Re: [Qemu-devel] [PATCH 6/7] tcg/mips: Support full movcond select operation

[Richard Henderson <rth@twiddle.net>]

On 10/01/2015 01:30 AM, James Hogan wrote:
> Adapt the MIPS movcond implementation to handle the full select
> operation using a pair of MOVN/MOVZ instructions.
>
> This allows the register alias constraint to be removed (which is what
> ensured v2 == dest), and allows it to be more easily extended to support
> the MIPS r6 instructions SELNEZ/SELEQZ which replace MOVN/MOVZ and
> require similar logic.
>
> For example, previously we only supported:
>   movcond_i32 dest, c1, c2, v1, v2=dest, cond
>
> With the host code:
>   MOV[ZN] dest, v1, [!](c1 cond c2)
>
> Meaning:
>   if (c1 cond c2)
>       dest = v1;
>
> But now v2 doesn't have to equal dest, so we can support:
>   movcond_i32 dest, c1, c2, v1, v2, cond
>
> With the host code:
>   #if dest != v1
>       MOV[ZN] dest, v1, [!](c1 cond c2)
>   #endif
>   #if dest != v2
>       MOV[NZ] dest, v1, ![!](c1 cond c2)
>   #endif
>
> Meaning:
>   #if dest != v1
>       if ([!](c1 cond c2))
>           dest = v1;
>   #endif
>   #if dest != v2
>       if (![!](c1 cond c2))
>           dest = v2;
>   #endif

I don't think this is a good change.  In the case of dest != v1 && dest != v2, 
we wind up with two conditional instructions.  On most targets that I'm 
familiar with, this is more expensive than a plain move.

If r6 was conditional, as opposed to something that we must perforce know about 
at compilation time, then I'd say go ahead but split this into a normal move 
followed by a conditional move.  But since that's not the case, I don't see the 
point in changing anything at all.


r~