Perform link-time optimizations on relocatable object files.
The post-optimizer performs a number of advanced performance optimizations on the binary object code at link-time. The optional value sets the level of optimizations performed, and must be 0, 1, or 2.
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Specifying the -xlinkopt flag without a value implies -xlinkopt=1.
These optimizations are performed at link time by analyzing the object binary code. The object files are not rewritten but the resulting executable code may differ from the original object codes.
This option is most effective when used to compile the whole program, and with profile feedback.
When compiling in separate steps, -xlinkopt must appear on both compile and link steps.
demo% f95 -c -xlinkopt a.f95 b.f95 demo% f95 -o myprog -xlinkopt=2 a.o b.o
Note that the level parameter is only used when the compiler is linking. In the example above, the post-optimization level used is 2 even though the object binaries were compiled with an implied level of 1.
The link-time post-optimizer cannot be used with the incremental linker, ild. The -xlinkopt flag will set the default linker to be ld. Enabling the incremental linker explicitly with the -xildon flag will disable the -xlinkopt option if both are specified together.
For the -xlinkopt option to be useful, at least some, but not necessarily all, of the routines in the program must be compiled with this option. The optimizer can still perform some limited optimizations on object binaries not compiled with -xlinkopt.
The -xlinkopt option optimizes code coming from static libraries that appear on the compiler command line, but does not optimize code coming from shared (dynamic) libraries that appear on the command line. You can also use -xlinkopt when building shared libraries (compiling with -G ).
The -xlinkopt option requires profile feedback (-xprofile) in order to optimize the program. Profiling reveals the most- and least-used parts of the code, which enables the optimizer to focus its effort accordingly. Link-time optimization is particularly important with large applications where optimal placement of code can substantially reduce instruction cache misses. Additionally, -xlinkopt is most effective when used to compile the whole program. Use this option as follows:
demo% f95 -o progt -xO5 -xprofile=collect:prog file.f95 demo% progt demo% f95 -o prog -xO5 -xprofile=use:prog -xlinkopt file.95
For details on using profile feedback, see the -xprofile option
Note that compiling with this option will increase link time slightly. Object file sizes also increase, but the size of the executable remains the same. Compiling with the -xlinkopt and -g flags increases the size of the executable by including debugging information.