1Linker Script implementation notes and policy
2=============================================
3
4LLD implements a large subset of the GNU ld linker script notation. The LLD
5implementation policy is to implement linker script features as they are
6documented in the ld `manual <https://sourceware.org/binutils/docs/ld/Scripts.html>`_
7We consider it a bug if the lld implementation does not agree with the manual
8and it is not mentioned in the exceptions below.
9
10The ld manual is not a complete specification, and is not sufficient to build
11an implementation. In particular some features are only defined by the
12implementation and have changed over time.
13
14The lld implementation policy for properties of linker scripts that are not
15defined by the documentation is to follow the GNU ld implementation wherever
16possible. We reserve the right to make different implementation choices where
17it is appropriate for LLD. Intentional deviations will be documented in this
18file.
19
20Symbol assignment
21~~~~~~~~~~~~~~~~~
22
23A symbol assignment looks like:
24
25::
26
27  symbol = expression;
28  symbol += expression;
29
30The first form defines ``symbol``. If ``symbol`` is already defined, it will be
31overridden. The other form requires ``symbol`` to be already defined.
32
33For a simple assignment like ``alias = aliasee;``, the ``st_type`` field is
34copied from the original symbol. Any arithmetic operation (e.g. ``+ 0`` will
35reset ``st_type`` to ``STT_NOTYPE``.
36
37The ``st_size`` field is set to 0.
38
39Output section description
40~~~~~~~~~~~~~~~~~~~~~~~~~~
41
42The description of an output section looks like:
43
44::
45
46  section [address] [(type)] : [AT(lma)] [ALIGN(section_align)] [SUBALIGN](subsection_align)] {
47    output-section-command
48    ...
49  } [>region] [AT>lma_region] [:phdr ...] [=fillexp] [,]
50
51Output section address
52----------------------
53
54When an *OutputSection* *S* has ``address``, LLD will set sh_addr to ``address``.
55
56The ELF specification says:
57
58> The value of sh_addr must be congruent to 0, modulo the value of sh_addralign.
59
60The presence of ``address`` can cause the condition unsatisfied. LLD will warn.
61GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that
62sh_addr=0 (modulo sh_addralign).
63
64Output section alignment
65------------------------
66
67sh_addralign of an *OutputSection* *S* is the maximum of
68``ALIGN(section_align)`` and the maximum alignment of the input sections in
69*S*.
70
71When an *OutputSection* *S* has both ``address`` and ``ALIGN(section_align)``,
72GNU ld will set sh_addralign to ``ALIGN(section_align)``.
73
74Output section LMA
75------------------
76
77A load address (LMA) can be specified by ``AT(lma)`` or ``AT>lma_region``.
78
79- ``AT(lma)`` specifies the exact load address. If the linker script does not
80  have a PHDRS command, then a new loadable segment will be generated.
81- ``AT>lma_region`` specifies the LMA region. The lack of ``AT>lma_region``
82  means the default region is used. Note, GNU ld propagates the previous LMA
83  memory region when ``address`` is not specified. The LMA is set to the
84  current location of the memory region aligned to the section alignment.
85  If the linker script does not have a PHDRS command, then if
86  ``lma_region`` is different from the ``lma_region`` for
87  the previous OutputSection a new loadable segment will be generated.
88
89The two keywords cannot be specified at the same time.
90
91If neither ``AT(lma)`` nor ``AT>lma_region`` is specified:
92
93- If the previous section is also in the default LMA region, and the two
94  section have the same memory regions, the difference between the LMA and the
95  VMA is computed to be the same as the previous difference.
96- Otherwise, the LMA is set to the VMA.
97