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Dead-Code Stripping

The static linker (ld) supports the removal of unused code and data blocks from executable files. This process (known as dead-code stripping) helps reduce the overall size of executables, which in turn improves performance by reducing the memory footprint of the executable. It also allows programs to link successfully in the situation where unused code refers to an undefined symbol, something that would normally result in a link error.

Dead-code stripping is not limited to removing only unused functions and executable code from a binary. The linker also removes any unused symbols and data that reside in data blocks. Such symbols might include global variables, static variables, and string data among others.

When dead-code stripping is enabled, the static linker searches for code that is unreachable from an initial set of live symbols and blocks. The initial list of live symbols and blocks may include the following:

• Symbols listed in an exports file; alternatively, the symbols absent from a list of items marked as not to be exported. For dynamic libraries or bundles without an exports file, all global symbols are part of the initial live list. See “Preventing the Stripping of Unused Symbols” for more information.

• The block representing the default entry point or the symbol listed after the -e linker option, either of which identifies the specific entry point for an executable. See the ld man page for more information on the -e option.

• The symbol listed after the -init linker option, which identifies the initialization routine for a shared library. See the ld man page for more information.

• Symbols whose declaration includes the used attribute. See “Preventing the Stripping of Unused Symbols” for more information.

• Objective-C runtime data.

• Symbols marked as being referenced dynamically (via the REFERENCED_DYNAMICALLY bit in /usr/include/mach-o/nlist.h).

In this section:

Enabling Dead-Code Stripping in Your Project
Identifying Stripped Symbols
Preventing the Stripping of Unused Symbols
Assembly Language Support

Enabling Dead-Code Stripping in Your Project

To enable dead-code stripping in your project, you must pass the appropriate command-line options to the linker. From Xcode, you add these options in the Build pane of the target inspector; otherwise, you must add these options to your makefile or build scripts. Table 28-2 lists the Xcode build settings that control dead-code stripping. Enabling either of these build settings causes Xcode to build with the corresponding linker option.

Table 28-1 lists the basic dead-code stripping options.

You must recompile all object files using the compiler included with Xcode 1.5 or later before dead-code stripping can be performed by the linker. You must also compile the object files with the -gfull option to ensure that the resulting binaries can be properly debugged. In Xcode, change the value of the Level of Debug Symbols (GCC_DEBUGGING_SYMBOLS) build setting to All Symbols (-gfull).

Identifying Stripped Symbols

If you want to know what symbols were stripped by the static linker, you can find out by examining the linker-generated load map. This map lists all of the segments and sections in the linked executable and also lists the dead-stripped symbols. To have the linker generate a load map, add the -M option to your linker command-line options. In Xcode, you can add this option to the Other Linker Flags build setting.

Preventing the Stripping of Unused Symbols

If your executable contains symbols that you know should not be stripped, you need to notify the linker that the symbol is actually used. You must prevent the stripping of symbols in situations where external code (such as plug-ins) use those symbols but local code does not.

There are two ways to tell the linker not to dead strip a symbol. You can include it in an exports file or mark the symbol declaration explicitly. To mark the declaration of a symbol, you include the used attribute as part of its definition. For example, you would mark a function as used by declaring it as follows:

void MyFunction(int param1) __attribute__((used));

Alternatively, you can provide an exports list for your executable that lists any symbols you expect to be used by plug-ins or other external code modules. To specify an exports file from Xcode, use the Exported Symbols File (EXPORTED_SYMBOLS_FILE) build setting; enter the path, relative to the project directory, to the exports file. To specify an exports file from the linker command line use the -exported_symbols_list. option. (You can also specify a list of symbols not to export using the -unexported_symbols_list option.)

If you are using an exports list and building either a shared library, or an executable that will be used with ld's -bundle_loader flag, you need to include the symbols for exception frame information in the exports list for your exported C++ symbols. Otherwise, they may be stripped. These symbols end with .eh; you can view them with the nm tool.

Assembly Language Support

If you are writing assembly language code, the assembler now recognizes some additional directives to preserve or enhance the dead-stripping of code and data. You can use these directives to flag individual symbols or entire sections of assembly code.

Preserving Individual Symbols

To prevent the dead stripping of an individual symbol, use the .no_dead_strip directive. For example, the following code prevents the specified string from being dead stripped:

.no_dead_strip _my_version_string

.cstring

_my_version_string:

.ascii "Version 1.1"

Preserving Sections

To prevent an entire section from being dead stripped, add the no_dead_strip attribute to the section declaration. The following example demonstrates the use of this attribute:

.section __OBJC, __image_info, regular, no_dead_strip

You can also add the live_support attribute to a section to prevent it from being dead stripped if it references other code that is live. This attribute prevents the dead stripping of some code that might actually be needed but not referenced in a detectable way. For example, the compiler adds this attribute to C++ exception frame information. In your code, you might use the attribute as follows:

.section __TEXT, __eh_frame, coalesced, no_toc+strip_static_syms+live_support

Dividing Blocks of Symbols

The .subsections_via_symbols directive notifies the assembler that the contents of sections may be safely divided into individual blocks prior to dead-code stripping. This directive makes it possible for individual symbols to be stripped out of a given section if they are not used. This directive applies to all section declarations in your assembly file and should be placed outside of any section declarations, as shown below:

.subsections_via_symbols

; Section declarations...

If you use this directive, make sure that each symbol in the section marks the beginning of a separate block of code. Implicit dependencies between blocks of code might result in the removal of needed code from the executable. For example, the following section contains three individual symbols, but execution of the code at _plus_three ends at the blr statement at the bottom of the code block.

.text

.globl _plus_three

_plus_three:

addi r3, r3, 1

.globl _plus_two

_plus_two:

addi r3, r3, 1

.global _plus_one

_plus_one:

addi r3, r3, 1

blr

If you were to use the .subsections_via_symbols directive on this code, the assembler would permit the stripping of the symbols _plus_two and _plus_one if they were not called by any other code. If this occurred, _plus_three would no longer return the correct value because part of its code would be missing. In addition, if _plus_one were dead stripped, the code might crash as it continued executing into the next block.

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