Efficient coding considered harmful?

[Walter Bright]

In article <7421 at ihlpl.ATT.COM> knudsen at ihlpl.ATT.COM (Knudsen) writes:
>In article <8775 at smoke.BRL.MIL>, gwyn at smoke.BRL.MIL (Doug Gwyn ) writes:
>> (But don't write something like
>> 	for (p = &a[j]; --p >= a; )
>> which is nonportable.)
>Why not portable?

If a was created by a call to malloc, and (sizeof(a[0])>2), then on the 8086
in large memory model, p can NEVER be less than a! The trouble results from
malloc creating a segment for a. Decrementing p past a causes a segment
wrap, exactly like decrementing an unsigned variable past 0.

This behavior is allowed for by ANSI C, and is the most reasonable way
of doing things on the 8086.