forked from OSchip/llvm-project
78 lines
3.0 KiB
ReStructuredText
78 lines
3.0 KiB
ReStructuredText
Linker Script implementation notes and policy
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=============================================
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LLD implements a large subset of the GNU ld linker script notation. The LLD
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implementation policy is to implement linker script features as they are
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documented in the ld `manual <https://sourceware.org/binutils/docs/ld/Scripts.html>`_
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We consider it a bug if the lld implementation does not agree with the manual
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and it is not mentioned in the exceptions below.
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The ld manual is not a complete specification, and is not sufficient to build
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an implementation. In particular some features are only defined by the
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implementation and have changed over time.
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The lld implementation policy for properties of linker scripts that are not
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defined by the documentation is to follow the GNU ld implementation wherever
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possible. We reserve the right to make different implementation choices where
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it is appropriate for LLD. Intentional deviations will be documented in this
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file.
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Output section description
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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The description of an output section looks like:
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::
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section [address] [(type)] : [AT(lma)] [ALIGN(section_align)] [SUBALIGN](subsection_align)] {
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output-section-command
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...
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} [>region] [AT>lma_region] [:phdr ...] [=fillexp] [,]
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Output section address
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----------------------
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When an *OutputSection* *S* has ``address``, LLD will set sh_addr to ``address``.
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The ELF specification says:
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> The value of sh_addr must be congruent to 0, modulo the value of sh_addralign.
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The presence of ``address`` can cause the condition unsatisfied. LLD will warn.
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GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that
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sh_addr=0 (modulo sh_addralign).
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Output section alignment
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------------------------
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sh_addralign of an *OutputSection* *S* is the maximum of
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``ALIGN(section_align)`` and the maximum alignment of the input sections in
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*S*.
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When an *OutputSection* *S* has both ``address`` and ``ALIGN(section_align)``,
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GNU ld will set sh_addralign to ``ALIGN(section_align)``.
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Output section LMA
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------------------
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A load address (LMA) can be specified by ``AT(lma)`` or ``AT>lma_region``.
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- ``AT(lma)`` specifies the exact load address. If the linker script does not
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have a PHDRS command, then a new loadable segment will be generated.
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- ``AT>lma_region`` specifies the LMA region. The lack of ``AT>lma_region``
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means the default region is used. Note, GNU ld propagates the previous LMA
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memory region when ``address`` is not specified. The LMA is set to the
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current location of the memory region aligned to the section alignment.
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If the linker script does not have a PHDRS command, then if
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``lma_region`` is different from the ``lma_region`` for
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the previous OutputSection a new loadable segment will be generated.
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The two keywords cannot be specified at the same time.
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If neither ``AT(lma)`` nor ``AT>lma_region`` is specified:
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- If the previous section is also in the default LMA region, the difference
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between the LMA and the VMA is computed to be the same as the previous
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difference.
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- Otherwise, the LMA is set to the VMA.
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