LLD's performance on PGO instrumented Windows binaries was still not
great even with the fix in D56955; out of the 2m41s linker runtime,
around 2 minutes were still being spent in ICF. I looked into this more
closely and discovered that the vast majority of the runtime was being
spent segregating .pdata sections with the following relocation chain:
.pdata -> identical .text -> unique PGO counter (not eligible for ICF)
This patch causes us to perform 2 rounds of relocation hash
propagation, which allows the hash for the .pdata sections to
incorporate the identifier from the PGO counter. With that, the amount
of time spent in ICF was reduced to about 2 seconds. I also found that
the same change led to a significant ICF performance improvement in a
regular release build of Chromium's chrome_child.dll, where ICF time
was reduced from around 1s to around 700ms.
With the same change applied to the ELF linker, median of 100 runs
for lld-speed-test/chrome reduced from 4.53s to 4.45s on my machine.
I also experimented with increasing the number of propagation rounds
further, but I did not observe any further significant performance
improvements linking Chromium or Firefox.
Differential Revision: https://reviews.llvm.org/D56986
llvm-svn: 351899
It turns out that sections in PGO instrumented object files on Windows
contain a large number of relocations pointing to themselves. With
r347429 this can cause many sections to receive the same hash (usually
zero) as a result of a section's hash being xor'ed with itself.
This patch causes the COFF and ELF linkers to avoid this problem
by adding the hash of the relocated section instead of xor'ing it.
On my machine this causes the regressing test case
provided by Mozilla to terminate in 2m41s.
Differential Revision: https://reviews.llvm.org/D56955
llvm-svn: 351898
to reflect the new license.
We understand that people may be surprised that we're moving the header
entirely to discuss the new license. We checked this carefully with the
Foundation's lawyer and we believe this is the correct approach.
Essentially, all code in the project is now made available by the LLVM
project under our new license, so you will see that the license headers
include that license only. Some of our contributors have contributed
code under our old license, and accordingly, we have retained a copy of
our old license notice in the top-level files in each project and
repository.
llvm-svn: 351636
On my machine this reduced median link time of lld-speed-test/chrome
from 2.68s to 2.41s. It also reduces link time of Chrome for Android
with a prototype compiler change that causes the compiler to create
large numbers of identical (modulo relocations) sections from >15
minutes to a few seconds.
Differential Revision: https://reviews.llvm.org/D54773
llvm-svn: 347594
Previously, we uncompress all compressed sections before doing anything.
That works, and that is conceptually simple, but that could results in
a waste of CPU time and memory if uncompressed sections are then
discarded or just copied to the output buffer.
In particular, if .debug_gnu_pub{names,types} are compressed and if no
-gdb-index option is given, we wasted CPU and memory because we
uncompress them into newly allocated bufers and then memcpy the buffers
to the output buffer. That temporary buffer was redundant.
This patch changes how to uncompress sections. Now, compressed sections
are uncompressed lazily. To do that, `Data` member of `InputSectionBase`
is now hidden from outside, and `data()` accessor automatically expands
an compressed buffer if necessary.
If no one calls `data()`, then `writeTo()` directly uncompresses
compressed data into the output buffer. That eliminates the redundant
memory allocation and redundant memcpy.
This patch significantly reduces memory consumption (20 GiB max RSS to
15 Gib) for an executable whose .debug_gnu_pub{names,types} are in total
5 GiB in an uncompressed form.
Differential Revision: https://reviews.llvm.org/D52917
llvm-svn: 343979
These symbols are declared early with the same value, so they otherwise
appear identical to ICF.
Differential Revision: https://reviews.llvm.org/D51376
llvm-svn: 340998
This patch does the same thing as r338153 for COFF.
Note that this patch affects only the order of log messages.
The output file is already deterministic.
Differential Revision: https://reviews.llvm.org/D50023
llvm-svn: 338406
The xxHash64 function has been made unsigned-char-independent, so
we can reland this change now.
Original commit message:
> The icf-safe.s test currently fails on 32-bit platforms because it uses
> the --print-icf-sections flag and depends on the output appearing in
> a specific order. However, this flag causes the output to depend on
> the order of the sections in the Sections array, which depends on the
> hash values returned from hash_combine, which happen to be different
> for that test between 32-bit and 64-bit platforms.
>
> This change makes the output deterministic by using xxHash64 instead of
> hash_combine.
Differential Revision: https://reviews.llvm.org/D49877
llvm-svn: 338153
The icf-safe.s test currently fails on 32-bit platforms because it uses
the --print-icf-sections flag and depends on the output appearing in
a specific order. However, this flag causes the output to depend on
the order of the sections in the Sections array, which depends on the
hash values returned from hash_combine, which happen to be different
for that test between 32-bit and 64-bit platforms.
This change makes the output deterministic by using xxHash64 instead of
hash_combine.
Differential Revision: https://reviews.llvm.org/D49877
llvm-svn: 338088
We are already ICF'ing these sections as a unit with their dependent
sections, so they don't need to be considered for ICF individually.
This change also "fixes" slowness caused by our quadratic-in-group-size
relocation segregation algorithm on 32-bit ARM platforms with unwind
data and ICF on rodata. In this scenario almost every function's
.ARM.exidx is identical except for the targets of the relocations
that refer to the function and its .ARM.extab, which causes almost
all of the program's .ARM.exidx sections to be initially added to the
same class, which causes us to compare every such section with every
other such section.
Differential Revision: https://reviews.llvm.org/D49716
llvm-svn: 337967
The gold behaviour with regard to --keep-unique is arguably a bug.
I also noticed a bug in my patch, which is that we mislink the
following program with --icf=safe by merging f3 and f4:
void f1() {}
void f2() {}
__attribute__((weak)) void* f3() { return f1; }
__attribute__((weak)) void* f4() { return f2; }
int main() {
printf("%p %p\n", f3(), f4());
}
llvm-svn: 337729
Under --icf=all we now only apply KeepUnique to non-executable
address-significant sections. This has the effect of making --icf=all
mean unsafe ICF for executable sections and safe ICF for non-executable
sections.
With this change the meaning of the KeepUnique bit changes to
"does the current ICF mode (together with the --keep-unique and
--ignore-data-address-equality flags) require this section to be
kept unique".
Differential Revision: https://reviews.llvm.org/D49626
llvm-svn: 337640
The only restriction is that we cannot merge more than one KeepUnique
section together. This matches gold's behaviour and reduces code size
when using --icf=safe.
Differential Revision: https://reviews.llvm.org/D49622
llvm-svn: 337638
Code is dead. We use only InputSections when building the list of
sections elegible for the ICF:
https://github.com/llvm-mirror/lld/blob/master/ELF/ICF.cpp#L439
And 'isEligible' filters out SyntheticSections as well for us.
That way the only Kind we have in the Sections vector is SectionBase::Regular,
so we do not need to check sections kind at all, it is always the same.
llvm-svn: 335460
Change removes the excessive comparsion of
the relocation arrays sizes.
This code was dead, because at the higer level,
equalsConstant function contains the following check:
`A->NumRelocations != B->NumRelocations`
where NumRelocations contains the size of the relocations array.
So removed check did the same job twice.
This was found with use of code coverage analysis.
llvm-svn: 335346
A user program may enumerate sections named with a C identifier using
__start_* and __stop_* symbols. We cannot ICF any such sections because
that could change program semantics.
Differential Revision: https://reviews.llvm.org/D47242
llvm-svn: 333054
Note that this doesn't do the right thing in the case where there is
a linker script. We probably need to move output section assignment
before ICF to get the correct behaviour here.
Differential Revision: https://reviews.llvm.org/D47241
llvm-svn: 333052
The --keep-unique <symbol> option is taken from gold. The intention is that
<symbol> will be prevented from being folded by ICF. Although not
specifically mentioned in the documentation <symbol> only matches
global symbols, with a warning if the symbol is not found.
The implementation finds the Section defining <symbol> and removes it from
the set of sections considered for ICF.
Differential Revision: https://reviews.llvm.org/D46755
llvm-svn: 332332
Summary:
r328610 fixed a data race in the COFF linker. This change makes a
similar fix to the ELF linker.
Reviewers: ruiu, pcc, rnk
Reviewed By: ruiu
Subscribers: emaste, llvm-commits, arichardson
Differential Revision: https://reviews.llvm.org/D45192
llvm-svn: 329088
The Data member of synthetic section's is not valid and empty. The Data
member is required to be valid by ICF as it is used by ICF to determine
the equality of section contents. Therefore, exclude synthetic sections
from ICF.
Fixes bug PR36910.
Differential Revision: https://reviews.llvm.org/D44923
llvm-svn: 328624
Latest patch version now.
Original commit message:
[ELF] - Do not crash with --emit-relocs and --icf=all together.
Previously we would crash because did not mark .rel[a] sections
as dead and they tried to access parent which was not live
after ICF and therefore was null.
Differential revision: https://reviews.llvm.org/D43241
llvm-svn: 325879
GNU gold doesn't print out ICF sections for -verbose. It only shows
them for -print-icf-sections. We printed out them for -verbose because
we didn't have -print-icf-sections. Now that we have the option, there's
no reason to print out for -verbose.
Differential Revision: https://reviews.llvm.org/D43100
llvm-svn: 324755
Previously, multiple chunks of --print-icf-sections messages were interleaved
and didn't make sense. This is because forEachClass is multi-threaded.
llvm-svn: 324683
Currently ICF information is output through stderr if the "--verbose"
flag is used. This differs to Gold for example, which uses an explicit
flag to output this to stdout. This commit adds the
"--print-icf-sections" and "--no-print-icf-sections" flags and changes
the output message format for clarity and consistency with
"--print-gc-sections". These messages are still output to stderr if
using the verbose flag. However to avoid intermingled message output to
console, this will not occur when the "--print-icf-sections" flag is
used.
Existing tests have been modified to expect the new message format from
stderr.
Patch by Owen Reynolds.
Differential Revision: https://reviews.llvm.org/D42375
Reviewers: ruiu, rafael
Reviewed by:
llvm-svn: 323976
When we have --icf=safe we should be able to define --icf=all as a
shorthand for --icf=safe --ignore-function-address-equality.
For now --ignore-function-address-equality is used only to control
access to non preemptable symbols in shared libraries.
llvm-svn: 322152
An internal linker has support for merging identical data and in some
cases it can be a significant win.
This is behind an off by default flag so it has to be requested
explicitly.
llvm-svn: 320448
Now that DefinedRegular is the only remaining derived class of
Defined, we can merge the two classes.
Differential Revision: https://reviews.llvm.org/D39667
llvm-svn: 317448
Now that we have only SymbolBody as the symbol class. So, "SymbolBody"
is a bit strange name now. This is a mechanical change generated by
perl -i -pe s/SymbolBody/Symbol/g $(git grep -l SymbolBody lld/ELF lld/COFF)
nd clang-format-diff.
Differential Revision: https://reviews.llvm.org/D39459
llvm-svn: 317370
Summary:
This will allow using the functionality from other linkers. It is also
a prerequisite for sharing the error logging code.
Reviewers: ruiu
Reviewed By: ruiu
Subscribers: emaste, mgorny, llvm-commits
Differential Revision: https://reviews.llvm.org/D38822
llvm-svn: 315725
Computing section content hashes early seems like a win in terms of
performance. It increases a chance that two different sections will get
different class IDs from the beginning.
Without threads, this patch improves Chromium link time by about 0.3
seconds. With threads, by 0.1 seconds. That's less than 1% time saving
but not bad for a small patch.
llvm-svn: 314644
Relocations referring to merge sections are considered equal if they
resolve to the same offset in the same output section.
Differential Revision: https://reviews.llvm.org/D34094
llvm-svn: 305177
This creates a new library called BinaryFormat that has all of
the headers from llvm/Support containing structure and layout
definitions for various types of binary formats like dwarf, coff,
elf, etc as well as the code for identifying a file from its
magic.
Differential Revision: https://reviews.llvm.org/D33843
llvm-svn: 304864
This is the only place we use threads for ICF. The intention of this code
was to split an input vector into 256 shards and process them in parallel.
What the code was actually doing was to split an input into 257 shards,
process the first 256 shards in parallel, and the remaining one in serial.
That means this code takes ceil(256/n)+1 instead of ceil(256/n) where n
is the number of available CPU cores. The former converges to 2 while
the latter converges to 1.
This patches fixes the above issue.
llvm-svn: 303797
ARM Exception Index Table sections .ARM.exidx have an implicit dependency
on code sections via SHF_LINK_ORDER. When code sections are folded by ICF
we must mark the unique .ARM.exidx table that describes it as not live
to prevent an illegal entry in the exception table.
Note that we do not try and follow the relocations from the .ARM.exidx
section to the .ARM.extab sections to mark these as not live. Leaving
these sections is not a correctness problem. In theory these could be
removed via an application of garbage collection.
Fixes https://bugs.llvm.org/show_bug.cgi?id=32614
Differential Revision: https://reviews.llvm.org/D31932
llvm-svn: 300182
Peter Collingbourne