[Mesa-dev] [PATCH] i965/vec4: Implement b2f and b2i using negation.

[Matt Turner]

On Sun, Oct 11, 2015 at 5:26 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Sat, Oct 10, 2015 at 1:17 PM, Matt Turner <mattst88 at gmail.com> wrote:
>> Curro added this in commit 3ee2daf23d (before the vec4/NIR backend was
>> added) but it was missed in the new NIR backend. Add it there as well.
>>
>> instructions in affected programs:     1857 -> 1810 (-2.53%)
>> helped:                                15
>> ---
>>  src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 8 +-------
>>  1 file changed, 1 insertion(+), 7 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
>> index 41bd80d..fdf767d 100644
>> --- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
>> @@ -1237,14 +1237,8 @@ vec4_visitor::nir_emit_alu(nir_alu_instr *instr)
>>        break;
>>
>>     case nir_op_b2i:
>> -      emit(AND(dst, op[0], src_reg(1)));
>> -      break;
>> -
>>     case nir_op_b2f:
>> -      op[0].type = BRW_REGISTER_TYPE_D;
>> -      dst.type = BRW_REGISTER_TYPE_D;
>> -      emit(AND(dst, op[0], src_reg(0x3f800000u)));
>> -      dst.type = BRW_REGISTER_TYPE_F;
>> +      emit(MOV(dst, negate(op[0])));
>
> For my own edification, could you explain how this works? Let's say
> you have a sequence like...
>
> bool b = ...; /* b contains 0 or ~0 "internally" */
> int x = floatBitsToInt((float)b) + 1;

b2f generates a type-converting MOV: mov dst:F, -src0:D.

Input of 0 is 0 negated, and 0.0f. Input of ~0 (-1) is 1 negated, and 1.0f.

Your sequence would generate

mov r0:F, -r1:D
add r2:D, r0:D, 1

and copy propagation won't mess things up because it only operates on
mixed-types in known-safe cases.

(I haven't confirmed this)