[Mesa-dev] [PATCH 05/16] i965: Fix bitcast operations with negate

[Matt Turner]

On Thu, Dec 11, 2014 at 2:34 PM, Eduardo Lima Mitev <elima at igalia.com> wrote:
> From: Iago Toral Quiroga <itoral at igalia.com>
>
> For code such as this in a vertex or fragment shader:
>
> uniform float in0;
> flat out float out0;
> ...
> out0 = ceil(in0)
>
> We generate this IR:
>
> (assign  (x) (var_ref packed:out0)
>    (expression int bitcast_f2i
>        (expression float ceil (var_ref in0) ) ) )
>
> In i965, this would eventually become:
>
> rndd(8)         g14<1>.xF       -g3<0,4,1>.xF
> mov(8)          g6<1>.xD        -g14<4,4,1>.xD
> mov(8)          g116<1>D        g6<4,4,1>D
>
> which does not produce the expected result.
>
> The problem has to do with the second instruction, that changes the
> type to D at the same time it does the negate. For this to work we
> need to negate first with the original type, then change the type
> to the bitcast destination so we produce code like this instead:
>
> rndd(8)         g14<1>.xF       -g3<0,4,1>.xF
> mov(8)          g6<1>.xF        -g14<4,4,1>.xF
> mov(8)          g116<1>D        g6<4,4,1>D

I found and fixed a problem (in commit 0ae9ca12a8) last year where we
would put move the source modifiers out of the bitcast_*(...).

In that commit, I just emit a MOV to do the source modifier as a
separate step, and then let copy propagation handle getting rid of it
when possible. It looks like I didn't notice that ceil also does the
in place op[0].negate = !op[0].negate.

Instead of special casing the bit cast functions, we should just emit
the extra MOV in ceil and let copy propagation clean it up when
possible.