Re: [Qemu-devel] [PATCH 0/5] tcg conditional set, round 4

[Richard Henderson <rth@twiddle.net>]

On 12/21/2009 01:13 AM, Laurent Desnogues wrote:
> The question for the generalized movcond is how useful is it?
> Which front-ends would need it and would the cost to generate
> code for it on some (most?) back-ends be amortized?

... Any front end that has a conditional move instruction?
Sparcv9, Mips32, Alpha, ARM...

That said, I think the *biggest* gains are to be had because with 
movcond -- at least on some targets -- we can have one BB per TB, and 
avoid any intermediate spilling of global registers back to memory.

> My guess (I use that word given that I didn't do any benchmark
> to sustain my claim) is that your implementation is too complex.

Too complex for what?  The message against which you are quoting has an 
implementation of 2 lines.

> Of course setcond can be implemented in terms of movcond,
> but my guess (again that word...) is that setcond could be
> enough and even faster in most cases.

To implement condition codes, yes, to implement compare instructions 
(e.g. mips slt, alpha cmp{eq,lt,lte}), yes.  To implement conditional 
moves, no.  At least not without using 5 instructions where 1 would suffice.

> Regarding your patches, I would like to see setcond put in
> mainline with a simplified version for i386.

Again, simplified from what?  The last setcond implementation was 2 lines.


r~