SPARC: 32-bit and 64-bit General Dynamic (GD) (Linker and Libraries Guide)

Linker and Libraries Guide

SPARC: 32-bit and 64-bit General Dynamic (GD)

This code sequence is the most general, and can be included in both shared objects and dynamic executables. This code sequence can also reference an external TLS variable in either a shared object or dynamic executable.

Table 8–2 SPARC: 32-bit and 64-bit General Dynamic Thread-Local Variable Access Codes

Code Sequence	Initial Relocations	Symbol
# %l7 - initialized to GOT pointer 0x00 sethi %hi(@dtlndx(x)), %o0 0x04 add %o0,%lo(@dtlndx(x)),%o0 0x08 add %l7, %o0, %o0 0x0c call x@TLSPLT # %o0 - contains address of TLS variable	R_SPARC_TLS_GD_HI22 R_SPARC_TLS_GD_LO22 R_SPARC_TLS_GD_ADD R_SPARC_TLS_GD_CALL	x x x x

Code Sequence

Initial Relocations

Symbol

# %l7 - initialized to GOT pointer

0x00 sethi %hi(@dtlndx(x)), %o0
0x04 add   %o0,%lo(@dtlndx(x)),%o0
0x08 add   %l7, %o0, %o0
0x0c call  x@TLSPLT
 
# %o0 - contains address of TLS variable

 
 
R_SPARC_TLS_GD_HI22
R_SPARC_TLS_GD_LO22
R_SPARC_TLS_GD_ADD
R_SPARC_TLS_GD_CALL

 
 
x
x
x
x


	Outstanding Relocations: 32-bit	Symbol
GOT[n] GOT[n + 1]	R_SPARC_TLS_DTPMOD32 R_SPARC_TLS_DTPOFF32	x x


	Outstanding Relocations: 64-bit	Symbol
GOT[n] GOT[n + 1]	R_SPARC_TLS_DTPMOD64 R_SPARC_TLS_DTPOFF64	x x

The sethi, and add instructions generate R_SPARC_TLS_GD_HI22 and R_SPARC_TLS_GD_LO10 relocations respectively. These relocations instruct the link-editor to allocate space in the global offset table 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_SPARC_TLS_DTPMOD32 and R_SPARC_TLS_DPTOFF32 relocations against the GOT for processing by the runtime linker.

The add instruction causes the generation of the R_SPARC_TLS_GD_ADD relocation. This relocation is used only if the GD code sequence is changed to another sequence by the link-editor.

The call instruction generates the R_SPARC_TLS_GD_CALL relocation. This relocation instructs the link-editor to bind the call to the __tls_get_addr() function, and associates the call instruction with the GD code sequence.

Note –

The add instruction must appear before the call instruction. It cannot be placed into the delay slot for the call. This is required as the code-transformations that can occur later require a known order.

The register used as the GOT-pointer for the add instruction tagged by the R_SPARC_TLS_GD_ADD relocation, must be the first register in the add instruction. This permits the link-editor to identify the GOT-pointer register during a code transformation.