Linker and Libraries Guide

32-bit x86: Initial Executable (IE)

This code sequence implements the IE model described in Thread-Local Storage Access Models.

Two code-sequences for the IE model exist. One sequence is for position independent code which uses a GOT-pointer. The other sequence is for position dependent code which does not use a GOT-pointer.

Table 8–10 32-bit x86: Initial Executable, Position Independent, Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
0x00 movl %gs:0, %eax 0x06 addl x@gotntpoff(%ebx), %eax # %eax - contains address of TLS variable	<none> R_386_TLS_GOTIE	x
	Outstanding Relocations	Symbol
GOT[n]	R_386_TLS_TPOFF	x

The addl instruction generates a R_386_TLS_GOTIE relocation. This relocation instructs the link–editor to create space in the GOT to store the static TLS offset for symbol x. A R_386_TLS_TPOFF relocation is left outstanding against the GOT table for the runtime linker to fill in with the static TLS offset for symbol x.

Table 8–11 32-bit x86: Initial Executable, Position Dependent, Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
0x00 movl %gs:0, %eax 0x06 addl x@indntpoff, %eax # %eax - contains address of TLS variable	<none> R_386_TLS_IE	x
	Outstanding Relocations	Symbol
GOT[n]	R_386_TLS_TPOFF	x

The addl instruction generates a R_386_TLS_IE relocation. This relocation instructs the link-editor to create space in the GOT to store the static TLS offset for symbol x. The main difference between this sequence and the position independent form, is that the instruction is bound directly to the GOT entry created, instead of using an offset off of the GOT-pointer register. A R_386_TLS_TPOFF relocation is left outstanding against the GOT for the runtime linker to fill in with the static TLS offset for symbol x.

The contents of variable x, rather than the address, can be loaded by embedding the offset directly into the memory reference as shown in the next two sequences.

Table 8–12 32-bit x86: Initial Executable, Position Independent, Dynamic Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
0x00 movl x@gotntpoff(%ebx), %eax 0x06 movl %gs:(%eax), %eax # %eax - contains address of TLS variable	R_386_TLS_GOTIE <none>	x
	Outstanding Relocations	Symbol
GOT[n]	R_386_TLS_TPOFF	x

Table 8–13 32-bit x86: Initial Executable, Position Independent, Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
0x00 movl x@indntpoff, %ecx 0x06 movl %gs:(%ecx), %eax # %eax - contains address of TLS variable	R_386_TLS_IE <none>	x
	Outstanding Relocations	Symbol
GOT[n]	R_386_TLS_TPOFF	x

In the last sequence, if the %eax register is used instead of the %ecx register, the first instruction can be either 5 or 6 bytes long.