HP-UX Linker and Libraries User's Guide

Long Calls
Normally, the compilers generate a single-instruction call sequence using the BL instruction. The
compilers can be forced to generate a long call sequence when the module is so large that the BL
is not guaranteed to reach the beginning of the subspace. In the latter case, the linker can insert
a stub. The existing long call sequence is three instructions, using an absolute target address:
LDIL L'target,%r1
BLE R'target(%sr4,%r1)
COPY %r1,%rp
When the PIC option is in effect, the compilers must generate the following instruction sequence,
which is PC-relative:
BL .+8,%rp ; get pc into rp
ADDIL L'target - $L0 + 4, %rp ; add pc-rel offset to rp
LDO R'target - $L1 + 8(%r1), %r1
$L0: LDSID (%r1), %r31
$L1: MTSP %r31, %sr0
BLE 0(%sr0,%r1)
COPY %r31,%rp
Long Branches and Switch Tables
Long branches are similar to long calls, but are only two instructions because the return pointer is
not needed:
LDIL L'target,%r1
BE R'target(%sr4,%r1)
For PIC, these two instructions must be transformed into four instructions, similar to the long call
sequence:
BL .+8,%r1 ; get pc into r1
ADDIL L'target-L,%r1 ; add pc-relative offset
L: LDO R'target-L,%r1 ; add pc-relative offset
BV,N 0(%r1) ; and branch
The only problem with this sequence occurs when the long branch is in a switch table, where each
switch table entry is restricted to two words. A long branch within a switch table must allocate a
linkage table entry and make an indirect branch:
LDW T'target(%r19),%r1 ; load LT entry
BV,N 0(%r1) ; branch indirect
Here, the T' operator indicates a new fixup request supported by the linker for linkage table entries.
Assigned GOTO Statements
ASSIGN statements in FORTRAN must be converted to a PC-relative form. The existing sequence
forms the absolute address in a register before storing it in the variable:
LDIL L'target,tmp
LDO R'target(tmp),tmp
This must be transformed into the following four-instruction sequence:
BL .+8,tmp ; get rp into tmp
DEPI 0,31,2,tmp ; zero out low-order 2 bits
L: ADDIL L'target-L,tmp ; get pc-rel offset
LDO R'target-L(%r1),tmp
Literal References
References to literals in the text space are handled exactly like ASSIGN statements (shown above).
The LDO instruction can be replaced with LDW as appropriate.
188 Writing and Generating Position-Independent Code