HP-UX Linker and Libraries User's Guide

X': ADDIL L'ltoff,%r19 ; get procedure entry point
LDW R'ltoff(%r1),%r21
LDW R'ltoff+4(%r1),%r19 ; get new r19 value
LDSID (%r21),%r1
MTSP %r1,%sr0
BE 0(%sr0,%r21) ; branch to target
STW %rp,-24(%sp) ; save rp
;Export Stub (Shared libs and Incomplete Executables)
X': BL,N X,%rp ; trap the return
NOP
LDW -24(%sp),%rp ; restore the original rp
LDSID (%rp),%r1
MTSP %r1,%sr0
BE,N 0(%sr0,%rp) ; inter-space return
For More Information: The remainder of this section describes how compilers generate PIC for the
following addressing situations:
• “PIC Requirements for Compilers and Assembly Code” (page 187)
• “Long Calls” (page 188)
• “Long Branches and Switch Tables” (page 188)
• “Assigned GOTO Statements ” (page 188)
• “Literal References” (page 188)
• “Global and Static Variable References” (page 189)
• “Procedure Labels” (page 189)
You can use these guidelines to write assembly language programs that generate PIC object code.
For details on assembly language, see the Assembler Reference Manual and PA-RISC 2.0
Architecture.
PIC Requirements for Compilers and Assembly Code
The linkage table pointer register, %r19, must be stored at %sp-32 by all PIC routines. This can
be done once on procedure entry. The %r19 linkage table pointer register must also be restored
on return from a procedure call. The value must be stored in %sp-32 (and possibly in a callee-saves
register). If the PIC routine makes several procedure calls, the routine copies %r19 into a callee-saves
register as well, to avoid a memory reference when restoring %r19 upon return from each procedure
call. Just like %r27 (%dp), the compilers treat %r19 as a reserved register whenever PIC mode
is in effect. In general, references to code are handled by the linker, and the compilers act differently
only in the few cases where they would have generated long calls or long branches. References
to data, however, need a new fixup request to identify indirect references through the linkage
table, and the code generated changes slightly.
NOTE: Any code which is PIC or which makes calls to PIC must follow the standard procedure
call mechanism.
When linking files produced by the assembler, the linker exports only those assembly language
routines that have been explicitly exported as entry (that is, symbols of type ST_ENTRY).
Compiler-generated assembly code does not explicitly export routines with the entry type specified.
So, the assembly language programmer must ensure that this is done with the .EXPORT pseudo-op.
For example, in assembly language, a symbol is exported using
.EXPORT foo, type
where type can be code, data, entry, and others. To ensure that foo is exported from
a shared library, the assembly statement must be:
.EXPORT foo,entry
Generating Position-Independent Code 187