflict even if the symbol is private to each file.
This patch fixes that bug.
llvm-svn: 240756
I split them in r240319 because I thought they are different enough
that we should treat them as different types. It turned out that
that was not a good idea. They are so similar that we ended up having
many duplicate code.
llvm-svn: 240706
Only SectionChunk can be dead-stripped. Previously,
all types of chunks implemented these functions,
but their functions were blank.
Likewise, only DefinedRegular and DefinedCOMDAT symbols
can be dead-stripped. markLive() function was implemented
for other symbol types, but they were blank.
I started thinking that the change I made in r240319 was
a mistake. I separated DefinedCOMDAT from DefinedRegular
because I thought that would make the code cleaner, but now
we want to handle them as the same type here. Maybe we
should roll it back.
This change should improve readability a bit as this removes
some dubious uses of reinterpret_cast. Previously, we
assumed that all COMDAT chunks are actually SectionChunks,
which was not very obvious.
llvm-svn: 240675
The change I made in r240620 was not correct. If a symbol foo is
defined, and if you use __imp_foo, __imp_foo symbol is automatically
defined as a pointer (not just an alias) to foo.
Now that we need to create a chunk for automatically-created symbols.
I defined LocalImportChunk class for them.
llvm-svn: 240622
Identical COMDAT Folding (ICF) is an optimization to reduce binary
size by merging COMDAT sections that contain the same metadata,
actual data and relocations. MSVC link.exe and many other linkers
have this feature. LLD achieves on per with MSVC in terms produced
binary size with this patch.
This technique is pretty effective. For example, LLD's size is
reduced from 64MB to 54MB by enaling this optimization.
The algorithm implemented in this patch is extremely inefficient.
It puts all COMDAT sections into a set to identify duplicates.
Time to self-link with/without ICF are 3.3 and 320 seconds,
respectively. So this option roughly makes LLD 100x slower.
But it's okay as I wanted to achieve correctness first.
LLD is still able to link itself with this optimization.
I'm going to make it more efficient in followup patches.
Note that this optimization is *not* entirely safe. C/C++ require
different functions have different addresses. If your program
relies on that property, your program wouldn't work with ICF.
However, it's not going to be an issue on Windows because MSVC
link.exe turns ICF on by default. As long as your program works
with default settings (or not passing /opt:noicf), your program
would work with LLD too.
llvm-svn: 240519
Before this change, you got to cast a symbol to DefinedRegular and then
call isCOMDAT() to determine if a given symbol is a COMDAT symbol.
Now you can just use isa<DefinedCOMDAT>().
As to the class definition of DefinedCOMDAT, I could remove duplicate
code from DefinedRegular and DefinedCOMDAT by introducing another base
class for them, but I chose to not do that to keep the class hierarchy
shallow. This amount of code duplication doesn't worth to define a new
class.
llvm-svn: 240319
This is a case that one mistake caused a very mysterious bug.
I made a mistake to calculate addresses of common symbols, so
each common symbol pointed not to the beginning of its location
but to the end of its location. (Ouch!)
Common symbols are aligned on 16 byte boundaries. If a common
symbol is small enough to fit between the end of its real
location and whatever comes next, this bug didn't cause any harm.
However, if a common symbol is larger than that, its memory
naturally overlapped with other symbols. That means some
uninitialized variables accidentally shared memory. Because
totally unrelated memory writes mutated other varaibles, it was
hard to debug.
It's surprising that LLD was able to link itself and all LLD
tests except gunit tests passed with this nasty bug.
With this fix, the new COFF linker is able to pass all tests
for LLVM, Clang and LLD if I use MSVC cl.exe as a compiler.
Only three tests are failing when used with clang-cl.
llvm-svn: 240216
We are currently handling all combinations of SymbolBody types directly.
This patch is to flip this and Other if Other->kind() < this->kind()
to reduce number of combinations. No functionality change intended.
llvm-svn: 239745
In the case where either a bitcode file and a regular file or two bitcode
files export a common or comdat symbol with the same name, the linker needs
to pick one of them following COFF semantics. This patch implements a design
for resolving such symbols that pushes most of the work onto either LLD's
regular mechanism for resolving common or comdat symbols or the IR linker's
mechanism for doing the same.
We modify SymbolBody::compare to always prefer non-bitcode symbols, so that
during the initial phase of symbol resolution, the symbol table always contains
a regular symbol in any case where we need to choose between a regular and
a bitcode symbol. In SymbolTable::addCombinedLTOObject, we force export
any bitcode symbols that were initially pre-empted by a regular symbol,
and later use SymbolBody::compare to choose between the regular symbol in
the symbol table and the regular symbol from the combined LTO object file.
This design seems to be sound, so long as the resolution mechanism is defined
to be commutative and associative modulo arbitrary choices between symbols
(which seems to be the case for COFF).
Differential Revision: http://reviews.llvm.org/D10329
llvm-svn: 239563
This change seems to make the linker about 10% faster.
Reading symbol name is not very cheap because it needs strlen()
on the string table. We were wasting time on reading non-external
symbol names that would never be used by the linker.
llvm-svn: 239332
Symbols exported by DLLs can be imported not by name but by
small number or ordinal. Usually, symbols have both ordinals
and names, and in that case ordinals are called "hints" and
used by the loader as hints.
However, symbols can have only ordinals. They are called
import-by-ordinal symbols. You need to manage ordinals by hand
so that they will never change if you choose to use the feature.
But it's supposed to make dynamic linking faster because
it needs no string comparison. Not sure if that claim still
stands in year 2015, though. Anyways, the feature exists,
and this patch implements that.
llvm-svn: 238780
Currently we set the field to zero, but as per the spec, we should
set numbers we read from import library files. The loader uses the
values as starting offsets for binary search when looking up imported
symbols from DLL.
llvm-svn: 238562
This is an initial patch for a section-based COFF linker.
The patch has 2300 lines of code including comments and blank lines.
Before diving into details, you want to start from reading README
because it should give you an overview of the design.
All important things are written in the README file, so I write
summary here.
- The linker is already able to self-link on Windows.
- It's significantly faster than the existing implementation.
The existing one takes 5 seconds to link LLD on my machine,
while the new one only takes 1.2 seconds, even though the new
one is not multi-threaded yet. (And a proof-of-concept multi-
threaded version was able to link it in 0.5 seconds.)
- It uses much less memory (250MB vs. 2GB virtual memory space
to self-host).
- IMHO the new code is much simpler and easier to read than
the existing PE/COFF port.
http://reviews.llvm.org/D10036
llvm-svn: 238458
Reid Kleckner