Hi Raphael,
Can you please verify this also with a larger array? IIRC sdcc checks the
array size (in bytes) and then decides if it can get away with 8 bit
calculations.
Greets,
Maarten
> Hi,
>
> SDCC (not restricted to the pic ports) seems to be buggy with array
> indices;
> the problem is visible in the iCode as well.
>
> <code>
> struct s { int a; int b; char c; } arr[5];
>
> char demo1(unsigned idx) { return arr[idx].c; }
> char demo2(unsigned idx) { return arr[idx - 32].c; }
>
> char demo3(unsigned char idx) { return arr[idx].c; }
> char demo4(unsigned char idx) { return arr[idx - 32].c; }
> char demo5(unsigned char idx) { return arr[(unsigned)(idx - 32)].c; }
> char demo6(unsigned char idx) { idx -= 32; return arr[idx].c; }
> char demo7(unsigned char idx) { idx -= 32; return arr[(unsigned)idx].c; }
> </code>
>
> The problem is the computation of the array index (idx * 5, 5 being
> sizeof(struct s)), which might overflow a byte and thus should be
> carried out on (unsigned) ints.
>
> demo1 and demo2 work nicely.
> demo3 and demo4 use 8x8->8 SUB and MUL and fail.
> demo5 implements 16x16->16 SUB (!) and MUL and works.
> demo6 implements 8x8->8 SUB and MUL and fails.
> demo7 implements 8x8->8 SUB and 16x16->16 MUL and works nicely.
>
> For now, I would recommend to go along demo7() and explicitly cast
> the array index to (a 16-bit) unsigned.
>
>> I think so the problem comes from ignoring __mulchar overflow (which
>> can and will overflow for r0x1003>51). Is this some kind of bug or
>> am I overlooking something? Thanks.
>
> I agree, this seems to be a bug. Array index computation must be
> carried out on (full-width) ints.
>
> Does anyone of the SDCC developers have a clue if (and where) that is
> to be fixed?!? Probably some AST optimization ...
>
>
> Best regards
>
> Raphael
>
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