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            Understanding Variations in Dhrystone Performance

          By Reinhold P. Weicker, Siemens AG, AUT E 51, Erlangen

                                April 1989

                      This article has appeared in:

        Microprocessor Report, May 1989 (Editor: M. Slater), pp. 16-17

Microprocessor manufacturers tend to credit all the  performance  measured  by

benchmarks to the speed of their processors, they often don't even mention the

programming language and compiler used. In their detailed  documents,  usually

called  "performance brief" or "performance report," they usually do give more

details. However, these details are often lost in the press releases and other

marketing  statements.  For serious performance evaluation, it is necessary to

study the code generated by the various compilers.

Dhrystone was originally published in Ada (Communications  of  the  ACM,  Oct.

1984).  However, since good Ada compilers were rare at this time and, together

with UNIX, C became more and more popular, the C version of Dhrystone  is  the

one  now  mainly  used in industry. There are "official" versions 2.1 for Ada,

Pascal, and C,  which  are  as  close  together  as  the  languages'  semantic

differences permit.

Dhrystone contains two statements  where  the  programming  language  and  its

translation play a major part in the execution time measured by the benchmark:

  o   String assignment (in procedure Proc_0 / main)

  o   String comparison (in function Func_2)

In Ada and Pascal, strings are arrays of characters where the  length  of  the

string  is  part  of the type information known at compile time. In C, strings

are also arrays of characters, but there  are  no  operators  defined  in  the

language  for  assignment  and  comparison  of  strings.   Instead,  functions

"strcpy" and "strcmp" are used. These functions are  defined  for  strings  of

arbitrary  length, and make use of the fact that strings in C have to end with

a terminating null byte. For general-purpose calls  to  these  functions,  the

implementor  can  assume  nothing  about  the  length and the alignment of the

strings involved.

The C version of Dhrystone spends a relatively large amount of time  in  these

two  functions.  Some  time  ago, I made measurements on a VAX 11/785 with the

Berkeley UNIX (4.2) compilers (often-used compilers,  but  certainly  not  the

most  advanced).  In  the  C  version, 23% of the time was spent in the string

functions; in the Pascal version, only 10%. On good RISC machines (where  less

time is spent in the procedure calling sequence than on a VAX) and with better

optimizing compilers, the percentage is higher; MIPS has reported 34%  for  an

R3000.   Because  of this effect, Pascal and Ada Dhrystone results are usually

better than C results (except when the optimization quality of the C  compiler

is considerably better than that of the other compilers).

Several people have noted that the string operations are  over-represented  in

Dhrystone,  mainly  because the strings occurring in Dhrystone are longer than

average strings. I admit that this is true, and have said  so  in  my  SIGPLAN

Notices  paper  (Aug.  1988);  however, I didn't want to generate confusion by

changing the string lengths from version 1 to version 2.

Even if they are somewhat over-represented in Dhrystone, string operations are

frequent  enough  that  it makes sense to implement them in the most efficient

way possible, not only for benchmarking purposes.  This means  that  they  can

and should be written in assembly language code. ANSI C also explicitly allows

the strings functions to be implemented as macros, i.e. by inline code.

There is also a third way to speed up the "strcpy" statement in Dhrystone: For

this  particular  "strcpy" statement, the source of the assignment is a string

constant. Therefore, in contrast to calls to "strcpy" in the general case, the

compiler  knows  the  length  and alignment of the strings involved at compile

time and can generate code in the same efficient  way  as  a  Pascal  compiler

(word instructions instead of byte instructions).

This is not allowed in the case of the "strcmp" call: Here, the addresses  are

formal  procedure  parameters, and no assumptions can be made about the length

or alignment of the strings.  Any such assumptions would indicate an incorrect

implementation.  They  might work for Dhrystone, where the strings are in fact

word-aligned  with  typical  compilers,  but  other  programs  would   deliver

incorrect results.

So, for an apple-to-apple  comparison  between  processors,  and  not  between

several  possible  (legal  or  illegal)  degrees of compiler optimization, one

should check that the systems are comparable with  respect  to  the  following

three points:

  (1) String functions in assembly language vs. in C

      Frequently used functions such as the string functions can and should be

      written  in  assembly language, and all serious C language systems known

      to me do this. (I list this point  for  completeness  only.)  Note  that

      processors  with an instruction that checks a word for a null byte (such

      as AMD's  29000  and  Intel's  80960)  have  an  advantage  here.  (This

      advantage  decreases  relatively if optimization (3) is applied.) Due to

      the length of the strings involved in Dhrystone, this advantage  may  be

      considered  too  high  in  perspective, but it is certainly legal to use

      such instructions - after all,  these  situations  are  what  they  were

      invented for.

  (2) String function code inline vs. as library functions.

      ANSI  C  has  created  a  new  situation,  compared   with   the   older

      Kernighan/Ritchie  C.  In  the  original C, the definition of the string

      function was not part of the  language.  Now  it  is,  and  inlining  is

      explicitly  allowed.  I  probably  should have stated more clearly in my

      SIGPLAN  Notices  paper  that  the  rule  "No  procedure  inlining   for

      Dhrystone"  referred  to  the  user level procedures only and not to the

      library routines.

  (3) Fixed-length and alignment assumptions for the strings

      Compilers should be allowed to optimize in these cases if (and only  if)

      it  is safe to do so. For Dhrystone, this is the "strcpy" statement, but

      not the  "strcmp"  statement  (unless,  of  course,  the  "strcmp"  code

      explicitly   checks   the  alignment  at  execution  time  and  branches

      accordingly).  A "Dhrystone switch" for the  compiler  that  causes  the

      generation  of  code  that  may  not work under certain circumstances is

      certainly inappropriate for comparisons. It has been reported in  Usenet

      that some C compilers provide such a compiler option; since I don't have

      access to all C compilers involved, I cannot verify this.

      If the fixed-length and word-alignment assumption can be  used,  a  wide

      bus  that permits fast multi-word load instructions certainly does help;

      however, this fact by itself should not make a really big difference.

A check of  these  points  -  something  that  is  necessary  for  a  thorough

evaluation  and  comparison  of  the  Dhrystone  performance claims - requires

object code listings as well as listings for  the  string  functions  (strcpy,

strcmp) that are possibly called by the program.

I don't pretend that Dhrystone is  a  perfect  tool  to  measure  the  integer

performance  of microprocessors. The more it is used and discussed, the more I

myself learn about aspects that I hadn't noticed yet when I wrote the program.

And  of  course,  the  very success of a benchmark program is a danger in that

people may tune their compilers and/or hardware to it, and  with  this  action

make it less useful.

Whetstone and Linpack have their critical points also:  The  Whetstone  rating

depends  heavily on the speed of the mathematical functions (sine, sqrt, ...),

and Linpack is sensitive to data alignment for some cache configurations.

Introduction of a standard set of public domain benchmark software  (something

the  SPEC  effort attempts) is certainly a worthwhile thing.  In the meantime,

people will continue to use whatever is available and widely distributed,  and

Dhrystone  ratings  are probably still better than MIPS ratings if these are -

as often in industry - based on  no  reproducible  derivation.   However,  any

serious  performance  evaluation  requires  more than just a comparison of raw

numbers; one has to make sure  that  the  numbers  have  been  obtained  in  a

comparable way.