Ola Fosheim Grøstad:
IMO muls should yield 2N bits of output for N bits input, then
the compiler should do strength reduction.
Adds should be done on N+1 bits types, using 33 bit output for
32 bits input, then strength reduce it to >=32 bit output if
both operands are 31 bits or less?
I copy here a comment I've written here:
https://github.com/D-Programming-Language/phobos/pull/1866#issuecomment-50216839
A core.checkedint function like muls() accepts two ints or two
longs. What if you have a uint and int? An example:
void main() {
import std.stdio, core.checkedint;
int x = -1;
uint y = 3_000_000_000;
writeln(x, " ", y);
writeln(x * y);
bool overflow = false;
immutable r1 = muls(x, y, overflow);
writeln(r1, " ", overflow);
overflow = false;
immutable r2 = mulu(x, y, overflow);
writeln(r2, " ", overflow);
}
It outputs:
-1 3000000000
1294967296
1294967296 false
1294967296 true
Here the overflow boolean from the muls() is false, but the
result is wrong (it's -3_000_000_000 that is not representable by
both ints and uints).
Bye,
bearophile
