[lld/ELF] Add documentation on large sections

lld/docs/ELF/large_sections.rst

@@ -0,0 +1,52 @@
+Large data sections
+===================
+
+When linking very large binaries, lld may report relocation overflows like
+
+::
+
+  relocation R_X86_64_PC32 out of range: 2158227201 is not in [-2147483648, 2147483647]
+
+This happens when running into architectural limitations. For example, in x86-64
+PIC code, a reference to a static global variable is typically done with a
+``R_X86_64_PC32`` relocation, which is a 32-bit signed offset from the PC. That
+means if the global variable is laid out further than 2GB (2^31 bytes) from the
+instruction referencing it, we run into a relocation overflow.
+
+lld normally lays out sections as follows:
+
+.. image:: section_layout.png
+
+The largest relocation pressure is usually from ``.text`` to the beginning of
+``.rodata`` or ``.text`` to the end of ``.bss``.
+
+Some code models offer a tradeoff between relocation pressure and performance.
+For example, x86-64's medium code model splits global variables into small and
+large globals depending on if their size is over a certain threshold. Large
+globals are placed further away from text and we use 64-bit references to refer
+to them.
+
+Large globals are placed in separate sections from small globals, and those
+sections have a "large" section flag, e.g. ``SHF_X86_64_LARGE`` for x86-64. The
+linker places large sections on the outer edges of the binary, making sure they
+do not affect affect the distance of small globals to text. The large versions
+of ``.rodata``, ``.bss``, and ``.data`` are ``.lrodata``, ``.lbss``, and
+``.ldata``, and they are laid out as follows:
+
+.. image:: large_section_layout_pic.png
+
+We try to keep the number of ``PT_LOAD`` segments to a minimum, so we place
+large sections next to the small sections with the same RWX permissions when
+possible.
+
+``.lbss`` is right after ``.bss`` so that they are merged together and we
+minimize the number of segments with ``p_memsz > p_filesz``.
+
+Note that the above applies to PIC code. For less common non-PIC code with
+absolute relocations instead of relative relocations, 32-bit relocations
+typically assume that symbols are in the lower 2GB of the address space. So for
+non-PIC code, large sections should be placed after all small sections to avoid
+``.lrodata`` pushing small symbols out of the lower 2GB of the address space.
+``-z lrodata-after-bss`` changes the layout to be:
+
+.. image:: large_section_layout_nopic.png

lld/docs/index.rst

@@ -166,6 +166,7 @@ document soon.
    error_handling_script
    Partitions
    ReleaseNotes
+   ELF/large_sections
    ELF/linker_script
    ELF/start-stop-gc
    ELF/warn_backrefs