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Plan for incremental linking with gold

I've worked out a sketch of how to implement incremental linking with
gold.  I hope to work on this at some point next year, though not
immediately.  I would be happy to hear any comments or suggestions.  I
would also be happy to hear if anybody is interested in tackling part
of this work.



The goal of incremental linking is to decrease link time by modifying
an existing executable or shared library rather than creating one from
scratch.  This is most effective when only a small number of input
files have changed since the output file was first linked.

Incremental linking is only designed to work when each link has the
same view of the file system, and file system timestamps are
consistent.  This will be true on a single machine, and should
normally be true when using various different machines with the same
NFS mounts.  Moving the incrementally linked output file to a machine
with a different filesystem view, and then running another incremental
link there, will generate correct output but will typically do a full
link rather than an incremental one.


The incremental linking interface is simple.  When the option
--incremental is passed to the linker, it checks whether the output
file already exists.  If it does, and if it contains incremental
linking information (described below), then the linker will check the
timestamp of the output file and of all input files.  The linker will
then only link in those input files which are newer than the
executable.  Whether the output file existed beforehand or not, the
linker will insert incremental linking information so that future
incremental links will succeed.

Thus during development it will be reasonable to always use the
--incremental option.  This should behave well in all cases, and
should significantly reduce link times when only a few input files
have changed.

For a release it will normally be desirable to do a full link without
the --incremental option.  You will not want to release a binary which
contains the incremental linking information, which can be sizeable.
Also, an incremental link produces output which is inherently
non-repeatable.  When it is desirable to get exactly the same output
for the same inputs, it is necessary to not use --incremental.

The goal is explicitly that whether or not you use --incremental on
the link line will not change the way the program behaves at runtime,
except for very minor performance differences.

That said, the --incremental option is incompatible with some options:
-r, --strip, --emit-relocs, possibly others.

For additional flexibility, the options --incremental-changed and
--incremental-unchanged may be used.  Every input file which appears
between --incremental-changed and --incremental-unchanged will be
assumed to have not changed since the last incremental link.  This may
be used by build systems which have extra knowledge about which files
have changed.

Internal implementation

We will implement gold to work efficiently in the normal case, and to
fall back to a normal complete link in less common cases.  We will
fall back to a normal link in these cases:

* The command line changes (except for uses of --incremental-changed
  and ---incremental-unchanged).

* A linker script changes.

* A global symbol which was previously defined in one input file is
  now defined in a different input file.

Note in particular that adding an input file to the link, or removing
one, will cause a fall back.

These restrictions could be sometimes avoided with some additional
work.  However, they are not the most important case, and the
algorithm is considerably simplified if we do not have to consider
changes to symbol overriding.

The linker will parse the command line as usual and build a list of
input files.  It will look for the output file, and extract the list
of input files from the last link.  If any linker script is specified
with the --script option, it will be treated as the first input file.

gold will walk through the input files as usual in the first pass.
For each input file, gold will check the timestamp (or rely on the
--incremental-unchanged option).

If input file is new or output file is dropped:
  * Recompute everything.

If timestamps identical:
  * If object or shared library:
    + Read file's global symbols from output file.
  * If archive:
    + All members will be unchanged; skip them all.
  * If linker script:
    + Read linker script from input file.

If timestamps differ:
  * If object or shared library:
    + If file changed from object to shared library or vice-versa,
      recompute everything.
    + Read symbol table from input file.
    + Read file's global symbols from output file.
    + For any newly defined symbols:
      - If the symbol was previously defined by a later input file,
        recompute everything.
    + For any symbols which used to be defined by this file but no
      longer are:
      - If the symbol was ever referenced, recompute everything.
  * If archive:
    + Treat each member as a changed file.
  * If linker script:
    + Recompute everything.

After this process, we have a list of input files which are changed,
and we have a list of global symbols which changed (because they are
defined by an input file which changed).

The next gold pass is to scan relocations.  gold must scan the
relocations of any input object files which changed.

Next is layout.  Unchanged input files keep the same layout.  For
changed files we walk through the input sections.  For each section we
are keeping, we use the same layout if there is room available.
Otherwise we must allocate new space in an appropriate segment.

In the final pass gold will process the relocations for every file
which changed, and for every file which refers to a symbol which
changed.  For speed of handling changed symbols, for each global
symbol, we record a list of relocations which must be recalculated.

Handling debugging information which changes size may require
modifications to gdb to support non-contiguous debugging information.
(Or maybe we will just always put the debugging information at the end
of the file).

To implement this algorithm, we need the following information:
  * A list of all input files with timestamps.
  * For each input object file or shared library:
    + A list of global symbols.
    + For each input object file:
      - A list of where input sections are in the output file, and how
        large they are.
  * For each global symbol:
    + A list of relocations to apply if the value changes.
  * For each segment:
    + Available space.

Data storage

The section .gnu_incremental_inputs will list the input files used for
the link.  It will have some specific section type (TBD).  The
SHF_ALLOC flag will be clear.  The sh_link field will refer to a
section .gnu_incremental_strtab which will have type SHT_STRTAB and
will be a normal string table, again with SHF_ALLOC clear.

.gnu_incremental_inputs data format:

4 byte version number
4 byte input file count
  * In the order in which the files are included in the link.
  * If --script is used, the first input file is the script.
4 byte offset of command line options in .gnu_incremental_strtab
4 byte padding

For each input file:
  4 byte offset of file name in .gnu_incremental_strtab section
    * This is the file name which the linker opens.
  4 byte offset of data in this section
    * offset from start of section to data for this file.
  8 byte modification time seconds
    * The value of the st_mtime field of a stat of the file (zero
      extended to eight bytes if necessary).
  4 byte modification time nanoseconds
    * If available.
  2 byte type of file
    * object, shared library, archive, or linker script.
  2 byte padding

The data found at the offset in this section:

For a linker script:
  * Nothing additional.  Offset may be zero.

For an object file or shared library:
  4 byte count of input sections
    * Always zero for a shared library.
  4 byte count of global symbols
  For each input section:
    4 byte offset to section name in .gnu_incremental_strtab section
    4 byte output section index
      * If input section is omitted, this is zero.
    8 byte offset of input section in output section
    8 byte input section size
  For each global symbol defined or referenced by the file:
    4 byte index into output global symbol table
    4 byte offset to next linked list entry
      * Linked list starts from global symbol list.
      * Offset is to entry in input file list.
    4 byte count of relocations for this symbol for this file
    4 byte offset to relocations in .gnu_incremental_relocs section

For an archive:  
  4 byte count of members
  For each member:
    4 byte offset of member in input file list
    4 byte padding

The section .gnu_incremental_symtab will list the global symbols.  It
will have some specific section type (TBD).  The SHF_ALLOC flag flag
will be clear.  The sh_link field will point to the
.gnu_incremental_inputs section.  The contents will exactly parallel
the global symbols in the output file symbol table.

For each global symbol:
  4 byte offset in .gnu_incremental_inputs section
    * This is the start of a linked list of object files.
    * Each object file which mentions the symbol will be on the list.

The section .gnu_incremental_relocs will list relocations.  It will
have some specific section type (TBD).  The SHF_ALLOC flag will be
clear.  The contents will be largely processor dependent.  Each object
which refers to a global symbol will have a relocation count and a
relocation offset into this section, as described above.  Typical
section contents will depend on whether this is a 32-bit or a 64-bit

For each relocation entry:
  * 4 or 8 byte relocation type
  * 4 or 8 byte VMA in output file
  * 4 or 8 byte addend

The typical relocation computation for a REL relocation will be to
first write the addend field into the output section contents,
replacing the current value, and then run the usual relocation
routine.  The typical relocation computation for a RELA relocation
will be unchanged.  However, specific processors may adopt different
implementations where appropriate.  The machine independent part of
the linker only needs to know the count and size of the relocation

To find free space in each segment, the incremental linker will looks
for gaps between input sections within segments.  The incremental
linker will normally allocate some extra space for each input section
when possible, in order to easily accomodate small increases in
section size.  When the incremental linker first creates an output
file it will also allocate some amount of space in such a section at
the end of each segment.  The incremental linker will create new
segments as needed to get more space; it may be necessary to disable
this feature on certain operating systems, in which case it will
fallback to a full relink.

In general the incremental linker should always compute essentially
the same .dynamic, .dynsym, and .dynstr sections.

Interaction with concurrent linking

In concurrent linking the linker does not assume that input files are
available until notified by an external process.  An incremental
concurrent link will only affect the first pass, in which the linker
determines what has changed.  The linker will not complete the first
pass until all input files are available.

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