Linker and Libraries Guide

32-bit x86: General Dynamic (GD)

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

Table 8–8 32-bit x86: General Dynamic Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
0x00 leal x@tlsgd(,%ebx,1), %eax 0x07 call x@tlsgdplt # %eax - contains address of TLS variable	R_386_TLS_GD R_386_TLS_GD_PLT	x x
	Outstanding Relocations	Symbol
GOT[n] GOT[n + 1]	R_386_TLS_DTPMOD32 R_386_TLS_DTPOFF32	x

The leal instruction generates a R_386_TLS_GD relocation which instructs the link-editor to allocate space in the GOT to hold a TLS_index structure for variable x. The link-editor processes this relocation by substituting the GOT-relative offset for the new GOT entry.

Since the load object index and TLS block index for x are not known until runtime, the link-editor places the R_386_TLS_DTPMOD32 and R_386_TLS_DTPOFF32 relocations against the GOT for processing by the runtime linker. The address of the generated GOT entry is loaded into register %eax for the call to ___tls_get_addr().

The call instruction causes the generation of the R_386_TLS_GD_PLT relocation. This instructs the link-editor to bind the call to the ___tls_get_addr() function and associates the call instruction with the GD code sequence.

The call instruction must immediately follow the leal instruction. This requirement is necessary to permit the code transformations.