as: Alpha-Relocs
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.
Info Catalog
as: Alpha-Regs
as: Alpha Syntax