Jonas Maebe wrote:
So the bugfix might be to make subrange types signed when the range allows (which would always be the case in valid ISO programs).
Frank
Note that doing this may cause problems with packed arrays and records, like we had in FPC. If you treat 0..3 as signed, then you can no longer pack it in 2 bits using the current format used by FPC and GPC. The reason is that if a type is signed, both compilers treat the uppermost bit of a packed value as the sign bit. So 2 and 3 would be interpreted as -2 and -1, respectively.
Indeed. But I was talking about unpacked types only. We have special code for subranges in packed arrays anyway, and that shouldn't change.
Of course, it is possible to change the format of packed values (e.g., always store them as unsigned, with a bias of -lower_bound in case of signed types), but this may cause backward compatibility issues.
No, I wouldn't like to do this. (Even if one could gain a bit occasionally, such as storing -1 .. 2 in 2 bits instead of 3 as now, but it doesn't seem worth the effort.)
Or, alternatively, you can of course do the sign extension only if the lower bound is < 0 rather than if the type is signed (maybe that even already happens today in gpc that way). Depending on how the gpc internals work, differentiating between "signed type" and "lower bound < 0" may cause some hairiness.
Yes, it does, and that's basically the problem here. In the original case, the lower bound was >= 0, but the type was treated as unsigned.
To be clear, I'm only talking about unpacked subrange types whose both bounds fit into (signed) "Integer". When the lower bound is < 0, it's signed anyway, no issue here. But when it's >= 0, the type should get signed as well, not unsigned like now.
OTOH, when the upper bound doesn't fit into "Integer" anymore (this case doesn't affect ISO compliance, as ISO doesn't have any bigger type than "Integer"), we should rather use an unsigned type than a bigger signed type, even when available (e.g., on IA32, use unsigned 32 bit rather than signed 64 bit, as long as the upper bound fits in unsigned 32 bits).
Frank