The linker supports the -O
option which performs the following optimizations at link time:
optimizes references to data by removing
unnecessary ADDIL
instructions from the object code.
removes procedures that can never be reached.
These optimizations can be separately enabled or disabled
with the +O[no]fastaccess
and +O[no]procelim
options respectively. The -O
linker option simply combines enabling of these into one option.
For example, the following ld
command enables linker optimizations and results in a smaller, faster
executable:
$ ld -O -o prog /usr/ccs/lib/crt0.o prog.o -lm -lc |
To enable one or the other optimization only, use the appropriate
+O option:
$ ld +Ofastaccess -o prog /usr/ccs/lib/crt0.o prog.o -lm -lc
$ ld +Oprocelim -o prog /usr/ccs/lib/crt0.o prog.o -lm -lc |
Incompatibilities
with other Options |
 |
The -O,
+Ofastaccess,
and +Oprocelim
options are incompatible with these linker options:
- -b
These options have no effect on position-independent
code, so they are not useful when building shared libraries with
ld -b.
- -A
Dynamic linking is incompatible with link-time optimization.
- -r
Relocatable linking is incompatible with link-time
optimization.
- -D
Setting the offset of the data space is incompatible
with link-time optimization.
The linker issues a warning when such conflicts occur. If
you require any of these features, do not use the linker optimization
options.
Unused Procedure Elimination with
+Oprocelim |
|
Unused or "dead" procedure elimination is
the process of removing unreferenced procedures from the $TEXT$
space of an executable or shared library to reduce the size of the
program or library.
Dead procedure elimination is performed after all symbols
have been resolved prior to any relocation. It works on a per subspace
basis. That is, only entire subspaces are removed and only if all
procedures in the subspace are unreferenced. Typically, if a relocatable
link (ld -r)
has not been performed and the code is not written in assembly,
every procedure is in its own subspace. Relocatable links may merge
subspaces. Merged subspaces can prevent the removal of dead procedures.
Therefore, it is optimal to have each procedure in its own subspace.
For complete executables, dead procedure
elimination removes any text subspaces that are not referenced from
another subspace. Self references, such as recursive procedures
or subspaces with multiple procedures that call each other, are
not considered outside references and are therefore candidates for
removal.
If the address of a procedure is taken, the subspace within
which it resides is not removed. If a subspace is referenced in
any way by a fixup representing a reference other than a PC-relative
call or an absolute call it is not removed.
For incomplete executables, dead procedure
elimination works the same as for complete executables except that
no exported symbols or their dependencies are removed. If an incomplete
executable contains a symbol that is to be referenced by a shared
library and is not exported, it is removed
if the other conditions discussed above hold.
In shared libraries only symbols that are not referenced and
not exported are removed. In shared libraries all symbols that are
not of local scope are exported. Therefore only locally scoped symbols
not referenced are removed.
When performing a relocatable link with the -r
option, dead procedure elimination is disabled since the only possible
gain would be the removal of unreferenced local procedures. Objects
resulting from a relocatable link are subject to dead procedure
elimination upon a final link.
Affects on Symbolic Debugging
Any procedure that has symbolic debug information associated
with it is not removed. Procedures that do not have symbolic debug
information associated with them but are included in a debug link
are removed if they are not referenced.
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