[fpc-devel] Unexpected behaviour of bit operators

[Marco Borsari]

In the code below

program test;
const n=12;
s=1 shl n;
var a,b,c,h1,h2:word;
begin
a:=77;
b:=0;
(*c:=(a XOR b)*(a SHL 5+b SHR 2);*)
h1:=((a XOR b)*(a SHL 5+b SHR 2)) SHR (16-n);
(*h1:=c SHR (16-n);*)
h2:=((a XOR b)*(a SHL 5+b SHR 2)) AND ((s-1) SHL (16-n)) SHR (16-n);
(*h2:=c AND ((s-1) SHL (16-n)) SHR (16-n);*)
writeln(h1,',',h2);
end.

the results of h1 and h2 (they are hashes) are different, and only h2
appears to be correct and inside the range.
If we precompute the hash value with c, then both h1 and h2 are
the same.
Does this is an effect of some multiplication overflow,
or is it a bug?

Regards, Marco Borsari
-- 
Simplex sigillum veri