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Linker and Libraries Guide

SPARC: General Dynamic (GD)

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

Table 8–2 SPARC: 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_LO10
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 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.

The load object index and TLS block index for x are not known until runtime. Therefore, 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 second 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 uses the special syntax, x@TLSPLT. This call references the TLS variable and 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. The add instruction can not be placed into the delay slot for the call. This requirement is necessary 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 requirement permits the link-editor to identify the GOT-pointer register during a code transformation.