intel/compiler: Document and assert some aspects of 8-bit integer lowering

In the vec4 compiler, 8-bit types should never exist.

In the scalar compiler, 8-bit types should only ever be able to exist on
Gfx ver 8 and 9.

Some instructions are handled in non-obvious ways.

Hopefully this will save the next person some time.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9025>

src/intel/compiler/brw_fs_nir.cpp

@@ -965,6 +965,39 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr,
    fs_reg op[NIR_MAX_VEC_COMPONENTS];
    fs_reg result = prepare_alu_destination_and_sources(bld, instr, op, need_dest);
 
+#ifndef NDEBUG
+   /* Everything except raw moves, some type conversions, iabs, and ineg
+    * should have 8-bit sources lowered by nir_lower_bit_size in
+    * brw_preprocess_nir or by brw_nir_lower_conversions in
+    * brw_postprocess_nir.
+    */
+   switch (instr->op) {
+   case nir_op_mov:
+   case nir_op_vec2:
+   case nir_op_vec3:
+   case nir_op_vec4:
+   case nir_op_vec8:
+   case nir_op_vec16:
+   case nir_op_i2f16:
+   case nir_op_i2f32:
+   case nir_op_i2i16:
+   case nir_op_i2i32:
+   case nir_op_u2f16:
+   case nir_op_u2f32:
+   case nir_op_u2u16:
+   case nir_op_u2u32:
+   case nir_op_iabs:
+   case nir_op_ineg:
+      break;
+
+   default:
+      for (unsigned i = 0; i < nir_op_infos[instr->op].num_inputs; i++) {
+         assert((devinfo->ver == 8 || devinfo->ver == 9) ||
+                type_sz(op[i].type) > 1);
+      }
+   }
+#endif
+
    switch (instr->op) {
    case nir_op_mov:
    case nir_op_vec2:

src/intel/compiler/brw_nir.c

@@ -645,6 +645,11 @@ lower_bit_size_callback(const nir_instr *instr, UNUSED void *data)
       if (alu->dest.dest.ssa.bit_size >= 32)
          return 0;
 
+      /* Note: nir_op_iabs and nir_op_ineg are not lowered here because the
+       * 8-bit ABS or NEG instruction should eventually get copy propagated
+       * into the MOV that does the type conversion.  This results in far
+       * fewer MOV instructions.
+       */
       switch (alu->op) {
       case nir_op_idiv:
       case nir_op_imod:
@@ -666,6 +671,9 @@ lower_bit_size_callback(const nir_instr *instr, UNUSED void *data)
       case nir_op_fsin:
       case nir_op_fcos:
          return devinfo->ver < 9 ? 32 : 0;
+      case nir_op_isign:
+         assert(!"Should have been lowered by nir_opt_algebraic.");
+         return 0;
       default:
          if (devinfo->ver >= 11) {
             if (nir_op_infos[alu->op].num_inputs >= 2 &&

src/intel/compiler/brw_vec4_nir.cpp

@@ -1145,6 +1145,14 @@ vec4_visitor::nir_emit_alu(nir_alu_instr *instr)
       op[i].swizzle = brw_swizzle_for_nir_swizzle(instr->src[i].swizzle);
    }
 
+#ifndef NDEBUG
+   /* On Gen7 and earlier, no functionality is exposed that should allow 8-bit
+    * integer types to ever exist.
+    */
+   for (unsigned i = 0; i < nir_op_infos[instr->op].num_inputs; i++)
+      assert(type_sz(op[i].type) > 1);
+#endif
+
    switch (instr->op) {
    case nir_op_mov:
       try_immediate_source(instr, &op[0], true);