Re: [Lightning] About 32 and 64 bits registers

[Paolo Bonzini]

On 08/30/2010 08:53 PM, Paulo César Pereira de Andrade wrote:
>    Err, what I mean't was, to follow a proper logic of not showing the 
> programmer
> register widths, it should mean:
>
> mul_i  r0(64) = r1(i32) * r2(i32)
> mul_ui r0(64) = r1(u32) * r2(u32)

It doesn't matter what you do.  Using mul_{i,ui} and relying on the top 
bits (e.g. by using a branch opcode with long type) is undefined anyway.

To get the high bits in a 32x32->64 multiplication, you need to use hmul 
or extend to long before doing the multiplication.  It's inefficient, 
but it's also not extremely common in generated code.

Paolo