Relocation Types

Relocation

Relocation Types

The AMD64 ABI adds one additional field:

**Figure:** Relocatable Fields
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word8 This specifies a 8-bit field occupying 1 byte.

word16 This specifies a 16-bit field occupying 2 bytes with arbitrary byte alignment. These values use the same byte order as other word values in the AMD64 architecture.

word32 This specifies a 32-bit field occupying 4 bytes with arbitrary byte alignment. These values use the same byte order as other word values in the AMD64 architecture.

word64 This specifies a 64-bit field occupying 8 bytes with arbitrary byte alignment. These values use the same byte order as other word values in the AMD64 architecture.

word8		This specifies a 8-bit field occupying 1 byte.
word16		This specifies a 16-bit field occupying 2 bytes with arbitrary byte alignment. These values use the same byte order as other word values in the AMD64 architecture.
word32		This specifies a 32-bit field occupying 4 bytes with arbitrary byte alignment. These values use the same byte order as other word values in the AMD64 architecture.
word64		This specifies a 64-bit field occupying 8 bytes with arbitrary byte alignment. These values use the same byte order as other word values in the AMD64 architecture.

The following notations are used for specifying relocations in table :

A: Represents the addend used to compute the value of the relocatable field.
B: Represents the base address at which a shared object has been loaded into memory during execution. Generally, a shared object is built with a 0 base virtual address, but the execution address will be different.
G: Represents the offset into the global offset table at which the relocation entry's symbol will reside during execution.
GOT: Represents the address of the global offset table.
L: Represents the place (section offset or address) of the Procedure Linkage Table entry for a symbol.
P: Represents the place (section offset or address) of the storage unit being relocated (computed using r_offset).
S: Represents the value of the symbol whose index resides in the relocation entry.

The AMD64 ABI architectures uses only Elf64_Rela relocation entries with explicit addends. The r_addend member serves as the relocation addend.

**Table:** Relocation Types
Name	Value	Field	Calculation
`R_X86_64_NONE`	0	none	none
`R_X86_64_64`	1	word64	`S + A`
`R_X86_64_PC32`	2	word32	`S + A - P`
`R_X86_64_GOT32`	3	word32	`G + A`
`R_X86_64_PLT32`	4	word32	`L + A - P`
`R_X86_64_COPY`	5	none	none
`R_X86_64_GLOB_DAT`	6	word64	`S`
`R_X86_64_JUMP_SLOT`	7	word64	`S`
`R_X86_64_RELATIVE`	8	word64	`B + A`
`R_X86_64_GOTPCREL`	9	word32	`G + GOT + A - P`
`R_X86_64_32`	10	word32	`S + A`
`R_X86_64_32S`	11	word32	`S + A`
`R_X86_64_16`	12	word16	`S + A`
`R_X86_64_PC16`	13	word16	`S + A - P`
`R_X86_64_8`	14	word8	`S + A`
`R_X86_64_PC8`	15	word8	`S + A - P`
`R_X86_64_DPTMOD64`	16	word64
`R_X86_64_DTPOFF64`	17	word64
`R_X86_64_TPOFF64`	18	word64
`R_X86_64_TLSGD`	19	word32
`R_X86_64_TLSLD`	20	word32
`R_X86_64_DTPOFF32`	21	word32
`R_X86_64_GOTTPOFF`	22	word32
`R_X86_64_TPOFF32`	23	word32

The special semantics for most of these relocation types are identical to those used for the Intel386 ABI. ^4.2 ^4.3

The R_X86_64_GOTPCREL relocation has different semantics from the i386 R_I386_GOTPC relocation. In particular, because the AMD64 architecture has an addressing mode relative to the instruction pointer, it is possible to load an address from the GOT using a single instruction. The calculation done by the R_X86_64_GOTPCREL relocation gives the difference between the location in the GOT where the symbol's address is given and the location where the relocation is applied.

The R_X86_64_32 and R_X86_64_32S relocations truncate the computed value to 32-bits. The linker must verify that the generated value for the R_X86_64_32 (R_X86_64_32S) relocation zero-extends (sign-extends) to the original 64-bit value.

A program or object file using R_X86_64_8, R_X86_64_16, R_X86_64_PC16 or R_X86_64_PC8 relocations is not conformant to this ABI, these relocations are only added for documentation purposes. The R_X86_64_16, and R_X86_64_8 relocations truncate the computed value to 16-bits resp. 8-bits.

The relocations R_X86_64_DPTMOD64, R_X86_64_DTPOFF64, R_X86_64_TPOFF64 , R_X86_64_TLSGD , R_X86_64_TLSLD , R_X86_64_DTPOFF32, R_X86_64_GOTTPOFF and R_X86_64_TPOFF32 are listed for completeness. They are part of the Thread-Local Storage ABI extensions and are documented in the document called ``ELF Handling for Thread-Local Storage''^4.4 .

Footnotes

... .^4.2: Even though the AMD64 architecture supports IP-relative addressing modes, a GOT is still required since the offset from a particular instruction to a particular data item cannot be known by the static linker.
...2 ^4.3: Note that the AMD64 architecture assumes that offsets into GOT are 32-bit values, not 64-bit values. This choice means that a maximum of 2³²/8 = 2²⁹ entries can be placed in the GOT. However, that should be more than enough for most programs. In the event that it is not enough, the linker could create multiple GOTs. Because 32-bit offsets are used, loads of global data do not require loading the offset into a displacement register; the base plus immediate displacement addressing form can be used.
... Storage''^4.4: This document is currently available via http://people.redhat.com/drepper/tls.pdf

Jan Hubicka 2003-05-04