Kenwood HP 9000 manual Assigned Goto Statements, Literal References, 266

Position-Independent Code

Generating Position-Independent Code

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.

An opportunity for optimization in both cases is to share a single label (L) throughout a procedure, and let the result of BL become a common sub-expression. Thus only the first literal reference within a procedure is expanded to three instructions; the rest remain two instructions.

266 Chapter 7