x64: Local Dynamic (LD)

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

0x00 leaq  x1@tlsld(%rip), %rdi
0x07 call  __tls_get_addr@plt

# %rax - contains address of TLS block

0x10 leaq  x1@dtpoff(%rax), %rcx

# %rcx - contains address of TLS variable x1

0x20 leaq  x2@dtpoff(%rax), %r9

# %rcx - contains address of TLS variable x2

R_AMD64_TLSLD
R_AMD64_PLT32
 


R_AMD64_DTOFF32
 


R_AMD64_DTOFF32

x1
__tls_get_addr



 
x1


 
x2

GOT[n]

R_AMD64_DTMOD64

x1

The first two instructions are equivalent to the code sequence used for the general dynamic model, although without any padding. The two instructions must be consecutive. The x1@tlsld(%rip) sequence generates a the tls_index entry for symbol x1. This index refers to the current module that contains x1 with an offset of zero. The link-editor creates one relocation for the object, R_AMD64_DTMOD64.

The R_AMD64_DTOFF32 relocation is unnecessary, because offsets are loaded separately. The x1@dtpoff expression is used to access the offset of the symbol x1. Using the instruction as address 0x10, the complete offset is loaded and added to the result of the __tls_get_addr() call in %rax to produce the result in %rcx. The x1@dtpoff expression creates the R_AMD64_DTPOFF32 relocation.

Instead of computing the address of the variable, the value of the variable can be loaded using the following instruction. This instruction creates the same relocation as the original leaq instruction.

movq  x1@dtpoff(%rax), %r11

Provided the base address of a TLS block is maintained within a register, loading, storing or computing the address of a protected thread-local variable requires one instruction.

Benefits exist in using the local dynamic model over the general dynamic model. Every additional thread-local variable access only requires three new instructions. In addition, no additional GOT entries, or runtime relocations are required.