Ways to Improve Performance

Profile-Based Optimization

The function _clear_counters is defined in icrt0.o. It is also defined as a stub (an empty function that does nothing) in crt0.o. This allows you to use the same source code without modification in the instrumented and un-instrumented versions of the program.

Optimizing Based on Profile Data (+P/-P)

The final step in PBO is optimizing a program using profile data created in the profiling phase. To do this, rebuild the program with the +P compiler option. As with the +I option, the +P option causes the compiler to generate an I-SOM .ofile, rather than the usual object code, for each source file.

Note that it is not really necessary to recompile the source files; you could, instead, specify the I-SOM .ofiles that were created during the instrumentation phase. For instance, suppose you have already created an I-SOM file named foo.o from foo.c using the +I compiler option; then the following commands are equivalent in effect:

cc+P foo.c cc +P foo.o

Both commands invoke the linker, but the second command doesn't compile before invoking the linker.

The -P Linker Option

After creating an I-SOM file for each source file, the compiler driver invokes the linker with the -Poption, causing the linker to optimize all the .o files. As with the +I option, the driver uses /opt/langtools/lbin/ucomp to generate code and perform various optimizations.

To see how the compiler invokes the linker, specify the -voption when compiling. For instance, suppose you have instrumented prog.c and gathered profile data into flow.data. The following example shows how the compiler driver invokes the linker when +P is specified:

$ cc -o prog -v +P prog.o

/usr/ccs/bin/ld /usr/ccs/lib/crt0.o -u main -o prog \ prog.o -P -lc

Notice how the program is now linked with /usr/ccs/lib/crt0.o instead of /opt/langtools/lib/icrt0.o because the profiling code is no longer needed.

Chapter 8

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Kenwood HP 9000 manual Optimizing Based on Profile Data +P/-P, P Linker Option
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