HP 3000 Manuals HP 3000 Manuals
Profile-Based Optimization Profile-based optimization (PBO) is a set of performance-improving code transformations based on the run-time characteristics of your application.
NOTE Profile-based optimization is only available on HP-UX.
There are three steps involved in performing this optimization: 1. Instrumentation - Insert data collection code into the object program. 2. Data Collection - Run the program with representative data to collect execution profile statistics. 3. Optimization - Generate optimized code based on the profile data. Invoke profile-based optimization through HP C by using any level of optimization and the +I and +P options on the cc command line. Compile times will be fast and link times will be slow when using PBO because code generation happens at link time. Instrumenting the Code To instrument your program, use the +I option as follows: cc -Aa +I -O -c sample.c Compile for instrumentation. cc -o sample.exe +I -O sample.o Link to make instrumented executable. The first command line uses the -O option to perform level 2 optimization and instruments the code. The -c option in the first command line suppresses linking and creates an intermediate object file called sample.o. The .o file can be used later in the optimization phase, avoiding a second compile. The second command line uses the -o option to link sample.o into sample.exe. The +I option instruments sample.exe with data collection code. Note that instrumented programs run slower than non-instrumented programs. Only use instrumented code to collect statistics for profile-based optimization. Collecting Data for Profiling To collect execution profile statistics, run your instrumented program with representative data as follows: sample.exe < input.file1 Collect execution profile data. sample.exe < input.file2 This step creates and logs the profile statistics to a file, by default called flow.data. The data collection file is a structured file that may be used to store the statistics from multiple test runs of different programs that you may have instrumented. Performing Profile-Based Optimization To optimize the program based on the previously collected run-time profile statistics, relink the program as follows: cc -o sample.exe +P -O sample.o An alternative to this procedure is to recompile the source file in the optimization step: cc -o sample.exe +I -0 sample.c instrumentation sample.exe < input.file1 data collection cc -o sample.exe +P -O sample.c optimization Maintaining Profile Data Files Profile-based optimization stores execution profile data in a disk file. By default, this file is called flow.data and is located in your current working directory. You can override the default name of the profile data file. This is useful when working on large programs or on projects with many different program files. The FLOW_DATA environment variable can be used to specify the name of the profile data file with either the +I or +P options. The +df command line option can be used to specify the name of the profile data file when used with the +P option. The +df option takes precedence over the FLOW_DATA environment variable. In the following example, the FLOW_DATA environment variable is used to override the flow.data file name. The profile data is stored instead in /users/profiles/prog.data. %setenv FLOW_DATA /users/profiles/prog.data %cc -Aa -c +I +O3 sample.c %cc -o sample.exe +I +03 sample.o %sample.exe < input.file1 %cc -o sample.exe +P +03 sample.o In the next example, the +df option is used to override the flow.data file name with the name /users/profiles/prog.data. %cc -Aa -c +I +O3 sample.c %cc -o sample.exe +I +03 sample.o %sample.exe < input.file1 %mv flow.data /users/profile/prog.data %cc -o sample.exe +df /users/profiles/prog.data +P +03 sample.o Maintaining Instrumented and Optimized Program Files You can maintain both instrumented and optimized versions of a program. You might keep an instrumented version of the program on hand for development use, and several optimized versions on hand for performance testing and program distribution. Care must be taken when maintaining different versions of the executable file because the instrumented program file name is used as the key identifier when storing execution profile data in the data file. The optimizer must know what this key identifier name is in order to find the execution profile data. By default, the key identifier name used to retrieve the profile data is the instrumented program file name used to run the program for data collection. When you optimize a program file and the optimized program file name is different from the instrumented program file name, you must use the +pgm option. Specify the instrumented program file name with this option. The optimizer uses this value as the key identifier to retrieve execution profile data. In the following example, the instrumented program file name is sample.inst. The optimized program file name is sample.opt. The +pgm name option is used to pass the instrumented program name to the optimizer: %cc -Aa -c +I +O3 sample.c %cc -o sample.inst +I +03 sample.o %sample.inst < input.file1 %cc -o sample.opt +P +03 +pgm sample.inst sample.o Profile-Based Optimization Notes When using profile-based optimization, please note the following: * Because the linker performs code generation for profile-based optimization, linking object files compiled with +I and +P takes more time than linking ordinary object files. However, compile-times will be relatively fast. This is because the compiler is only generating the intermediate code. * Profile-based optimization has a greater impact on application performance at each higher level of optimization. * Profile-based optimization should be enabled during the final stages of application development. To obtain the best performance, re-profile and re-optimize your application after making source code changes. For more information on profile-based optimization, see the manual Programming on HP-UX.