fdpr Command

Purpose

A performance tuning utility for improving execution time and real memory utilization of user-level application programs.

Syntax

Most Common Usage:

fdpr -p ProgramName -x Command

Use with Phases 1 and 3 Flags:

fdpr -p ProgramName [-M Segnum ] [-o OutputFile ] [-nI ] [[-Rn ]|[-R0 |-R1 |-R2 |-R3 ]] [-v ] -s [-1 |-3 ] [-x Command ]

Use With Phase 2 Flag:

fdpr -p ProgramName [-M Segnum ] [-o OutputFile ] [-nI ] [[-Rn ]|[-R0 |-R1 |-R2 |-R3 ]] [-v ] [-s [-2 |-12|-23] -x Command

Description

The fdpr command (Feedback Directed Program Restructuring) is a performance-tuning utility that may help improve the execution time and the real memory utilization of user-level application programs. The fdpr program optimizes the executable image of a program by collecting information on the behavior of the program while the program is used for some typical workload, and then the program creates a new version that is optimized for that workload. The new program generated by fdpr typically runs faster and uses less real memory.

Attention: The fdpr command applies advanced optimization techniques to a program that may result in programs that do not behave as expected; programs that are reordered using this tool should be used with due caution and should be rigorously retested with, at a minimum, the same test suite used to test the original program in order to verify expected functionality. The reordered program is not supported.

The fdpr command builds an optimized executable program in 3 distinct phases:

• Phase 1: Create an instrumented executable program.
• Phase 2: Run the instrumented program and create the profile data.
• Phase 3: Generate the optimized executable program file.

These phases can be run separately or in partial or full combination, but must be run in order (i.e., -1 then -2 then -3 or -12 then -3) and by the same user. The default is to run all three phases.

Note: The instrumented executable, created in phase 1 and run in phase 2, typically runs several times slower than the original program. Due to the increased execution time required by the instrumented program, the executable should be invoked in such a way as to minimize execution duration, while still fully exercising the desired code areas. The fdpr command user should also attempt to eliminate, where feasible, any time dependent aspects of the program.

Flags

Optimization

The fdpr command provides four levels of optimization. The flags -R1, -R2, and -R3 provide the most aggressive optimization along with the greatest potential speedups. However, in some cases, using these optimization levels may result in an executable that does not behave as expected. Programs that contain assembler code (in particular, code that performs dynamic branch calculations) or programs derived from nonstandard compilers are prone to these types of reordering-induced anomalies. In addition, the -R1 and -R3 flags produce executables that do not include debug information and are therefore not supported by the dbx command.

Use of the -R0 flag can result in a slightly reduced performance improvement as this flag attempts to preserve functionality and debug capability by maintaining the original program structure and by eliminating branch table and function descriptor pointer adjustments. Functional errors are much less likely, though still possible. Also, this option produces a reordered executable that is typically 20-40% larger than the original program.

Both the -R0 and -R2 flags utilize a program-reordering technique in which the original structure of the program, including traceback entries, is preserved. The reordered code, which represents the highly-executed code paths through the program, is appended to the end of the executable. This technique provides near optimal performance improvement by allowing global code reordering (independent of procedure boundaries and absent of interleaved traceback entries) while preserving the debug capability. In addition, program functionality is maintained for a larger class of programs using the original program structure as a "safety net" to catch undetected and/or unmodified dynamically (runtime) computed branch instructions). The -R2 flag attempts to fix all dynamically computed branches that branch to moved code. However, for some programs (especially assembler programs), it is difficult to correctly identify these dynamic branches and using the -R2 flag for this class of programs can result in unexpected functional errors. Also, reordering programs that utilize any form of self-modifying code will probably result in unexpected functional errors.

Executables built with the -qfdpr compiler flag contain information to assist fdpr in producing reordered programs with guaranteed functionality. When this compiler flag is used, the functionality advantage of the fdpr option -R0 is extended to options -R1, -R2, and -R3. However, if -qfdpr is used, only those object modules built with this flag are reordered. If the -qfdpr flag is used, it should be used for all object modules in a program. Static linking will not improve performance if the -qfdpr flag is used.

Additional performance enhancements can be realized by using static linking when building the program to be reordered. Since the fdpr program only reorders the instructions within the executable program specified, any dynamically linked shared library routines called by the program are not reordered. Statically linking these library routines to the executable allows for reordering both the instructions in the program and all library routines used by the program. There are other advantages as well as disadvantages to building a statically linked program. See "Dynamic and Static Linking" in the AIX Versions 3.2 and 4 Performance Tuning Guide for further information.

Output Files

All files created by the fdpr command are stored in the current directory with the exception of any files that may be created by running the command specified in the -x flag. During the optimization process, the original program is saved by renaming the program, and is only restored to the original program name upon successful completion of the final phase.

The profile file created by the fdpr command explicitly uses the name of the current directory since scripts used to run the program may change the working directory before executing the program.

The files created and/or used by the fdpr command are:

Debug Support

The use of the optimization flags -R0 and -R2 results in an executable that has additional information included in the program file for use by the dbx debug program. This additional information allows dbx to provide limited debug support by mapping reordered instruction addresses to their original locations and by maintaining traceback entries in the original text section. The dbx command maps most reordered instruction addresses to the corresponding addresses in the original executable as follows:

0xRRRRRRRR = fdpr[0xYYYYYYYY]

where 0xRRRRRRRR indicates the reordered address and fdpr[0xYYYYYYYY] indicates the original address. Also, dbx uses the traceback entries in the original instruction area to find associated procedure names and during stack traceback. See the "Examples" section for further details.

Enhanced Debugging Capabilities

In order to enable a certain degree of debugging capability for optimized programs, fdpr updates the Symbol Table to reflect the changes that were made in the .text section.

Entry fields in the Symbol Table that specify addresses of symbols that were relocated during the reordering of fdpr, are modified to point to their new addresses in the .text section.

In addition, in the case where functions or files are split during reordering, fdpr creates new entries in the Symbol Table for each new part of the split function/file. These new parts of the same function are given new symbol names in the Symbol Table according to the following naming convention:

<original function name> [<number of function's part>] [fdpr|orig

For optimization flags -R0/-R2, which append all new reordered code to the end of the .text section, the suffix string [fdpr] indicates that the new entry refers to an address in the appended text area whereas the [orig] string refers to an address in the original text area. For optimization flags -R1/-R3 all the new entries are suffixed with the [fdpr] string.

For example: Originally, if a function was split into 3 parts, it would have 3 entries in the Symbol Table; one for each part:

  [Index] m   Value       Scn     Aux   Sclass    Type    Name
Original Entry:
   [456]  m  0x00000230    2       1     0x02    0x0000   .main

Restructured Entries:
   [456]  m  0x00000304    2       1     0x02    0x0000   .main
  [1447]  m  0x00003328    2       1     0x02    0x0000   .main[1] [fdpr]
  [1453]  m  0x000033b4    2       1     0x02    0x0000   .main[2] [fdpr]

Examples

The following are typical usage examples of the fdpr command.

1. This example allows the user to run all three phases. In this example, test1 is the unstripped executable and test2 is a shell script that invokes test1. The current working directory is /tmp/fdpr.

   test2 script file:
   
   # code to exercise test1
   test1 -expand 100 -root $PATH file.jpg -quit
   # the end of test2

   Run the fdpr command (using the default optimization):

   fdpr -p test1 -x test2

   This results in the new reordered executable test1.fdpr.
2. To run one phase at a time, execute phase one of fdpr and save the necessary temporary files.

   fdpr -s -1 -p test1

   This command string renames the original program to __test1.save and creates an instrumented version with the name test1.

   To execute phase two and save temporary files:

   fdpr -s -2 -p test1 -x test2

   This command string executes the script file test2 that runs the instrumented version of test1 to collect the profile data.

   To execute phase three, saving temporary files:

   fdpr -s -3 -p test1

   Again, this results in the new reordered executable test1.fdpr.
3. To run the first two phases followed by phase three, execute phase one and two, saving temporary files.

   fdpr -s -12 -p test1 -x test2

   Execute phase three, while saving temporary files and using optimization level three.

   fdpr -s -3 -R3 -p test1

4. If an error occurs while running an fdpr reordered program built with the -R0 or -R2 optimization flags, the dbx command can be used to determine what procedure the error occurred in (either in the original text section or in the reordered text section) as follows:

   dbx program.fdpr

   which produces the output similar to the following:

   Type 'help' for help.
   reading symbolic information ...warning: no source compiled with -g
    
   [using memory image in core]
    
   Segmentation fault in proc_d at 0x10000634 = fdpr[0x10000290]
   0x10000634 (???) 98640000        stb   r3,0x0(r4)
   (dbx)

   The address mapping information 0x10000634 = fdpr[0x10000290] indicates that the instruction at address 0x10000634 is in the reordered text section and originally resided (in the original program) at address 0x10000290 in proc_d. Running dbx on the original program and using the mapped addresses (0x10000290 in the above example) may provide additional information to aid in debugging.

   A stack traceback, which is used to determine how the program arrived at the current location, is produced as follows:

   (dbx) where

   which produces the following output:

   proc_d(0x0) at 0x10000634
   proc_c(0x0) at 0x10000604
   proc_b(0x0) at 0x100005d0
   proc_a(0x0) at 0x1000059c
   main(0x2, 0x2ff7fba4) at 0x1000055c
   (dbx)

5. The dbx subcommand stepi may also be used to single step through the instructions of a reordered executable program as follows:

   (dbx) stepi

   which produces the following output:

   stopped in proc_d at 0x1000061c = fdpr[0x10000278]
   0x1000061c (???) 9421ffc0       stwu   r1,-64(r1)
   (dbx)

   In this example, dbx indicates that the program stopped in routine proc_d at address 0x1000061c in the reordered text section (originally located at address 0x10000278).

Files

Related Information

The dbx command.

Restructuring Executables with fdpr in AIX Versions 3.2 and 4 Performance Tuning Guide.