nir: track if var copies lowering was called

In general we should only call it once, and then we should avoid to
call any lowering that introduce back copies. So far we were tracking
that manually out of the nir shader on several places.

Ideally we would like to add a nir_validate rule, but right now there
are some exceptions to this rule. For example right now the Intel
compiler calls nir_lower_io_to_temporaries as part of linking
tess_ctrl/mesh/task sahders.

One option would be to allow drivers to reset the value, but for now
let's not add that validation rule.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19338>

src/compiler/nir/nir_lower_var_copies.c

@@ -164,6 +164,8 @@ nir_lower_var_copies(nir_shader *shader)
 {
    bool progress = false;
 
+   shader->info.var_copies_lowered = true;
+
    nir_foreach_function(function, shader) {
       if (function->impl)
          progress |= lower_var_copies_impl(function->impl);

src/compiler/nir/nir_validate.c

@@ -643,6 +643,12 @@ validate_intrinsic_instr(nir_intrinsic_instr *instr, validate_state *state)
       validate_assert(state, glsl_get_bare_type(dst->type) ==
                              glsl_get_bare_type(src->type));
       validate_assert(state, !nir_deref_mode_may_be(dst, nir_var_read_only_modes));
+      /* FIXME: now that we track if the var copies were lowered, it would be
+       * good to validate here that no new copy derefs were added. Right now
+       * we can't as there are some specific cases where copies are added even
+       * after the lowering. One example is the Intel compiler, that calls
+       * nir_lower_io_to_temporaries when linking some shader stages.
+       */
       break;
    }
 

src/compiler/shader_info.h

@@ -297,6 +297,11 @@ typedef struct shader_info {
     */
    bool io_lowered:1;
 
+   /** Has nir_lower_var_copies called. To avoid calling any
+    * lowering/optimization that would introduce any copy_deref later.
+    */
+   bool var_copies_lowered:1;
+
    /* Whether the shader writes memory, including transform feedback. */
    bool writes_memory:1;