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kbuild: allow archs to select link dead code/data elimination

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Introduce LD_DEAD_CODE_DATA_ELIMINATION option for architectures to
select to build with -ffunction-sections, -fdata-sections, and link
with --gc-sections. It requires some work (documented) to ensure all
unreferenced entrypoints are live, and requires toolchain and build
verification, so it is made a per-arch option for now.

On a random powerpc64le build, this yelds a significant size saving,
it boots and runs fine, but there is a lot I haven't tested as yet, so
these savings may be reduced if there are bugs in the link.

    text      data        bss        dec   filename
11169741   1180744    1923176	14273661   vmlinux
10445269   1004127    1919707	13369103   vmlinux.dce

~700K text, ~170K data, 6% removed from kernel image size.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>
This commit is contained in:
Nicholas Piggin 2016-08-24 22:29:20 +10:00 • committed by Michal Marek
parent a5967db9af
commit b67067f117
7 changed files with 104 additions and 63 deletions
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23
include/linux/compiler.h
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@ -182,6 +182,29 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
# define unreachable() do { } while (1)
#endif
/*
* KENTRY - kernel entry point
* This can be used to annotate symbols (functions or data) that are used
* without their linker symbol being referenced explicitly. For example,
* interrupt vector handlers, or functions in the kernel image that are found
* programatically.
*
* Not required for symbols exported with EXPORT_SYMBOL, or initcalls. Those
* are handled in their own way (with KEEP() in linker scripts).
*
* KENTRY can be avoided if the symbols in question are marked as KEEP() in the
* linker script. For example an architecture could KEEP() its entire
* boot/exception vector code rather than annotate each function and data.
*/
#ifndef KENTRY
# define KENTRY(sym) \
extern typeof(sym) sym; \
static const unsigned long __kentry_##sym \
__used \
__attribute__((section("___kentry" "+" #sym ), used)) \
= (unsigned long)&sym;
#endif
#ifndef RELOC_HIDE
# define RELOC_HIDE(ptr, off) \
({ unsigned long __ptr; \