From b67067f1176df6ee727450546b58704e4b588563 Mon Sep 17 00:00:00 2001 From: Nicholas Piggin Date: Wed, 24 Aug 2016 22:29:20 +1000 Subject: kbuild: allow archs to select link dead code/data elimination 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 Signed-off-by: Michal Marek --- include/linux/compiler.h | 23 +++++++++++++++++++++++ include/linux/export.h | 30 +++++++++++++++--------------- include/linux/init.h | 38 +++++++++++++------------------------- 3 files changed, 51 insertions(+), 40 deletions(-) (limited to 'include/linux') diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 1bb954842725..86130cded110 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -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; \ diff --git a/include/linux/export.h b/include/linux/export.h index c565f87f005e..337cb90f3668 100644 --- a/include/linux/export.h +++ b/include/linux/export.h @@ -1,5 +1,6 @@ #ifndef _LINUX_EXPORT_H #define _LINUX_EXPORT_H + /* * Export symbols from the kernel to modules. Forked from module.h * to reduce the amount of pointless cruft we feed to gcc when only @@ -42,27 +43,26 @@ extern struct module __this_module; #ifdef CONFIG_MODVERSIONS /* Mark the CRC weak since genksyms apparently decides not to * generate a checksums for some symbols */ -#define __CRC_SYMBOL(sym, sec) \ - extern __visible void *__crc_##sym __attribute__((weak)); \ - static const unsigned long __kcrctab_##sym \ - __used \ - __attribute__((section("___kcrctab" sec "+" #sym), unused)) \ +#define __CRC_SYMBOL(sym, sec) \ + extern __visible void *__crc_##sym __attribute__((weak)); \ + static const unsigned long __kcrctab_##sym \ + __used \ + __attribute__((section("___kcrctab" sec "+" #sym), used)) \ = (unsigned long) &__crc_##sym; #else #define __CRC_SYMBOL(sym, sec) #endif /* For every exported symbol, place a struct in the __ksymtab section */ -#define ___EXPORT_SYMBOL(sym, sec) \ - extern typeof(sym) sym; \ - __CRC_SYMBOL(sym, sec) \ - static const char __kstrtab_##sym[] \ - __attribute__((section("__ksymtab_strings"), aligned(1))) \ - = VMLINUX_SYMBOL_STR(sym); \ - extern const struct kernel_symbol __ksymtab_##sym; \ - __visible const struct kernel_symbol __ksymtab_##sym \ - __used \ - __attribute__((section("___ksymtab" sec "+" #sym), unused)) \ +#define ___EXPORT_SYMBOL(sym, sec) \ + extern typeof(sym) sym; \ + __CRC_SYMBOL(sym, sec) \ + static const char __kstrtab_##sym[] \ + __attribute__((section("__ksymtab_strings"), aligned(1))) \ + = VMLINUX_SYMBOL_STR(sym); \ + static const struct kernel_symbol __ksymtab_##sym \ + __used \ + __attribute__((section("___ksymtab" sec "+" #sym), used)) \ = { (unsigned long)&sym, __kstrtab_##sym } #if defined(__KSYM_DEPS__) diff --git a/include/linux/init.h b/include/linux/init.h index 6935d02474aa..e571fec4bb28 100644 --- a/include/linux/init.h +++ b/include/linux/init.h @@ -150,24 +150,8 @@ extern bool initcall_debug; #ifndef __ASSEMBLY__ -#ifdef CONFIG_LTO -/* Work around a LTO gcc problem: when there is no reference to a variable - * in a module it will be moved to the end of the program. This causes - * reordering of initcalls which the kernel does not like. - * Add a dummy reference function to avoid this. The function is - * deleted by the linker. - */ -#define LTO_REFERENCE_INITCALL(x) \ - ; /* yes this is needed */ \ - static __used __exit void *reference_##x(void) \ - { \ - return &x; \ - } -#else -#define LTO_REFERENCE_INITCALL(x) -#endif - -/* initcalls are now grouped by functionality into separate +/* + * initcalls are now grouped by functionality into separate * subsections. Ordering inside the subsections is determined * by link order. * For backwards compatibility, initcall() puts the call in @@ -175,12 +159,16 @@ extern bool initcall_debug; * * The `id' arg to __define_initcall() is needed so that multiple initcalls * can point at the same handler without causing duplicate-symbol build errors. + * + * Initcalls are run by placing pointers in initcall sections that the + * kernel iterates at runtime. The linker can do dead code / data elimination + * and remove that completely, so the initcall sections have to be marked + * as KEEP() in the linker script. */ #define __define_initcall(fn, id) \ static initcall_t __initcall_##fn##id __used \ - __attribute__((__section__(".initcall" #id ".init"))) = fn; \ - LTO_REFERENCE_INITCALL(__initcall_##fn##id) + __attribute__((__section__(".initcall" #id ".init"))) = fn; /* * Early initcalls run before initializing SMP. @@ -216,15 +204,15 @@ extern bool initcall_debug; #define __initcall(fn) device_initcall(fn) -#define __exitcall(fn) \ +#define __exitcall(fn) \ static exitcall_t __exitcall_##fn __exit_call = fn -#define console_initcall(fn) \ - static initcall_t __initcall_##fn \ +#define console_initcall(fn) \ + static initcall_t __initcall_##fn \ __used __section(.con_initcall.init) = fn -#define security_initcall(fn) \ - static initcall_t __initcall_##fn \ +#define security_initcall(fn) \ + static initcall_t __initcall_##fn \ __used __section(.security_initcall.init) = fn struct obs_kernel_param { -- cgit v1.2.3