agx: Flip point coordinates because OpenGL

Fixes dEQP-GLES2.functional.shaders.builtin_variable.pointcoord

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11818>

src/asahi/compiler/agx_compile.c

@@ -1103,6 +1103,39 @@ agx_lower_front_face(struct nir_builder *b,
    return true;
 }
 
+static bool
+agx_lower_point_coord(struct nir_builder *b,
+                      nir_instr *instr, UNUSED void *data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+
+   if (intr->intrinsic != nir_intrinsic_load_deref)
+      return false;
+
+   nir_deref_instr *deref = nir_src_as_deref(intr->src[0]);
+   nir_variable *var = nir_deref_instr_get_variable(deref);
+
+   if (var->data.mode != nir_var_shader_in)
+      return false;
+
+   if (var->data.location != VARYING_SLOT_PNTC)
+      return false;
+
+   assert(intr->dest.is_ssa);
+   assert(intr->dest.ssa.num_components == 2);
+
+   b->cursor = nir_after_instr(&intr->instr);
+   nir_ssa_def *def = nir_load_deref(b, deref);
+   nir_ssa_def *y = nir_channel(b, def, 1);
+   nir_ssa_def *flipped_y = nir_fadd_imm(b, nir_fneg(b, y), 1.0);
+   nir_ssa_def *flipped = nir_vec2(b, nir_channel(b, def, 0), flipped_y);
+   nir_ssa_def_rewrite_uses(&intr->dest.ssa, flipped);
+   return true;
+}
+
 static void
 agx_optimize_nir(nir_shader *nir)
 {
@@ -1304,6 +1337,11 @@ agx_compile_shader_nir(nir_shader *nir,
       /* Lower from OpenGL [-1, 1] to [0, 1] if half-z is not set */
       if (!key->vs.clip_halfz)
          NIR_PASS_V(nir, nir_lower_clip_halfz);
+   } else if (ctx->stage == MESA_SHADER_FRAGMENT) {
+      /* Flip point coordinate since OpenGL and Metal disagree */
+      NIR_PASS_V(nir, nir_shader_instructions_pass,
+            agx_lower_point_coord,
+            nir_metadata_block_index | nir_metadata_dominance, NULL);
    }
 
    NIR_PASS_V(nir, nir_split_var_copies);