[Oberon] Numerical CASE Statements in Project Oberon

[John Stout]

Jan

I wasn't suggesting that we NOT implement it, just that compiling it (implementing it) to a
series of compiled IF THEN statements may be more in keeping with NW's view.

Suppose we have (taken from the Oberon-07 report):

CASE k OF
 0: x := x + y
 | 1: x := x − y
 | 2: x := x * y
 | 3: x := x / y
END

This would compile, using IF THEN equivalents, to code like this (I've assumed that R14 is a base register):

LDR0, R14, k_offset
SUBR1, R0, 0
BNECase_1
code for x := x + y
BEnd_Case
Case_1:
SUBR1, R0, 1
BNECase_2
Code for x := x - y
BEnd_Case
Case_2:
SUBR1, R0, 2
BNECase_3
Code for x := x * y
BEnd_Case
Case_3:
SUBR1, R0, 3
BNEEnd_Case
Code for x := x / y
End_Case:

This is simple and clear. There is a clear maximum execution path length. Adding label ranges is a bit longer, but still (I think) simple and clear, e.g.,

CASE k OF
0 .. 10: x := x − y
 | 11 .. 1000: x := x * y
 | 1001 .. 2047 : x := x / y
END

could compile to something like this:

LDR0, R14, k_offset
SUBR1, R0, 0
BLTCase_1
SUBR1, R0, 10
BGTCase_1
code for x := x - y
BEnd_Case
Case_1:
SUBR1, R0, 11
BLTCase_1
SUBR1, R0, 1000
BGTCase_2
Code for x := x * y
BEnd_Case
Case_2:
SUBR1, R0, 1001
BLTCase_1
SUBR1, R0, 2047
BNEEnd_Case
Code for x := x / y
End_Case:

This will work even for very large ranges. The size of the generated code (no data needed) depends on the number of cases, not the range of the constants in the case label list or case label range.

Adding label lists:

CASE k OF
0, 10: x := x − y
 | 11, 1000: x := x * y
 | 1001, 2047, 19 : x := x / y
END

could compile to something like this:

LDR0, R14, k_offset
SUBR1, R0, 0
BEQC_1
SUBR1, R0, 10
BNECase_1
C_0:code for x := x - y
BEnd_Case
Case_1:
SUBR1, R0, 11
BEQC_1
SUBR1, R0, 1000
BNECase_2
C_1:Code for x := x * y
BEnd_Case
Case_2:
SUBR1, R0, 1001
BEQC_3
SUBR1, R0, 2047
BEQC_3
SUBR1, R0, 19
BNEEnd_Case
C_3:Code for x := x / y
End_Case:

John

>> Am 16.11.2015 um 09:31 schrieb Jan de Kruyf <jan.de.kruyf at gmail.com>:
>>
>> Yes,
>> The old man has been right most of the time, I agree. And I will have
>> a royal battle to reduce the compiler construction back down to
>> Wirthian elegance, that I also realize. But at the same time the code
>> I produce is very elegant and sweet. On the basis of that I would say
>> you are wrong in your statement that IF THEN can be made as efficient
>> as a good case construct. Besides, a Case statement allows ranges
>> like A..Z, a..z and so on, this has never been part of IF THEN, while
>> in a properly constructed bin-tree it resolves itself.
>>
>> So since Wirth was first and foremost a teacher all his life, we
>> could say that there are 2 learning opportunities here 1. to produce
>> fast code.
>> 2. to construct an elegant way of doing so.
>>
>>  I think that the master himself also realized the issues you and I
>> are bringing up, and did not see his way out of the predicament yet.
>> And since he is about 80 now and since he still got a few irons in
>> the fire this was left for a bit.
>>
>> Lastly Dijkstra taught that you should never limit yourself because
>> of inefficient machines. But you should strive to generalize any
>> language design solution to as many cases as possible, even if that
>> slows your program execution down a bit. There were big arguments in
>> the Algol design meetings about that. Some people then felt that you
>> had to live with the equipment given and use it as efficiently as
>> possible. Wirth no doubt knows that whole history even better than I
>> do. In the mean time of course, the machine limitations of those days
>> (it was the time of the vacuum tubes) have been solved a hundred
>> times over, but we still are trying to live with the language limitations of some of the older languages.
>> Ergo the numerical case statement is in the O-7 language report, but
>> is has not been implemented yet.
>>
>> So I feel fully justified implementing it. The way how to, we can
>> talk about. If the Oberon community is alive and well ten I am sure
>> that the last word has not been spoken yet.
>>
>>
>> Enjoy your day
>>
>> j.
>>

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John Stout
Management Information Systems Coordinator
Tel: 01704 530601
E-Mail: JSS at kgv.ac.uk
Web: www.kgv.ac.uk


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