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(as.info)Alpha-Relocs

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Relocations
...........

   Some of these relocations are available for ECOFF, but mostly only
for ELF.  They are modeled after the relocation format introduced in
Digial 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_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

`!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.