3.4.122.1 Position-Independent Code:

Use -xcode=pic13 or -xcode=pic32 when creating dynamic shared libraries to improve runtime performance.

While the code within a dynamic executable is usually tied to a fixed address in memory, position-independent code can be loaded anywhere in the address space of the process.

When you use position-independent code, relocatable references are generated as an indirect reference through a global offset table. Frequently accessed items in a shared object will benefit from compiling with -xcode=pic13 or -xcode=pic32 by not requiring the large number of relocations imposed by code that is not position-independent.

The size of the global offset table is limited to 8Kb.

There are two nominal performance costs with -xcode={pic13|pic32} :

• A routine compiled with either -xcode=pic13 or -xcode=pic32 executes a few extra instructions upon entry to set a register to point at the global offset table used for accessing a shared library’s global or static variables.

• Each access to a global or static variable involves an extra indirect memory reference through the global offset table. If the compile is done with pic32, there are two additional instructions per global and static memory reference.

When considering the above costs, remember that the use of -xcode=pic13 or -xcode=pic32 can significantly reduce system memory requirements, due to the effect of library code sharing. Every page of code in a shared library compiled -xcode=pic13 or -xcode=pic32 can be shared by every process that uses the library. If a page of code in a shared library contains even a single non-pic (that is, absolute) memory reference, the page becomes nonsharable, and a copy of the page must be created each time a program using the library is executed.

The easiest way to tell whether or not a .o file has been compiled with -xcode=pic13 or -xcode=pic32 is with the nm command:

nm file.o | grep _GLOBAL_OFFSET_TABLE_

A .o file containing position-independent code will contain an unresolved external reference to _GLOBAL_OFFSET_TABLE_ as marked by the letter U.

To determine whether to use -xcode=pic13 or -xcode=pic32, check the size of the Global Offset Table (GOT) by using elfdump -c (see the elfdump(1) man page for more information) and for the section header, sh_name: .got. The sh_size value is the size of the GOT. If the GOT is less than 8,192 bytes, specify -xcode=pic13, otherwise specify -xcode=pic32.

In general, use the following guidelines to determine how you should use -xcode:

• If you are building an executable you should not use -xcode=pic13 or -xcode=pic32.

• If you are building an archive library only for linking into executables you should not use -xcode=pic13 or -xcode=pic32.

• If you are building a shared library, start with -xcode=pic13 and once the GOT size exceed 8,192 bytes, use -xcode=pic32.

• If you are building an archive library for linking into shared libraries you should just use -xcode=pic32.

Compiling with the -xcode=pic13 or pic32 (or -pic or -PIC) options is recommended when building dynamic libraries. See the Solaris Linker and Libraries Guide.