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    Subject[PATCH 0/3] build system: section garbage collection for vmlinux
    Build system: section garbage collection for vmlinux

    Newer gcc and binutils can do dead code and data removal
    at link time. It is achieved using combination of
    -ffunction-sections -fdata-sections options for gcc and
    --gc-sections for ld.

    Theory of operation:

    Option -ffunction-sections instructs gcc to place each function
    (including static ones) in it's own section named .text.function_name
    instead of placing all functions in one big .text section.

    At link time, ld normally coalesce all such sections into one
    output section .text again. It is achieved by having *(.text.*) spec
    along with *(.text) spec in built-in linker scripts.

    If ld is invoked with --gc-sections, it tracks references, starting
    from entry point and marks all input sections which are reachable
    from there. Then it discards all input sections which are not marked.

    This isn't buying much if you have one big .text section per .o module,
    because even one referenced function will pull in entire section.
    You need -ffunction-sections in order to split .text into per-function
    sections and make --gc-sections much more useful.

    -fdata-sections is analogous: it places each global or static variable
    into .data.variable_name, .rodata.variable_name or .bss.variable_name.

    How to use it in kernel:

    First, we need to adapt existing code for new section names.
    Basically, we need to stop using section names of the form
    in the kernel for - otherwise section placement done by kernel's
    custom linker scripts produces broken vmlinux and vdso images.

    Second, kernel linker scripts need to be adapted by adding KEEP(xxx)
    directives around sections which are not directly referenced, but are
    nevertheless used (initcalls, altinstructions, etc).

    These patches fix section names and add
    unconditionally because only newest binutils have
    ld --gc-sections which is stable enough for kernel use.
    IOW: this is an experimental feature for now.

    Patches are conservative and mark a lot of things with
    KEEP() directive in linker script, inhibiting GC for them.
    Size savings are modest:

    text data bss dec hex filename
    5159478 1005139 406784 6571401 644589
    5131822 996090 401439 6529351 63a147 linux-2.6.23-rc4.gc/vmlinux

    In this particular case, 402 objects were discarded, saving 42 kb.

    Linker is unable to discard more because current infrastructure
    is a bit flawed in this regard. It prevents some unused code
    from being detected. In particular:
    KEEP(__ex_table) -> .fixup -> get_user and friends
    KEEP(.smp_locks) -> lock prefixes

    This is an experimental build where KEEPs for them were removed:

    text data bss dec hex filename
    5131822 996090 401439 6529351 63a147 vmlinux
    5065681 996090 401439 6463210 629eea vmlinux.sans_KEEP

    52k of difference are due to __ex_table and .smp_locks being removed,
    the remaining ~13k is genuinely unused .text.* sections.

    Patches were run-tested on x86_64, and likely do not work on any other arch
    (need to add KEEP() to arch/*/kernel/ for each arch).
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      Last update: 2007-09-05 15:45    [W:0.030 / U:184.576 seconds]
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