[Oberon] Dynamic local array implementation
luke.boasso at gmail.com
Fri Oct 13 01:35:52 CEST 2017
> The Astrobe compiler does use a frame pointer. Given the local variable
> 'a' declared as:
> a: ARRAY OF INTEGER;
> The code generated by NEW(a, 10) is as follows:
> MOV r10,10
> STR r10,[fp,-8]
> MOV r10,r10 lsl 2
> SUB sp,sp,r10
> STR sp,[fp,-4]
I see, that is similar to what I was expecting.
If my understanding is correct, [fp, -8] and [fp, -4] represent an array
descriptor, where you store the length and address on the stack
respectively. SP is decremented by length*element size to make space for
the array.Very neat!
Is [fp,-4] the beginning or the end of the array "a"?
I have checked "An Oberon Compiler for the ARM Processor" p. 26 and it
seems like in that compiler dynamic arrays are allocated on the heap:
VAR k: INTEGER;
a: ARRAY OF INTEGER;
BEGIN NEW(a, 10); a[k] := 100
The code generated by NEW(a, 10) is as follows:
SUB R11 FP 8 adr a
MOV R10 R0 10 len
MOV R9 R0 4 element size
BL 0 NEW (updated by loader)
Do you know if that system had a garbage collector? I am surprised by
that implementation, I thought it was doing something like your Astrobe
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