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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 modules 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.
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