intel/compiler: Use nir_split_conversions()

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34266>

src/intel/compiler/intel_nir.c

@@ -5,6 +5,56 @@
 
 #include "intel_nir.h"
 
+unsigned
+intel_nir_split_conversions_cb(const nir_instr *instr, void *data)
+{
+   nir_alu_instr *alu = nir_instr_as_alu(instr);
+
+   unsigned src_bit_size = nir_src_bit_size(alu->src[0].src);
+   nir_alu_type src_type = nir_op_infos[alu->op].input_types[0];
+   nir_alu_type src_full_type = (nir_alu_type) (src_type | src_bit_size);
+
+   unsigned dst_bit_size = alu->def.bit_size;
+   nir_alu_type dst_full_type = nir_op_infos[alu->op].output_type;
+
+   /* BDW PRM, vol02, Command Reference Instructions, mov - MOVE:
+    *
+    *   "There is no direct conversion from HF to DF or DF to HF.
+    *    Use two instructions and F (Float) as an intermediate type.
+    *
+    *    There is no direct conversion from HF to Q/UQ or Q/UQ to HF.
+    *    Use two instructions and F (Float) or a word integer type
+    *    or a DWord integer type as an intermediate type."
+    *
+    * It is important that the intermediate conversion happens through a
+    * 32-bit float type so we don't lose range when we convert from
+    * a 64-bit integer.
+    */
+   if ((src_full_type == nir_type_float16 && dst_bit_size == 64) ||
+       (src_bit_size == 64 && dst_full_type == nir_type_float16))
+      return 32;
+
+   /* SKL PRM, vol 02a, Command Reference: Instructions, Move:
+    *
+    *   "There is no direct conversion from B/UB to DF or DF to B/UB. Use
+    *    two instructions and a word or DWord intermediate type."
+    *
+    *   "There is no direct conversion from B/UB to Q/UQ or Q/UQ to B/UB.
+    *    Use two instructions and a word or DWord intermediate integer
+    *    type."
+    *
+    * It is important that we use a 32-bit integer matching the sign of the
+    * destination as the intermediate type so we avoid any chance of rtne
+    * rounding happening before the conversion to integer (which is expected
+    * to round towards zero) in double to byte conversions.
+    */
+   if ((src_bit_size == 8 && dst_bit_size == 64) ||
+       (src_bit_size == 64 && dst_bit_size == 8))
+      return 32;
+
+   return 0;
+}
+
 bool
 intel_nir_pulls_at_sample(nir_shader *shader)
 {