nir/linker: disable varying from uniform lowering by default

This fixes performance regression for Specviewperf/Energy
on AMD GPU. Other GPUs passing varying by memory may choose
to re-enable it as need.

Fixes: 2604625043 ("nir/linker: support uniform when optimizing varying")
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15341>

src/compiler/nir/nir.h

@@ -3578,6 +3578,15 @@ typedef struct nir_shader_compiler_options {
     */
    uint8_t support_indirect_inputs;
    uint8_t support_indirect_outputs;
+
+   /**
+    * Remove varying loaded from uniform, let fragment shader load the
+    * uniform directly. GPU passing varying by memory can benifit from it
+    * for sure; but GPU passing varying by on chip resource may not.
+    * Because it saves on chip resource but may increase memory pressure when
+    * fragment task is far more than vertex one, so better left it disabled.
+    */
+   bool lower_varying_from_uniform;
 } nir_shader_compiler_options;
 
 typedef struct nir_shader {

src/compiler/nir/nir_linking_helpers.c

@@ -1377,7 +1377,8 @@ nir_link_opt_varyings(nir_shader *producer, nir_shader *consumer)
       nir_ssa_def *ssa = intr->src[1].ssa;
       if (ssa->parent_instr->type == nir_instr_type_load_const) {
          progress |= replace_varying_input_by_constant_load(consumer, intr);
-      } else if (is_direct_uniform_load(ssa, &uni_scalar)) {
+      } else if (consumer->options->lower_varying_from_uniform &&
+                 is_direct_uniform_load(ssa, &uni_scalar)) {
          progress |= replace_varying_input_by_uniform_load(consumer, intr,
                                                            &uni_scalar);
       } else {