Optimizing Itanium-Based Applications (May 2011)

Optimizing Itanium-Based Applications

cc -o sample.exe +Oprofile=use -O sample.o Link for optimization.

The +Oprofile=use option is supported at optimization level 2 (-O or +O2) and above.

Note: 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.

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. To select an alternate profile data file X, use the +Oprofile=use:X command line option. You

can also use the FLOW_DATA environment variable to specify the name of the profile data file with the

+Oprofile=use compile. The [:file] qualifier takes precedence over the FLOW_DATA environment

variable. In the following example, the FLOW_DATA environment variable is set to override the flow.data

file name. The profile data is stored instead in /users/profiles/prog.data.

% cc -Aa -c +Oprofile=collect sample.c

% cc -o sample.exe +Oprofile=collect sample.o

% setenv FLOW_DATA /users/profiles/prog.data

% sample.exe < input.file1

% cc -o sample.exe +Oprofile=use +03 sample.o

In the next example, the Oprofile=use: option uses /users/profiles/prog.data to override

the flow.data file name.

% cc -Aa -c +Oprofile=collect sample.c

% cc -o sample.exe +Oprofile=collect sample.o

% sample.exe < input.file1

% mv flow.data /users/profile/prog.data

% cc -o sample.exe +Oprofile=use:/users/profiles/prog.data +03 sample.o

merging profile data files

Execution profile data files from different runs of an instrumented executable can be merged together into

a single file. This merging can take place implicitly during data collection, or it may be performed

explicitly by the user using the “fdm” command, This example shows an implicit merge:

% aCC +O3 -c +Oprofile=collect program.C

% aCC -o program.exe +Oprofile=collect program.o

% setenv FLOW_DATA /tmp/program.flow

% sample.exe < A.input Profile for input “A.input” written to /tmp/program.flow

% sample.exe < B.input Profile for input “B.input” merged into /tmp/program.flow

During the second run of the instrumented executable, the execution profile data derived from running the

program on “B.input” is merged with (added to) the existing profile database /tmp/program.flow. Profile

databases may also be merged explicitly using the tool /opt/langtools/bin/fdm. Here is an

example of an explicit merge:

% unset FLOW_DATA ; rm flow.data