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docs: Update comdat section attribute

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Signed-off-by: "Glücksmann, Igor" <igor.glucksmann@avast.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
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Igor Glucksmann 2021-09-17 16:54:03 +02:00 • committed by Cyrill Gorcunov
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commit 3f9fc2a3a7
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doc/nasmdoc.src
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@ -5896,6 +5896,44 @@ for data (and BSS) sections.
Informational sections get a default alignment of 1 byte (no
alignment), though the value does not matter.
\b \I{comdat, win32 attribute}\c{comdat=}, followed by a number
("selection"), colon (acting as a separator) and a name,
marks the section as a \I{COMDAT section, in win32}"COMDAT section".
It allows Microsoft linkers to perform function-level linking,
to deal with multiply defined symbols, to eliminate dead code/data.
The "selection" number should be one of the
\c{IMAGE_COMDAT_SELECT_*} constants from
\W{https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src/Debug/pe-format.md#comdat-sections-object-only}\c{COFF format specification};
this value controls if the linker allows multiply defined symbols
and how it handles them.
The name is the \I{COMDAT symbol, in win32}"COMDAT symbol"
- basically a new name for the section. So even though you have one
section given by the main name (e.g. \c{.text}), it can actually
consist of hundreds of COMDAT sections having their own name
(and alignment).
When the "selection" is IMAGE_COMDAT_SELECT_ASSOCIATIVE (5),
the following name is the "COMDAT symbol" of the associated COMDAT
section; this way you can link a piece of code or data only when
another piece of code or data gets actually linked.
\> So, when linking a NASM-compiled file with some C code,
the source may be structured as follows.
Note that the default \c{.text} section in handled in a special
way and it doesn't work well with \c{comdat}; you may want to append
a \c{$} character and an arbitrary suffix to the section name.
It will get linked into the \c{.text} section anyway - see the info on
\W{https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src/Debug/pe-format.md#grouped-sections-object-only}\c{Grouped Sections}.
\c section .text$1 align=16 comdat=1:FirstFnc
\c ... ; Code linked only if referenced from C
\c
\c section .text$1 align=16 comdat=1:SecondFnc
\c ... ; Code linked only if referenced from C
\c
\c section .rdata align=32 comdat=5:FirstFnc
\c ... ; Data linked only if the related code (FirstFnc) is linked
\c
The defaults assumed by NASM if you do not specify the above
qualifiers are: