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Dhrystone Benchmark: Rationale for Version 2 and Measurement Rules
Reinhold P. Weicker
Siemens AG, E STE 35
Postfach 3240
D-8520 Erlangen
Germany (West)
1. 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:
o 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.
o 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