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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!
Nldq
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!
Nldl
) 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!
Njsr
) 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!
Nlituse_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!
Nextbl
) 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!
Nldq
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__tls_get_addr
used to compute the
address of the thread-local storage variable whose descriptor was
loaded with !tlsgd!
N.
!lituse_tlsldm!
N__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!
Nldah
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
ldah
instruction to add the high 16 bits of a
32-bit displacement from the GP.
!gprellow
!gprel
!samegp
$27
or perform a standard GP load in the first two instructions via the
.prologue
directive.
!tlsgd
!tlsgd!
Nlda
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!
Nlda
instruction to load the address of a TLS
descriptor for the current module in the GOT.
Similar in other respects to tlsgd
.
!gotdtprel
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
gprel
relocations except they compute dtp-relative offsets.
!gottprel
ldq
instruction to load the offset of the TLS
symbol from the thread pointer. Also known as the tp-relative offset.
!tprelhi
!tprello
!tprel
gprel
relocations except they compute tp-relative offsets.