mulsw etc functions not working correctly?

[rjobling]

I've been trying to use mulsw to make sure the compiler does a 16x16=32 multiplication. But as things are the compiler seems to assume mulsw results can be truncated to 16bits. I'm seeing generated code that does this. I think its because the inline asm used 'a' for the result and 'a' is a short. So it optimizes out the additional bits you might hope for in the returning int.

Well, anyway I think this alternative works:

inline int mulsw(short a, short b) {
int r = a;
asm("mulsw %1,%0":"+d"(r): "mid"(b): "cc");
return r;
}

But you will probably want to check that yourself and do the same for muluw.

[rjobling]

One downside of my solution seems to be that the compiler will add ext.l before the muls because 'a' is copied to 'r' which is an int.

So far I haven't found a solution that doesn't either add some extra unnecessary code or remove necessary upper bits.

[rjobling]

I ran into this again because I needed to remove some 32bit multiplies.

The support code as it is doesn't return the full 32bit result unless you use the version I provided above.

[BartmanAbyss]

I haven't really found a satisfying solution either. It's always either non-optimal code or breaking code.

[BartmanAbyss]

see also #31 and #13

[rjobling]

The current code has the problem that if the result doesn't fit in 16bits then you get the wrong value.