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Slide 1

1 Title

bchop Utility - Title

R.K.Owen,Ph.D.,

National Energy Research Scientific Computer Center

NERSC / MS 50C
One Cyclotron Road
Berkeley, CA 94720

(510)604-5935

rk@owen.sj.ca.us
rkowen@nersc.gov

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TITLE

"So Optimization Breaks Your Code ..."

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Slide 2

1.1 Abstract

bchop Utility - Abstract

TITLE

"So Optimization Breaks Your Code ..."

ABSTRACT

Many compiler options can have an adverse affect on a code. Most notably, various levels of optimization can "break" a code, such that the program gives erroneous results. One option is to compile for the lowest common denominator. The other option is to identify which routines are adversely affected. The local utility, bchop performs a binary chop between object files in two directories (./good and ./bad) compares the output and isolates which object files are causing the differences in output. This helps isolate problems due to changing compiler or preprocessor options. The number of runs performed is approximately 2 E log₂(N), where N is the number of object files and E is the number of object modules causing errors. This utility, bchop, is similar to CRI's atchop for isolating multitasking problems.

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1.2 Why?

bchop Utility - Why?

Scenario

• User gets different results depending on compiler options (debugging vs. optimization)
• Code has 117 compilation units
• Each execution takes several minutes of C90 CPU time
• Not a multitasking problem

Other Scenarios

• f90 vs. cf77
• Different compiler options
• Can't run interactively

Solution

• Write a general utility bchop

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Slide 4

1.3 Atchop

bchop Utility - atchop

bchop just a atchop knock-off?

atchop	bchop
Finds multitasking problems only Can isolate problem loops Recompiles & loads code each time Specific to cf77 CPU intensive Not easy to use	Finds which modules are bad Modules compiled only once Loads code each time Independent of compiler Less CPU intensive easier to use Portable

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Slide 5

2 Binary Decomposition

bchop Utility - Binary Decomposition

AKA "interval halving"

• Each object file = endpoint in binary tree
• O(log(N)) to traverse tree
• Order object files
• Select according to LSBs
• Take subset from "bad"
• Take remainder from "good"

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2.1 Simple Example

bchop Utility - Binary Decomposition Example

• Order object files
• Select "bad" according to LSBs
• Increase selection level if overall failure

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2.2 Single Error

bchop Utility - Single Error

• Single error (E = 1)
• Number of compilation units (N = 16)
• Number of comparisons = 2 E log ₂ (N)

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Slide 8

2.3 Multiple Errors

bchop Utility - Multiple Errors

• Multiple errors (E = 2)
• Number of compilation units (N = 16)
• Number of comparisons < 2 E log ₂ (N)

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Slide 9

3 Number of Trials

bchop Utility - Number of Trials

Generally,

Number of Trials <= 2Elog₂(N) Where

• E = Number of errant object files
• N = Number of object files

Efficient if E << N

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Slide 10

3.1 Data plot

bchop Utility - Data

• 1,2, and 3 errors
• Up to 140 object files

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4 How to Use

bchop Utility - How to Use

Compare the "good" and the "bad"

The "good"

• Compile the code with the "good" options
• Make a directory - ./good
• Move the object files to the ./good directory

The "bad"

• Compile the code with the "bad" options
• Make a directory - ./bad
• Move the object files to the ./bad directory

The "ugly"

• Create a file listing all affected object files
• If necessary create a script to compare "good" & "bad" output
• Collect all the necessary input files together

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4.1 Options

bchop Utility - Options

Can specify the following:

• Custom script for output comparison
• Which output for comparison
• File for stdin
• Executable command line options
• Loader & options

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4.2 Command Synopsis

bchop Utility - Command Synopsis

 

usage: bchop [-v][-V][-1][-d diff_script] \
	[-o outfile][-i infile] \
        [-C cmd_line][-c load_cmd] \
	[-l libs][-L libdirs][-O okobjects] \
        -f objlistfile

 -v              verbose output
 -V              print version
 -1              no binary chopping...go one-by-one
 -d diff_script  script file takes 2 arguments and
                 exits 0 if no difference.
                 def: "diff -b $1 $2 > /dev/null"
 -o outfile      output file to compare.
                 def: stdout
 -i infile       stdin file for runs.
                 e.g. "./a.out < infile"
 -C cmd_line     command line arguments for runs
 -c load_cmd     command to link objects.
                 def: "cc -o "
 -llibs          added libraries for linking objects
 -Llibdirs       additional directories to search
                 for libraries
 -O okobjects    added objects to link with
 -f objlistfile  list of objects to link together
 

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Slide 14

5 Implementation

bchop Utility - Implementation

Written in "Standard C"
Compiles on

• Crays
• Workstations
• PCs running Linux

Heavy use of system() function
bchop

• Parsing of customization options
• Bookkeeping of "good" and "bad" objects
• Pasting strings together for commands
• Clean-up

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Slide 15

5.1 Future Work

bchop Utility - Future Work

• Restart capabilities
• Tighten SIGINT handling
• Multiple -l & -L options
• Report Timings (-t)
• Use "bad" (but OK) objects (-b)
• ?

Send suggestions (or code patches) to rk@owen.sj.ca.us

Source available by anonymous ftp at

ftp://ftp.owentrek.com/

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Slide 16

6 Conclusion

bchop Utility - Conclusion

• Effective & simple tool, finds objects affected by
  • Change in compiler options
  • Upgrades of compiler
  • Change in operating system
  • User environment
• Easy to use
• Not CPU intensive
• Was written in ~ 3 days

In original task ... found an object that caused differences not even considered by user.