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Chapter 7 267
Position-Independent Code
Generating Position-Independent Code
Global and Static Variable References
References to global or static variables currently require two instructions
either to form the address of a variable, or to load or store the contents of
the variable:
; to form the address of a variable
ADDIL L'var-$global$+x,%dp
LDO R'var-$global$+x(%r1),tmp
; to load the contents of a variable
ADDIL L'var-$global$+x,%dp
LDW R'var-$global$+x(%r1),tmp
These sequences must be converted to equivalent sequences using the
linkage table pointer in %r19:
; to form the address of a variable
LDW T'var(%r19),tmp1
LDO x(tmp1),tmp2 ; omit if x == 0
; to load the contents of a variable
LDW T'var(%r19),tmp1
LDW x(tmp1),tmp2
Note that the T' fixup on the LDW instruction allows for a 14-bit signed
offset, which restricts the DLT to be 16Kb. Because %r19 points to the
middle of the DLT, we can take advantage of both positive and negative
offsets. The T' fixup specifier should generate a DLT_REL fixup proceeded
by an FSEL override fixup. If the FSEL override fixup is not generated,
the linker assumes that the fixup mode is LD/RD for DLT_REL fixups. In
order to support larger DLT table sizes, the following long form of the
above data reference must be generated to reference tables that are
larger. If the DLT table grows beyond the 16Kb limit, the linker emits an
error indicating that the user must recompile using the +Z option which
produces the following long-load sequences for data reference:
; form the address of a variable
ADDIL LT'var,%r19
LDW RT'var(%r1),tmp1
LDO x(tmp1),tmp2 ; omit if x == 0
; load the contents of a variable
ADDIL LT'var,%r19
LDW RT'var(%r1),tmp1
LDW x(tmp1),tmp2
Procedure Labels
The compilers already mark procedure label constructs so that the linker
can process them properly. No changes are needed to the compilers.