[Mesa-dev] [PATCH 17/32] i965/vec4: Fix constant propagation across different types.

[Francisco Jerez]

Matt Turner <mattst88 at gmail.com> writes:

> On Fri, Feb 6, 2015 at 6:42 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>> If the source type differs from the original type of the constant we
>> need to bit-cast it before propagating, otherwise the original type
>> information will be lost.  If the constant was a vector float there
>> isn't much we can do, because the result of bit-casting the component
>> values of a vector float cannot itself be represented as an immediate.
>> ---
>>  src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp b/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
>> index 81567d2..0a961ce 100644
>> --- a/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
>> @@ -113,6 +113,16 @@ try_constant_propagate(struct brw_context *brw, vec4_instruction *inst,
>>     if (value.file != IMM)
>>        return false;
>>
>> +   if (value.type == BRW_REGISTER_TYPE_VF) {
>> +      /* The result of bit-casting the component values of a vector float
>> +       * cannot in general be represented as an immediate.
>> +       */
>> +      if (inst->src[arg].type != BRW_REGISTER_TYPE_F)
>> +         return false;
>> +   } else {
>> +      value.type = inst->src[arg].type;
>> +   }
>> +
>>     if (inst->src[arg].abs) {
>>        if (!brw_abs_immediate(value.type, &value.fixed_hw_reg)) {
>>           return false;
>
> Wow. Did you hit this in practice?
>
I hit the non-VF case in practice (constant propagation forgetting the
original type of the source when propagating a bit-cast constant of a
different type).  I haven't seen the VF case in the wild but my addition
wouldn't have seemed correct without taking vector immediates into
account somehow.

> Actually, I suspect this might be the cause of
> https://bugs.freedesktop.org/show_bug.cgi?id=86811 (only reacquired a
> BDW recently, so never investigated)
>
> Hm, but running the test with INTEL_DEVID_OVERRIDE and
> INTEL_DEBUG=vec4vs doesn't show the VF immediate being propagated into
> the shift, so perhaps it's not the problem.

Hm, yes, I guess that might be something else...
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