[Mesa-dev] [PATCH 04/10] i965: Don't convert if/else to conditional adds on Gen6.

[Kenneth Graunke]

On 12/05/2011 09:40 AM, Paul Berry wrote:
> Normally when outputting instructions in SPF (single program flow)
> mode, we convert IF and ELSE instructions to conditional ADD
> instructions applied to the IP register, since this lets us avoid
> having to emit an ENDIF instruction (and, in Gen4, lets us avoid using
> up precious space in the MaskStack).
> 
> However, according to the SandyBridge PRM (Volume 4 part 2, p79):
> 
>    [Errata DevSNB{WA}] - When SPF is ON, IP may not be updated by
>    non-flow control instructions.
> 
> So we have to disable this optimization on Gen6.
> 
> The reason we never noticed this problem before is that so far we
> haven't needed to use SPF mode on Gen6.  However, later patches in
> this series will introduce a Gen6 GS program which uses SPF mode.

I'm wondering if this is broken on Gen7+ as well.  It might be better to
just disable this optimization on intel->gen >= 6 altogether.  I'm
honestly not sure it provides much benefit post-Ironlake.

We could also test it on Ivybridge; if it works, I'm not opposed to
leaving it enabled either.