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.

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Chapter 7

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

HP 9000 specifications

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