Function sections - Revision history

Cschalle: Add category

2011-10-28T10:33:43Z

Add category

Peter Huewe at 22:26, 27 October 2011

2011-10-27T22:26:32Z

Tim Bird: Create page describing function sections feature

2008-07-02T03:03:23Z

Create page describing function sections feature

New page

== Introduction ==
"Function sections" is a technique for reducing the size of the kernel image.
It does this by placing each function into it's own linker section, which then
allows the linker to do better dead code removal.

Denys reported that usage of this technique got him about a 10% reduction in kernel
size.

== Theory of operation ==
Denys Vlasenko wrote:

-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 coalesces 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.

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

This doesn't buy much if you have one big .text section per .o module,
because even one referenced function will pull in entire section.
However, if you use -ffunction-sections to split .text into per-function
sections it makes --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.

== Status ==
Denys submitted patches in July 2008 to make the kernel compilable using
"gcc -ffunction-sections -fdata-sections". See http://lkml.org/lkml/2008/7/1/499

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	Denys submitted patches in July 2008 to make the kernel compilable using		Denys submitted patches in July 2008 to make the kernel compilable using
	"gcc -ffunction-sections -fdata-sections". See http://lkml.org/lkml/2008/7/1/499		"gcc -ffunction-sections -fdata-sections". See http://lkml.org/lkml/2008/7/1/499
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 == Introduction ==
 "Function sections" is a technique for reducing the size of the kernel image.
-It does this by placing each function into it's own linker section, which then
+It does this by placing each function into its own linker section, which then
 allows the linker to do better dead code removal.
@@ Line 10: / Line 10: @@
 Denys Vlasenko wrote:
--ffunction-sections instructs gcc to place each function
+ -ffunction-sections instructs gcc to place each function
-(including static ones) in it's own section named .text.function_name
+ (including static ones) in its own section named .text.function_name
-instead of placing all functions in one big .text section.
+ instead of placing all functions in one big .text section.
-At link time, ld normally coalesces all such sections into one
+ At link time, ld normally coalesces all such sections into one
-output section .text again. It is achieved by having *(.text.*) spec
+ output section .text again. It is achieved by having *(.text.*) spec
-along with *(.text) spec in built-in linker scripts.
+ along with *(.text) spec in built-in linker scripts.
-However, if ld is invoked with the option --gc-sections, it tracks references, starting
+ However, if ld is invoked with the option --gc-sections, it tracks references, starting
-from the entry point and marks all input sections which are reachable
+ from the entry point and marks all input sections which are reachable
-from there. Then it discards all input sections which are not marked.
+ from there. Then it discards all input sections which are not marked.
-This doesn't buy much if you have one big .text section per .o module,
+ This doesn't buy much if you have one big .text section per .o module,
-because even one referenced function will pull in entire section.
+ because even one referenced function will pull in entire section.
-However, if you use -ffunction-sections to split .text into per-function
+ However, if you use -ffunction-sections to split .text into per-function
-sections it makes --gc-sections much more useful.
+ sections it makes --gc-sections much more useful.
--fdata-sections is analogous: it places each global or static variable
+ -fdata-sections is analogous: it places each global or static variable
-into .data.variable_name, .rodata.variable_name or .bss.variable_name.
+ into .data.variable_name, .rodata.variable_name or .bss.variable_name.
 == Status ==
 Denys submitted patches in July 2008 to make the kernel compilable using
 "gcc -ffunction-sections -fdata-sections".  See http://lkml.org/lkml/2008/7/1/499