spirv: Emit NIR deref instructions on-the-fly

This simplifies our deref handling by emitting the actual NIR deref
instructions on-the-fly instead of of building up a deref chain and then
emitting them at the last moment.  In order for this to work with the
parts of the compiler that assume they can chase deref chains, we have
to run nir_rematerialize_derefs_in_use_blocks_impl to put the derefs
back in the right places.  Otherwise, in cases such as loop continues
where the SPIR-V blocks are not in the same order as the NIR blocks, we
may end up with a deref chain with a parent that does not dominate it's
child and nir_repair_ssa_impl will insert phis in the deref chain.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/compiler/spirv/vtn_cfg.c

@@ -1053,6 +1053,8 @@ vtn_function_emit(struct vtn_builder *b, struct vtn_function *func,
    vtn_foreach_instruction(b, func->start_block->label, func->end,
                            vtn_handle_phi_second_pass);
 
+   nir_rematerialize_derefs_in_use_blocks_impl(func->impl);
+
    /* Continue blocks for loops get inserted before the body of the loop
     * but instructions in the continue may use SSA defs in the loop body.
     * Therefore, we need to repair SSA to insert the needed phi nodes.

src/compiler/spirv/vtn_private.h

@@ -443,20 +443,9 @@ struct vtn_pointer {
     */
    struct vtn_variable *var;
 
-   /** The deref at the base of the chain
+   /** The NIR deref corresponding to this pointer */
-    *
-    * This field may be NULL if the pointer uses a (block_index, offset) pair
-    * instead of an access chain or if the access chain starts at a variable.
-    */
    nir_deref_instr *deref;
 
-   /** An access chain describing how to get from var to the referenced data
-    *
-    * This field may be NULL if the pointer references the entire variable or
-    * if a (block_index, offset) pair is used instead of an access chain.
-    */
-   struct vtn_access_chain *chain;
-
    /** A (block_index, offset) pair representing a UBO or SSBO position. */
    struct nir_ssa_def *block_index;
    struct nir_ssa_def *offset;

src/compiler/spirv/vtn_variables.c

@@ -43,21 +43,6 @@ vtn_access_chain_create(struct vtn_builder *b, unsigned length)
    return chain;
 }
 
-static struct vtn_access_chain *
-vtn_access_chain_extend(struct vtn_builder *b, struct vtn_access_chain *old,
-                        unsigned new_ids)
-{
-   struct vtn_access_chain *chain;
-
-   unsigned old_len = old ? old->length : 0;
-   chain = vtn_access_chain_create(b, old_len + new_ids);
-
-   for (unsigned i = 0; i < old_len; i++)
-      chain->link[i] = old->link[i];
-
-   return chain;
-}
-
 static bool
 vtn_pointer_uses_ssa_offset(struct vtn_builder *b,
                             struct vtn_pointer *ptr)
@@ -82,29 +67,42 @@ vtn_pointer_is_external_block(struct vtn_builder *b,
 
 /* Dereference the given base pointer by the access chain */
 static struct vtn_pointer *
-vtn_access_chain_pointer_dereference(struct vtn_builder *b,
+vtn_nir_deref_pointer_dereference(struct vtn_builder *b,
-                                     struct vtn_pointer *base,
+                                  struct vtn_pointer *base,
-                                     struct vtn_access_chain *deref_chain)
+                                  struct vtn_access_chain *deref_chain)
 {
-   struct vtn_access_chain *chain =
-      vtn_access_chain_extend(b, base->chain, deref_chain->length);
    struct vtn_type *type = base->type;
    enum gl_access_qualifier access = base->access;
 
+   nir_deref_instr *tail;
+   if (base->deref) {
+      tail = base->deref;
+   } else {
+      assert(base->var && base->var->var);
+      tail = nir_build_deref_var(&b->nb, base->var->var);
+   }
+
    /* OpPtrAccessChain is only allowed on things which support variable
     * pointers.  For everything else, the client is expected to just pass us
     * the right access chain.
     */
    vtn_assert(!deref_chain->ptr_as_array);
 
-   unsigned start = base->chain ? base->chain->length : 0;
    for (unsigned i = 0; i < deref_chain->length; i++) {
-      chain->link[start + i] = deref_chain->link[i];
-
       if (glsl_type_is_struct(type->type)) {
          vtn_assert(deref_chain->link[i].mode == vtn_access_mode_literal);
-         type = type->members[deref_chain->link[i].id];
+         unsigned idx = deref_chain->link[i].id;
+         tail = nir_build_deref_struct(&b->nb, tail, idx);
+         type = type->members[idx];
       } else {
+         nir_ssa_def *index;
+         if (deref_chain->link[i].mode == vtn_access_mode_literal) {
+            index = nir_imm_int(&b->nb, deref_chain->link[i].id);
+         } else {
+            vtn_assert(deref_chain->link[i].mode == vtn_access_mode_id);
+            index = vtn_ssa_value(b, deref_chain->link[i].id)->def;
+         }
+         tail = nir_build_deref_array(&b->nb, tail, index);
          type = type->array_element;
       }
 
@@ -115,8 +113,7 @@ vtn_access_chain_pointer_dereference(struct vtn_builder *b,
    ptr->mode = base->mode;
    ptr->type = type;
    ptr->var = base->var;
-   ptr->deref = base->deref;
+   ptr->deref = tail;
-   ptr->chain = chain;
    ptr->access = access;
 
    return ptr;
@@ -362,7 +359,7 @@ vtn_pointer_dereference(struct vtn_builder *b,
    if (vtn_pointer_uses_ssa_offset(b, base)) {
       return vtn_ssa_offset_pointer_dereference(b, base, deref_chain);
    } else {
-      return vtn_access_chain_pointer_dereference(b, base, deref_chain);
+      return vtn_nir_deref_pointer_dereference(b, base, deref_chain);
    }
 }
 
@@ -412,39 +409,15 @@ vtn_pointer_to_deref(struct vtn_builder *b, struct vtn_pointer *ptr)
    if (ptr->var && ptr->var->copy_prop_sampler)
       return vtn_pointer_to_deref(b, ptr->var->copy_prop_sampler);
 
-   nir_deref_instr *tail;
+   vtn_assert(!vtn_pointer_uses_ssa_offset(b, ptr));
-   if (ptr->deref) {
+   if (!ptr->deref) {
-      tail = ptr->deref;
+      struct vtn_access_chain chain = {
-   } else {
+         .length = 0,
-      assert(ptr->var && ptr->var->var);
+      };
-      tail = nir_build_deref_var(&b->nb, ptr->var->var);
+      ptr = vtn_nir_deref_pointer_dereference(b, ptr, &chain);
    }
 
-   /* Raw variable access */
+   return ptr->deref;
-   if (!ptr->chain)
-      return tail;
-
-   struct vtn_access_chain *chain = ptr->chain;
-   vtn_assert(chain);
-
-   for (unsigned i = 0; i < chain->length; i++) {
-      if (glsl_type_is_struct(tail->type)) {
-         vtn_assert(chain->link[i].mode == vtn_access_mode_literal);
-         unsigned idx = chain->link[i].id;
-         tail = nir_build_deref_struct(&b->nb, tail, idx);
-      } else {
-         nir_ssa_def *index;
-         if (chain->link[i].mode == vtn_access_mode_literal) {
-            index = nir_imm_int(&b->nb, chain->link[i].id);
-         } else {
-            vtn_assert(chain->link[i].mode == vtn_access_mode_id);
-            index = vtn_ssa_value(b, chain->link[i].id)->def;
-         }
-         tail = nir_build_deref_array(&b->nb, tail, index);
-      }
-   }
-
-   return tail;
 }
 
 static void
@@ -1447,7 +1420,6 @@ var_decoration_cb(struct vtn_builder *b, struct vtn_value *val, int member,
 
    if (val->value_type == vtn_value_type_pointer) {
       assert(val->pointer->var == void_var);
-      assert(val->pointer->chain == NULL);
       assert(member == -1);
    } else {
       assert(val->value_type == vtn_value_type_type);