9.2.3.3 Relocations

Some of these relocations are available for ECOFF, but mostly only for ELF. They are modeled after the relocation format introduced in Digital Unix 4.0, but there are additions.

The format is ‘!tag’ or ‘!tag!number’ where tag is the name of the relocation. In some cases number is used to relate specific instructions.

The relocation is placed at the end of the instruction like so:

ldah  $0,a($29)    !gprelhigh
lda   $0,a($0)     !gprellow
ldq   $1,b($29)    !literal!100
ldl   $2,0($1)     !lituse_base!100

!literal

!literal!N

Used with an ldq instruction to load the address of a symbol from the GOT.

A sequence number N is optional, and if present is used to pair lituse relocations with this literal relocation. The lituse relocations are used by the linker to optimize the code based on the final location of the symbol.

Note that these optimizations are dependent on the data flow of the program. Therefore, if any lituse is paired with a literal relocation, then all uses of the register set by the literal instruction must also be marked with lituse relocations. This is because the original literal instruction may be deleted or transformed into another instruction.

Also note that there may be a one-to-many relationship between literal and lituse, but not a many-to-one. That is, if there are two code paths that load up the same address and feed the value to a single use, then the use may not use a lituse relocation.

!lituse_base!N

Used with any memory format instruction (e.g. ldl) to indicate that the literal is used for an address load. The offset field of the instruction must be zero. During relaxation, the code may be altered to use a gp-relative load.

!lituse_jsr!N

Used with a register branch format instruction (e.g. jsr) to indicate that the literal is used for a call. During relaxation, the code may be altered to use a direct branch (e.g. bsr).

!lituse_jsrdirect!N

Similar to lituse_jsr, but also that this call cannot be vectored through a PLT entry. This is useful for functions with special calling conventions which do not allow the normal call-clobbered registers to be clobbered.

!lituse_bytoff!N

Used with a byte mask instruction (e.g. extbl) to indicate that only the low 3 bits of the address are relevant. During relaxation, the code may be altered to use an immediate instead of a register shift.

!lituse_addr!N

Used with any other instruction to indicate that the original address is in fact used, and the original ldq instruction may not be altered or deleted. This is useful in conjunction with lituse_jsr to test whether a weak symbol is defined.

ldq  $27,foo($29)   !literal!1
beq  $27,is_undef   !lituse_addr!1
jsr  $26,($27),foo  !lituse_jsr!1

!lituse_tlsgd!N

Used with a register branch format instruction to indicate that the literal is the call to __tls_get_addr used to compute the address of the thread-local storage variable whose descriptor was loaded with !tlsgd!N.

!lituse_tlsldm!N

Used with a register branch format instruction to indicate that the literal is the call to __tls_get_addr used to compute the address of the base of the thread-local storage block for the current module. The descriptor for the module must have been loaded with !tlsldm!N.

!gpdisp!N

Used with ldah and lda to load the GP from the current address, a-la the ldgp macro. The source register for the ldah instruction must contain the address of the ldah instruction. There must be exactly one lda instruction paired with the ldah instruction, though it may appear anywhere in the instruction stream. The immediate operands must be zero.

bsr  $26,foo
ldah $29,0($26)     !gpdisp!1
lda  $29,0($29)     !gpdisp!1

!gprelhigh

Used with an ldah instruction to add the high 16 bits of a 32-bit displacement from the GP.

!gprellow

Used with any memory format instruction to add the low 16 bits of a 32-bit displacement from the GP.

!gprel

Used with any memory format instruction to add a 16-bit displacement from the GP.

!samegp

Used with any branch format instruction to skip the GP load at the target address. The referenced symbol must have the same GP as the source object file, and it must be declared to either not use $27 or perform a standard GP load in the first two instructions via the .prologue directive.

!tlsgd

!tlsgd!N

Used with an lda instruction to load the address of a TLS descriptor for a symbol in the GOT.

The sequence number N is optional, and if present it used to pair the descriptor load with both the literal loading the address of the __tls_get_addr function and the lituse_tlsgd marking the call to that function.

For proper relaxation, both the tlsgd, literal and lituse relocations must be in the same extended basic block. That is, the relocation with the lowest address must be executed first at runtime.

!tlsldm

!tlsldm!N

Used with an lda instruction to load the address of a TLS descriptor for the current module in the GOT.

Similar in other respects to tlsgd.

!gotdtprel

Used with an ldq instruction to load the offset of the TLS symbol within its module’s thread-local storage block. Also known as the dynamic thread pointer offset or dtp-relative offset.

!dtprelhi

!dtprello

!dtprel

Like gprel relocations except they compute dtp-relative offsets.

!gottprel

Used with an ldq instruction to load the offset of the TLS symbol from the thread pointer. Also known as the tp-relative offset.

!tprelhi

!tprello

!tprel

Like gprel relocations except they compute tp-relative offsets.