x86: Local Dynamic (LD)

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

Table 16-9 32-bit x86: Local Dynamic Thread-Local Variable Access Codes

Code Sequence	Initial Relocations	Symbol
0x00 leal x1@tlsldm(%ebx), %eax 0x06 call x1@tlsldmplt # %eax - contains address of TLS block of current object 0x10 leal x1@dtpoff(%eax), %edx # %edx - contains address of local TLS variable x1 0x20 leal x2@dtpoff(%eax), %edx # %edx - contains address of local TLS variable x2	R_386_TLS_LDM R_386_TLS_LDM_PLT R_386_TLS_LDO_32 R_386_TLS_LDO_32	x1 x1 x1 x2
→	Outstanding Relocations	Symbol
GOT[n] GOT[n + 1]	R_386_TLS_DTPMOD32 <none>	x

The first leal instruction generates a R_386_TLS_LDM relocation. This relocation instructs the link-editor to allocate space in the GOT to hold a TLS_index structure for the current object. The link-editor process this relocation by substituting the GOT -relative offset for the new linkage table entry.

The load object index is not known until runtime. Therefore, a R_386_TLS_DTPMOD32 relocation is created, and the ti_tlsoffset field of the structure is zero filled. The call instruction is tagged with the R_386_TLS_LDM_PLT relocation.

The TLS offset for each local symbol is known at link-edit time so the link-editor fills these values in directly.

When a procedure references more than one local symbol, the compiler generates code to obtain the base address of the TLS block once. This base address is then used to calculate the address of each symbol without a separate library call.