Intention of change is to get rid of code duplication.
Decompressor was introduced in D28105.

Change allows to get rid of few methods relative to decompression.

Differential revision: https://reviews.llvm.org/D28106

llvm-svn: 291758

The two overloaded functions hid each other. This patch merges them.

llvm-svn: 291222

This value is used only once, and we can compute a value.
So we don't need to save it.

llvm-svn: 290164

Use of CachedHashStringRef makes sense only when we reuse hash values.
Sprinkling it to all DenseMap has no benefits and just complicates data types.
Basically we shouldn't use CachedHashStringRef unless there is a strong
reason to to do so.

llvm-svn: 290076

This change seems to make LLD 0.6% faster when linking Clang with
debug info. I don't want us to have lots of local optimizations,
but this function is very hot, and the improvement is small but
not negligible, so I think it's worth doing.

llvm-svn: 288757

Also add a citation to GNU gold safe ICF paper.

Differential Revision: https://reviews.llvm.org/D27398

llvm-svn: 288684

Use "color" instead of "group id" to describe the ICF algorithm.

llvm-svn: 288409

ICF is short for Identical Code Folding. It is a size optimization to
identify two or more functions that happened to have the same contents
to merges them. It usually reduces output size by a few percent.

ICF is slow because it is computationally intensive process. I tried
to paralellize it before but failed because I couldn't make a
parallelized version produce consistent outputs. Although it didn't
create broken executables, every invocation of the linker generated
slightly different output, and I couldn't figure out why.

I think I now understand what was going on, and also came up with a
simple algorithm to fix it. So is this patch.

The result is very exciting. Chromium for example has 780,662 input
sections in which 20,774 are reducible by ICF. LLD previously took
7.980 seconds for ICF. Now it finishes in 1.065 seconds.

As a result, LLD can now link a Chromium binary (output size 1.59 GB)
in 10.28 seconds on my machine with ICF enabled. Compared to gold
which takes 40.94 seconds to do the same thing, this is an amazing
number.

From here, I'll describe what we are doing for ICF, what was the
previous problem, and what I did in this patch.

In ICF, two sections are considered identical if they have the same
section flags, section data, and relocations. Relocations are tricky,
becuase two relocations are considered the same if they have the same
relocation type, values, and if they point to the same section _in
terms of ICF_.

Here is an example. If foo and bar defined below are compiled to the
same machine instructions, ICF can (and should) merge the two,
although their relocations point to each other.

  void foo() { bar(); }
  void bar() { foo(); }

This is not an easy problem to solve.

What we are doing in LLD is some sort of coloring algorithm. We color
non-identical sections using different colors repeatedly, and sections
in the same color when the algorithm terminates are considered
identical. Here is the details:

  1. First, we color all sections using their hash values of section
  types, section contents, and numbers of relocations. At this moment,
  relocation targets are not taken into account. We just color
  sections that apparently differ in different colors.

  2. Next, for each color C, we visit sections having color C to see
  if their relocations are the same. Relocations are considered equal
  if their targets have the same color. We then recolor sections that
  have different relocation targets in new colors.

  3. If we recolor some section in step 2, relocations that were
  previously pointing to the same color targets may now be pointing to
  different colors. Therefore, repeat 2 until a convergence is
  obtained.

Step 2 is a heavy operation. For Chromium, the first iteration of step
2 takes 2.882 seconds, and the second iteration takes 1.038 seconds,
and in total it needs 23 iterations.

Parallelizing step 1 is easy because we can color each section
independently. This patch does that.

Parallelizing step 2 is tricky. We could work on each color
independently, but we cannot recolor sections in place, because it
will break the invariance that two possibly-identical sections must
have the same color at any moment.

Consider sections S1, S2, S3, S4 in the same color C, where S1 and S2
are identical, S3 and S4 are identical, but S2 and S3 are not. Thread
A is about to recolor S1 and S2 in C'. After thread A recolor S1 in
C', but before recolor S2 in C', other thread B might observe S1 and
S2. Then thread B will conclude that S1 and S2 are different, and it
will split thread B's sections into smaller groups wrongly. Over-
splitting doesn't produce broken results, but it loses a chance to
merge some identical sections. That was the cause of indeterminism.

To fix the problem, I made sections have two colors, namely current
color and next color. At the beginning of each iteration, both colors
are the same. Each thread reads from current color and writes to next
color. In this way, we can avoid threads from reading partial
results. After each iteration, we flip current and next.

This is a very simple solution and is implemented in less than 50
lines of code.

I tested this patch with Chromium and confirmed that this parallelized
ICF produces the identical output as the non-parallelized one.

Differential Revision: https://reviews.llvm.org/D27247

llvm-svn: 288373

They return new vectors, but at the same time they mutate other vectors,
so returning values doesn't make much sense. We should just mutate two
vectors.

llvm-svn: 287979

The function was used only within Relocations.cpp, but now we are
using it in many places, so this patch moves it to a file that fits
to the functionality.

llvm-svn: 287943

We have different functions to stringize objects to construct
error messages. For InputFile, we have getFilename, and for
InputSection, we have getName. You had to memorize them.

I think this is the case where the function overloading comes in handy.

This patch defines toString() functions that are overloaded for all these
types, so that you just call it in error().

Differential Revision: https://reviews.llvm.org/D27030

llvm-svn: 287787

Differential revision: https://reviews.llvm.org/D26914

llvm-svn: 287750

Previously, we set (uintptr_t)-1 to InputSectionBase::OutSec to record
that a section has already been set to be assigned to some output section
by linker scripts. Later, we restored nullptr to the pointer to use
the field for the original purpose. That overloading is not very easy to
understand.

This patch adds a bit flag for that purpose, so that we don't need
to piggyback the flag on an unrelated pointer.

llvm-svn: 287508

Also this patch uses file-scope functions instead of class member function.

Now that ICF class is not visible from outside, InputSection class
can no longer be "friend" of it. So I removed the friend relation
and just make it expose the features to public.

llvm-svn: 287480

MergeOutputSection class was a bit hard to use because it provdes
a series of finalize functions that have to be called in a right way
at a right time. It also intereacted with MergeInputSection, and the
logic was somewhat entangled between the two classes.

This patch simplifies it by providing only one finalize function.
Now, all you have to do is to call MergeOutputSection::finalize
when you have added all sections to the output section. Then, it
internally merges strings and initliazes StringPiece objects.
I think this is much easier to understand.

This patch also adds comments.

llvm-svn: 287314

llvm-svn: 286805

llvm-svn: 286557

Relocations are the last thing that we wore storing a raw section
pointer to and parsing on demand.

With this patch we parse it only once and store a pointer to the
actual data.

The patch also changes where we store it. It is now in
InputSectionBase. Not all sections have relocations, but most do and
this simplifies the logic. It also means that we now only support one
relocation section per section. Given that that constraint is
maintained even with -r with gold bfd and lld, I think it is OK.

llvm-svn: 286459

Differential revision: https://reviews.llvm.org/D26349

llvm-svn: 286443

The disadvantage is that we use uint64_t instad of uint32_t for some
value in 32 bit files. The advantage is a substantially simpler code,
faster builds and less code duplication.

llvm-svn: 286414

Previously, we have both input and output section for .MIPS.abiflags.
Now we have only one class for .MIPS.abiflags, which is MipsAbiFlagsSection.
This class is a synthetic input section.

.MIPS.abiflags sections are handled as regular sections until
the control reaches Writer. Writer then aggregates all sections
whose type is SHT_MIPS_ABIFLAGS to create a single synthesized
input section. The synthesized section is then processed normally
as if it came from an input file.

llvm-svn: 286398

Previously, we have both input and output sections for .reginfo and
.MIPS.options. Now for each such sections we have one synthetic input
sections: MipsReginfoSection and MipsOptionsSection respectively.

Both sections are handled as regular sections until the control reaches
Writer. Writer then aggregates all sections whose type is SHT_MIPS_REGINFO
or SHT_MIPS_OPTIONS to create a single synthesized input section. In that
moment Writer also save GP0 value to the MipsGp0 field of the corresponding
ObjectFile. This value required for R_MIPS_GPREL16 and R_MIPS_GPREL32
relocations calculation.

Differential revision: https://reviews.llvm.org/D26444

llvm-svn: 286397

It was quite confusing that it had SectionKind of Regular, but was not
actually a InputSection.

llvm-svn: 286379

llvm-svn: 286370

This reverts commit r286100.

This saves 8 bytes of every InputSection.

llvm-svn: 286235

Differential revision: https://reviews.llvm.org/D26281

llvm-svn: 286100

A CommonInputSection is a section containing all common symbols.
That was an input section but was abstracted in a different way
than the synthetic input sections because it was written before
the synthetic input section was invented.

This patch rewrites CommonInputSection as a synthetic input section
so that it behaves better with other sections.

llvm-svn: 286053

We are going to have many more classes for linker-synthesized
input sections, so it's worth to be added to a separate file
than to the file for regular input sections.

llvm-svn: 285740

It was only used by build-id and that can easily compute it.

llvm-svn: 285691

llvm-svn: 285683

Differential revision: https://reviews.llvm.org/D25627

llvm-svn: 285682

Differential revision: https://reviews.llvm.org/D26070

llvm-svn: 285680

Now that it is easy to create input section and symbols, this is
simple.

llvm-svn: 285322

This allows a non template class to hold input sections.

llvm-svn: 285221

llvm-svn: 285190

Instead of storing a pointer, store the members we need.

The reason for doing this is that it makes it far easier to create
synthetic sections. It also avoids reading data from files multiple
times., which might help with cross endian linking and host
architectures with slow unaligned access.

There are obvious compacting opportunities, but this already has mixed
results even on native x86_64 linking.

There is also the possibility of better refactoring the code for
handling common symbols, but this already shows that a custom class is
not necessary.

llvm-svn: 285148

llvm-svn: 285082

We were fairly inconsistent as to what information should be accessed
with getSectionHdr and what information (like alignment) was stored
elsewhere.

Now all section info has a dedicated getter. The code is also a bit
more compact.

llvm-svn: 285079

Builds were failing with:

  InputSection.h(139): error C2338: SectionPiece is too big

because MSVC does record layout differently, probably not packing the
'OutputOff' and 'Live' bitfields because their types are of different
size. Using size_t for 'Live' seems to fix it.

llvm-svn: 284740

We allocate a lot of these when linking debug info. This speeds up the
link of debug programs by 1% to 2%.

llvm-svn: 284716