diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
index 6e100367997..b8217b714ff 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/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();
 }