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 .text.xxxx
.data.xxxx
.rodata.xxxx
.bss.xxxx
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 CONFIG_DISCARD_UNUSED_SECTIONS. It is not enabled 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 linux-2.6.23-rc4.org/vmlinux
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/vmlinux.lds.S for each arch). --
vda