Specifications

The example below shows the PLT entry when the PLT GOT is close enough to the small data area for a
relative jump.
Example 7-22: PLT Entry Near Small Data Area
.PLTn:
ldw r15, %gprel(plt_got_slot_address)(gp)
jmp r15
Example 7-23: Initial PLT Entry
res_0:
br .PLTresolve
...
.PLTresolve:
orhi r14, r0, %hiadj(res_0)
addi r14, r14, %lo(res_0)
sub r15, r15, r14
orhi r13, %hiadj(_GLOBAL_OFFSET_TABLE_)
ldw r14, %lo(_GLOBAL_OFFSET_TABLE_+4)(r13)
ldw r13, %lo(_GLOBAL_OFFSET_TABLE_+8)(r13)
jmp r13
In front of the initial PLT entry, a series of branches start of the initial entry (the nextpc instruction).
There is one branch for each PLT entry, labelled res_0 through res_N. The last several branches may be
replaced by nop instructions to improve performance. The link editor arranges for the Nth PLT entry to
point to the Nth branch; res_N – res_0 is four times the index into the .rela.plt section for the
corresponding R_JUMP_SLOT relocation.
The dynamic linker initializes GOT[1] to a unique identifier for each library and GOT[2] to the address of
the runtime resolver routine. In order for the two loads in .PLTresolve to share the same %hiadj,
_GLOBAL_OFFSET_TABLE_ must be aligned to a 16-byte boundary.
The runtime resolver receives the original function arguments in r4 through r7, the shared library
identifier from GOT[1] in r14, and the relocation index times four in r15. The resolver updates the
corresponding PLT GOT entry so that the PLT entry transfers control directly to the target in the future,
and then transfers control to the target.
In shared objects, the call and jmpi instructions can not be used because the library load address is not
known at link time. Calls to functions outside the current shared object must pass through the GOT. The
program loads function addresses using %call, and the link editor may arrange for such entries to be
lazily bound. Because PLT entries are only used for lazy binding, shared object PLTs are smaller, as shown
below.
Example 7-24: Shared Object PLT
.PLTn:
orhi r15, r0, %hiadj(index * 4)
addi r15, r15, %lo(index * 4)
br .PLTresolve
NII51016
2015.04.02
Procedure Linkage Table
7-21
Application Binary Interface
Altera Corporation
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