intel/fs: consider UNDEF as non-partial write

A few titles show max live register reductions, but nothing
significant in instruction count or other stats.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24282>

src/intel/compiler/brw_fs.cpp

@@ -658,6 +658,17 @@ fs_inst::is_partial_write() const
    if (this->opcode == SHADER_OPCODE_SEND)
       return false;
 
+   /* Special case UNDEF since a lot of places in the backend do things like this :
+    *
+    *  fs_builder ubld = bld.exec_all().group(1, 0);
+    *  fs_reg tmp = ubld.vgrf(BRW_REGISTER_TYPE_UD);
+    *  ubld.UNDEF(tmp); <- partial write, even if the whole register is concerned
+    */
+   if (this->opcode == SHADER_OPCODE_UNDEF) {
+      assert(this->dst.is_contiguous());
+      return this->size_written < 32;
+   }
+
    return this->exec_size * type_sz(this->dst.type) < 32 ||
           !this->dst.is_contiguous();
 }