Linker and Libraries Guide

SPARC: Thread-Local Variable Access

On SPARC, the following code sequence models are available for accessing thread-local variables.

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.

SPARC: Local Dynamic (LD)

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

Table 8–3 SPARC: Local Dynamic Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
# %l7 - initialized to GOT pointer 0x00 sethi %hi(@tmndx(x1)), %o0 0x04 add %o0, %lo(@tmndx(x1)), %o0 0x08 add %l7, %o0, %o0 0x0c call x@TLSPLT # %o0 - contains address of TLS block of current object 0x10 sethi %hi(@dtpoff(x1)), %l1 0x14 xor %l1, %lo(@dtpoff(x1)), %l1 0x18 add %o0, %l1, %l1 # %l1 - contains address of local TLS variable x1 0x20 sethi %hi(@dtpoff(x2)), %l2 0x24 xor %l2, %lo(@dtpoff(x2)), %l2 0x28 add %o0, %l2, %l2 # %l2 - contains address of local TLS variable x2	R_SPARC_TLS_LDM_HI22 R_SPARC_TLS_LDM_LO10 R_SPARC_TLS_LDM_ADD R_SPARC_TLS_LDM_CALL R_SPARC_TLS_LDO_HIX22 R_SPARC_TLS_LDO_LOX10 R_SPARC_TLS_LDO_ADD R_SPARC_TLS_LDO_HIX22 R_SPARC_TLS_LDO_LOX10 R_SPARC_TLS_LDO_ADD	x1 x1 x1 x1 x1 x1 x1 x2 x2 x2
	Outstanding Relocations: 32–bit	Symbol
GOT[n] GOT[n + 1]	R_SPARC_TLS_DTPMOD32 <none>	x1
	Outstanding Relocations: 64–bit	Symbol
GOT[n] GOT[n + 1]	R_SPARC_TLS_DTPMOD64 <none>	x1

The first sethi instruction and add instruction generate R_SPARC_TLS_LDM_HI22 and R_SPARC_TLS_LDM_LO10 relocations respectively. These relocations instruct the link-editor to allocate space in the GOT to hold a TLS_index structure for the current object. The link-editor processes this relocation by substituting the GOT -relative offset for the new GOT entry.

The load object index is not known until runtime. Therefore, a R_SPARC_TLS_DTPMOD32 relocation is created, and the ti_tlsoffset field of the TLS_index structure is zero filled.

The second add and the call instruction are tagged with the R_SPARC_TLS_LDM_ADD and R_SPARC_TLS_LDM_CALL relocations respectively.

The following sethi instruction and xor instruction generate the R_SPARC_LDO_HIX22 and R_SPARC_TLS_LDO_LOX10 relocations, respectively. The TLS offset for each local symbol is known at link-edit time, therefore these values are filled in directly. The add instruction is tagged with the R_SPARC_TLS_LDO_ADD relocation.

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.

Note –

The register containing the TLS object address in the add instruction tagged by the R_SPARC_TLS_LDO_ADD must be the first register in the instruction sequence. This requirement permits the link-editor to identify the register during a code transformation.

32-bit SPARC: Initial Executable (IE)

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

Table 8–4 32-bit SPARC: Initial Executable Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
# %l7 - initialized to GOT pointer, %g7 - thread pointer 0x00 sethi %hi(@tpoff(x)), %o0 0x04 or %o0, %lo(@tpoff(x)), %o0 0x08 ld [%l7 + %o0], %o0 0x0c add %g7, %o0, %o0 # %o0 - contains address of TLS variable	R_SPARC_TLS_IE_HI22 R_SPARC_TLS_IE_LO10 R_SPARC_TLS_IE_LD R_SPARC_TLS_IE_ADD	x x x x
	Outstanding Relocations	Symbol
GOT[n]	R_SPARC_TLS_TPOFF32	x

The sethi instruction and or instruction generate R_SPARC_TLS_IE_HI22 and R_SPARC_TLS_IE_LO10 relocations, respectively. These relocations instruct the link-editor to create space in the GOT to store the static TLS offset for symbol x. An R_SPARC_TLS_TPOFF32 relocation is left outstanding against the GOT for the runtime linker to fill in with the negative static TLS offset for symbol x. The ld and the add instructions are tagged with the R_SPARC_TLS_IE_LD and R_SPARC_TLS_IE_ADD relocations respectively.

Note –

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

64-bit SPARC: Initial Executable (IE)

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

Table 8–5 64-bit SPARC: Initial Executable Thread-Local Variable Access Codes


Code Sequence	Initial Relocations	Symbol
# %l7 - initialized to GOT pointer, %g7 - thread pointer 0x00 sethi %hi(@tpoff(x)), %o0 0x04 or %o0, %lo(@tpoff(x)), %o0 0x08 ldx [%l7 + %o0], %o0 0x0c add %g7, %o0, %o0 # %o0 - contains address of TLS variable	R_SPARC_TLS_IE_HI22 R_SPARC_TLS_IE_LO10 R_SPARC_TLS_IE_LD R_SPARC_TLS_IE_ADD	x x x x
	Outstanding Relocations	Symbol
GOT[n]	R_SPARC_TLS_TPOFF64	x

SPARC: Local Executable (LE)

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

Table 8–6 SPARC: Local Executable Thread-Local Variable Access Codes

Code Sequence

Initial Relocations

Symbol

# %g7 - thread pointer

0x00 sethi %hix(@tpoff(x)), %o0
0x04 xor   %o0,%lo(@tpoff(x)),%o0
0x08 add   %g7, %o0, %o0
 
# %o0 - contains address of TLS variable

 
 
R_SPARC_TLS_LE_HIX22
R_SPARC_TLS_LE_LOX10
<none>

x
x

The sethi and xor instructions generate R_SPARC_TLS_LE_HIX22 and R_SPARC_TLS_LE_LOX10 relocations respectively. The link-editor binds these relocations directly to the static TLS offset for the symbol defined in the executable. No relocation processing is required at runtime.