Revision as of 15:35, 14 June 2011

Dhrystone Benchmark: Rationale for Version 2 and Measurement Rules

Reinhold P. Weicker
Siemens AG, E STE 35
Postfach 3240
D-8520 Erlangen
Germany (West)

Why a Version 2 of Dhrystone?

The Dhrystone benchmark program [1] has become a popular benchmark for CPU/compiler performance measurement, in particular in the area of minicomputers, workstations, PC's and microprocesors. It apparently satisfies a need for an easy-to-use integer benchmark; it gives a first performance indication which is more meaningful than MIPS numbers which, in their literal meaning (million instructions per second), cannot be used across different instruction sets (e.g. RISC vs. CISC). With the increasing use of the benchmark, it seems necessary to reconsider the benchmark and to check whether it can still fulfill this function. Version 2 of Dhrystone is the result of such a re-evaluation, it has been made for two reasons:

a) Dhrystone has been published in Ada [1], and Versions in Ada, Pascal and C have  been  distributed  by  Reinhold Weicker via floppy disk.  However, the version that was used most often for benchmarking has been the version  made by  Rick  Richardson  by another translation from the Ada version into the C programming language, this has been the version  distributed  via  the  UNIX network Usenet [2].

There is an obvious need for a common C version of Dhrystone, since C is  at present  the  most  popular  system  programming  language  for the class of systems (microcomputers, minicomputers,  workstations)  where  Dhrystone  is used  most.   There  should  be,  as  far as possible, only one C version of Dhrystone such that results can be compared  without  restrictions.  In  the past,  the  C  versions  distributed by Rick Richardson (Version 1.1) and by Reinhold Weicker had small (though not significant) differences.

Together with the new C version, the  Ada  and  Pascal  versions  have  been updated as well.

b) As far as it is possible without changes to the Dhrystone statistics,optimizing compilers should be prevented from removing  significant statements. It has turned out in the past that optimizing compilers suppressed code generation for too many statements (by "dead code removal" or  "dead  variable  elimination").   This  has  lead  to  the  danger  that benchmarking  results obtained by a naive application of Dhrystone - without inspection of the code that was generated - could become meaningless.

The overall policiy for version 2 has been that the distribution of statements, operand types and operand locality described in [1] should remain unchanged as much as possible. (Very few changes were necessary; their impact should be negligible.) Also, the order of statements should remain unchanged. Although I am aware of some critical remarks on the benchmark - I agree with several of them - and know some suggestions for improvement, I didn't want to change the benchmark into something different from what has become known as "Dhrystone"; the confusion generated by such a change would probably outweight the benefits. If I were to write a new benchmark program, I wouldn't give it the name "Dhrystone" since this denotes the program published in [1]. However, I do recognize the need for a larger number of representative programs that can be used as benchmarks; users should always be encouraged to use more than just one benchmark. The new versions (version 2.1 for C, Pascal and Ada) will be distributed as widely as possible. (Version 2.1 differs from version 2.0 distributed via the UNIX Network Usenet in March 1988 only in a few corrections for minor deficiencies found by users of version 2.0.) Readers who want to use the benchmark for their own measurements can obtain a copy in machine-readable form on floppy disk (MS-DOS or XENIX format) from the author.

2. Overall Characteristics of Version 2

In general, version 2 follows - in the parts that are significant for performance measurement, i.e. within the measurement loop - the published (Ada) version and the C versions previously distributed. Where the versions distributed by Rick Richardson [2] and Reinhold Weicker have been different, it follows the version distributed by Reinhold Weicker. (However, the differences have been so small that their impact on execution time in all likelihood has been negligible.) The initialization and UNIX instrumentation part - which had been omitted in [1] - follows mostly the ideas of Rick Richardson [2]. However, any changes in the initialization part and in the printing of the result have no impact on performance measurement since they are outside the measaurement loop. As a concession to older compilers, names have been made unique within the first 8 characters for the C version.

The original publication of Dhrystone did not contain any statements for time measurement since they are necessarily system-dependent. However, it turned out that it is not enough just to inclose the main procedure of Dhrystone in a loop and to measure the execution time. If the variables that are computed are not used somehow, there is the danger that the compiler considers them as "dead variables" and suppresses code generation for a part of the statements. Therefore in version 2 all variables of "main" are printed at the end of the program. This also permits some plausibility control for correct execution of the benchmark.

At several places in the benchmark, code has been added, but only in branches that are not executed. The intention is that optimizing compilers should be prevented from moving code out of the measurement loop, or from removing code altogether. Statements that are executed have been changed in very few places only. In these cases, only the role of some operands has been changed, and it was made sure that the numbers defining the "Dhrystone distribution" (distribution of statements, operand types and locality) still hold as much as possible. Except for sophisticated optimizing compilers, execution times for version 2.1 should be the same as for previous versions.

Because of the self-imposed limitation that the order and distribution of the executed statements should not be changed, there are still cases where optimizing compilers may not generate code for some statements. To a certain degree, this is unavoidable for small synthetic benchmarks. Users of the benchmark are advised to check code listings whether code is generated for all statements of Dhrystone.

Contrary to the suggestion in the published paper and its realization in the versions previously distributed, no attempt has been made to subtract the time for the measurement loop overhead. (This calculation has proven difficult to implement in a correct way, and its omission makes the program simpler.) However, since the loop check is now part of the benchmark, this does have an impact - though a very minor one - on the distribution statistics which have been updated for this version.

3. Discussion of Individual Changes

In this section, all changes are described that affect the measurement loop and that are not just renamings of variables. All remarks refer to the C version; the other language versions have been updated similarly.

In addition to adding the measurement loop and the printout statements, changes have been made at the following places:

o In procedure "main", three statements have been added in the non-executed

 "then" part of the statement

       if (Enum_Loc == Func_1 (Ch_Index, 'C'))

 they are

       strcpy (Str_2_Loc, "DHRYSTONE PROGRAM, 3'RD STRING");
      Int_2_Loc = Run_Index;
      Int_Glob = Run_Index;

 The string assignment prevents  movement  of  the  preceding  assignment  to
Str_2_Loc  (5'th  statement  of  "main")  out  of the measurement loop (This
probably will not happen for the C version, but it did happen  with  another
language   and  compiler.)   The  assignment  to  Int_2_Loc  prevents  value
propagation for Int_2_Loc, and the assignment to Int_Glob makes the value of
Int_Glob possibly dependent from the value of Run_Index.

o In the three arithmetic computations at the end of the measurement loop in

 "main  ",  the  role  of  some  variables has been exchanged, to prevent the
division from just cancelling out the multiplication as it was  in  [1].   A
very   smart  compiler  might  have  recognized  this  and  suppressed  code
generation for the division.

o For Proc_2, no code has been changed, but the values of the actual parameter

 have changed due to changes in "main".

o In Proc_4, the second assignment has been changed from

       Bool_Loc = Bool_Loc | Bool_Glob;

 to

       Bool_Glob = Bool_Loc | Bool_Glob;

 It now assigns a value to a global variable  instead  of  a  local  variable
(Bool_Loc);   Bool_Loc  would  be  a  "dead  variable"  which  is  not  used
afterwards.

o In Func_1, the statement

       Ch_1_Glob = Ch_1_Loc;

 was added in the non-executed "else" part of the "if" statement, to  prevent
the suppression of code generation for the assignment to Ch_1_Loc.

o In Func_2, the second character comparison statement has been changed to

       if (Ch_Loc == 'R')

 ('R' instead of 'X') because  a  comparison  with  'X'  is  implied  in  the
preceding "if" statement.

 Also in Func_2, the statement

       Int_Glob = Int_Loc;

 has been added in the non-executed part of the last "if" statement, in order
to prevent Int_Loc from becoming a dead variable.

o In Func_3, a non-executed "else" part has been added to the "if" statement.

 While  the  program  would  not be incorrect without this "else" part, it is
considered bad programming practice if a function  can  be  left  without  a
return value.

 To compensate for this change, the (non-executed) "else" part  in  the  "if"
statement of Proc_3 was removed.

The distribution statistics have been changed only by the addition of the measurement loop iteration (1 additional statement, 4 additional local integer operands) and by the change in Proc_4 (one operand changed from local to global). The distribution statistics in the comment headers have been updated accordingly.

4. String Operations

The string operations (string assignment and string comparison) have not been changed, to keep the program consistent with the original version.

There has been some concern that the string operations are over-represented in the program, and that execution time is dominated by these operations. This was true in particular when optimizing compilers removed too much code in the main part of the program, this should have been mitigated in version 2.

It should be noted that this is a language-dependent issue: Dhrystone was first published in Ada, and with Ada or Pascal semantics, the time spent in the string operations is, at least in all implementations known to me, considerably smaller. In Ada and Pascal, assignment and comparison of strings are operators defined in the language, and the upper bounds of the strings occuring in Dhrystone are part of the type information known at compilation time. The compilers can therefore generate efficient inline code. In C, string assignemt and comparisons are not part of the language, so the string operations must be expressed in terms of the C library functions "strcpy" and "strcmp". (ANSI C allows an implementation to use inline code for these functions.) In addition to the overhead caused by additional function calls, these functions are defined for null-terminated strings where the length of the strings is not known at compilation time; the function has to check every byte for the termination condition (the null byte).

Obviously, a C library which includes efficiently coded "strcpy" and "strcmp" functions helps to obtain good Dhrystone results. However, I don't think that this is unfair since string functions do occur quite frequently in real programs (editors, command interpreters, etc.). If the strings functions are implemented efficiently, this helps real programs as well as benchmark programs.

I admit that the string comparison in Dhrystone terminates later (after scanning 20 characters) than most string comparisons in real programs. For consistency with the original benchmark, I didn't change the program despite this weakness.

5. Intended Use of Dhrystone

When Dhrystone is used, the following "ground rules" apply:

o Separate compilation (Ada and C versions)

 As mentioned in [1], Dhrystone was written  to  reflect  actual  programming
practice  in  systems  programming.   The  division into several compilation
units (5 in the Ada version, 2 in the C version)  is  intended,  as  is  the
distribution of inter-module and intra-module subprogram calls.  Although on
many systems there will be no difference in execution time  to  a  Dhrystone
version  where  all  compilation units are merged into one file, the rule is
that separate compilation should  be  used.   The  intention  is  that  real
programming  practice,  where  programs  consist  of  several  independently
compiled units, should  be  reflected.   This  also  has  implies  that  the
compiler,  while  compiling  one  unit,  has no information about the use of
variables, register allocation etc.  occuring in  other  compilation  units.
Although  in  real  life  compilation  units  will  probably  be larger, the
intention is that these effects  of  separate  compilation  are  modeled  in
Dhrystone.

 A few language systems have post-linkage optimization available (e.g., final
register allocation is performed after linkage).  This is a borderline case:
Post-linkage  optimization  involves  additional  program  preparation  time
(although  not  as  much  as  compilation in one unit) which may prevent its
general use in practical programming.  I think that  since  it  defeats  the
intentions given above, it should not be used for Dhrystone.

 Unfortunately, ISO/ANSI  Pascal  does  not  contain  language  features  for
separate  compilation.   Although  most  commercial Pascal compilers provide
separate compilation in some way, we cannot use it for Dhrystone since  such
a  version  would  not  be portable.  Therefore, no attempt has been made to
provide a Pascal version with several compilation units.

o No procedure merging

 Although Dhrystone contains some very short procedures where execution would
benefit  from  procedure  merging (inlining, macro expansion of procedures),
procedure merging is not to be used.  The reason is that the  percentage  of
procedure  and  function  calls  is  part of the "Dhrystone distribution" of
statements contained in [1].  This restriction does not hold for the  string
functions  of  the  C  version  since ANSI C allows an implementation to use
inline code for these functions.

o Other optimizations are allowed, but they should be indicated

 It is often hard to draw an exact line between "normal code generation"  and
"optimization"  in  compilers:  Some compilers perform operations by default
that are invoked in other compilers only  when  optimization  is  explicitly
requested.  Also, we cannot avoid that in benchmarking people try to achieve
results that look as good as possible.  Therefore,  optimizations  performed
by  compilers  -  other  than  those  listed  above - are not forbidden when
Dhrystone execution times are measured.  Dhrystone is  not  intended  to  be
non-optimizable  but  is  intended  to  be  similarly  optimizable as normal
programs.   For  example,  there  are  several  places  in  Dhrystone  where
performance   benefits   from   optimizations   like   common  subexpression
elimination, value  propagation  etc.,  but  normal  programs  usually  also
benefit  from  these  optimizations.   Therefore,  no  effort  was  made  to
artificially  prevent  such  optimizations.   However,  measurement  reports
should  indicate  which  compiler  optimization  levels  have been used, and
reporting results with different levels of  compiler  optimization  for  the
same hardware is encouraged.

o Default results are those without "register" declarations (C version)

 When Dhrystone results are quoted  without  additional  qualification,  they
should  be  understood  as  results  obtained  without use of the "register"
attribute. Good compilers should be able to make good use of registers  even
without explicit register declarations ([3], p. 193).

Of course, for experimental purposes, post-linkage optimization, procedure merging and/or compilation in one unit can be done to determine their effects. However, Dhrystone numbers obtained under these conditions should be explicitly marked as such; "normal" Dhrystone results should be understood as results obtained following the ground rules listed above.

In any case, for serious performance evaluation, users are advised to ask for code listings and to check them carefully. In this way, when results for different systems are compared, the reader can get a feeling how much performance difference is due to compiler optimization and how much is due to hardware speed.

6. Acknowledgements

The C version 2.1 of Dhrystone has been developed in cooperation with Rick Richardson (Tinton Falls, NJ), it incorporates many ideas from the "Version 1.1" distributed previously by him over the UNIX network Usenet. Through his activity with Usenet, Rick Richardson has made a very valuable contribution to the dissemination of the benchmark. I also thank Chaim Benedelac (National Semiconductor), David Ditzel (SUN), Earl Killian and John Mashey (MIPS), Alan Smith and Rafael Saavedra-Barrera (UC at Berkeley) for their help with comments on earlier versions of the benchmark.

7. Bibliography

[1]

  Reinhold P. Weicker: Dhrystone: A Synthetic Systems Programming Benchmark.
 Communications of the ACM 27, 10 (Oct. 1984), 1013-1030

[2]

  Rick Richardson: Dhrystone 1.1 Benchmark Summary (and Program Text)
 Informal Distribution via "Usenet", Last Version Known  to  me:  Sept.  21,
 1987

[3]

  Brian W. Kernighan and Dennis M. Ritchie:  The C Programming Language.
 Prentice-Hall, Englewood Cliffs (NJ) 1978