[Mesa-dev] [PATCH 16/26] i965: Implement the new imod and irem opcodes

[Matt Turner]

On Fri, Mar 25, 2016 at 4:12 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> ---
>  src/mesa/drivers/dri/i965/brw_fs_nir.cpp   | 32 ++++++++++++++++++++++++++++++
>  src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 32 ++++++++++++++++++++++++++++++
>  2 files changed, 64 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> index 14480fb..131f50e 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> @@ -844,8 +844,40 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr)
>        unreachable("Should have been lowered by borrow_to_arith().");
>
>     case nir_op_umod:
> +   case nir_op_irem:
> +      /* According to the sign table for INT DIV in the Ivy Bridge PRM, it
> +       * appears that our hardware just does the right thing for signed
> +       * remainder.
> +       */
> +      bld.emit(SHADER_OPCODE_INT_REMAINDER, result, op[0], op[1]);
> +      break;
> +
> +   case nir_op_imod: {
> +      /* Get a regular C-style remainder.  If a % b == 0, set the predicate. */
>        bld.emit(SHADER_OPCODE_INT_REMAINDER, result, op[0], op[1]);
> +
> +      /* Math instructions don't support conditional mod */
> +      inst = bld.MOV(bld.null_reg_d(), result);
> +      inst->conditional_mod = BRW_CONDITIONAL_NZ;
> +
> +      /* Now, we need to determine if signs of the sources are different.
> +       * When we XOR the sources, the top bit is 0 if they are the same and 1
> +       * if they are different.  We can then use a conditional modifier to
> +       * turn that into a predicate.  This leads us to an XOR.l instruction.
> +       */
> +      fs_reg tmp = bld.vgrf(BRW_REGISTER_TYPE_D);
> +      inst = bld.XOR(tmp, op[0], op[1]);
> +      inst->predicate = BRW_PREDICATE_NORMAL;
> +      inst->conditional_mod = BRW_CONDITIONAL_L;

This goes against the PRM:

"This operation does not produce sign or overflow conditions. Only the
.e/.z or .ne/.nz conditional modifiers should be used."