[Mesa-dev] [PATCH 9/9] glsl: Convert mul/div by power-of-two factors to shift expressions.

[Kenneth Graunke]

On 04/07/2014 11:00 AM, Matt Turner wrote:
> On Sun, Apr 6, 2014 at 11:49 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>> Integer shifts are basically always well supported and efficient; that
>> isn't always true of integer division, and sometimes even integer
>> multiplication isn't without issues.
>>
>> On some Intel hardware, INTDIV can't be used in SIMD16 mode.  It also
>> doesn't support immediate operands (on any generation), while ASR can.
>>
>> On Haswell, this cuts the number of instructions in dolphin/efb2ram by
>> 7.94%.  It also removes a single MOV in dolphin/realxfb (due to ASR
>> supporting immediates), and gains SIMD16 support (due to no INTDIV).
> 
> Just a note that the upstream shaders have already been changed to not
> do integer division. That's the reason I didn't bother fixing up my
> patch a couple of weeks ago.

Sure.  I was actually looking at a different program (some webgl demo)
when I started implementing this, though.  Seems generally useful.

[snip]
>> +   ir_constant_data shift_data;
>> +   for (int i = 0; i < const_op->type->vector_elements; i++) {
>> +      if (const_op->type->base_type == GLSL_TYPE_INT &&
>> +          const_op->value.i[i] <= 0) {
>> +         /* Negative values aren't powers of two. */
> 
> I was going to comment on the is_power_of_two() function returning
> true for 0 and how this was going to break things, but I see this
> check for <= 0 should handle it.

Right.

> I don't know what is_power_of_two() should return for zero. The
> comment "Negative values" would be more correct if we returned false
> from is_power_of_two() though.

Not sure either.  I just moved the function from i965, and also found
that Gallium's u_math.h has a util_is_power_of_two() function which is
implemented identically - that's actually where the comment comes from.

--Ken

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