diff --git a/src/cmd/go/internal/work/gc.go b/src/cmd/go/internal/work/gc.go
index f1d08e0..6031897 100644
--- a/src/cmd/go/internal/work/gc.go
+++ b/src/cmd/go/internal/work/gc.go
@@ -259,6 +259,15 @@
 			}
 		}
 	}
+	if p.ImportPath == "runtime" && objabi.Regabi_enabled != 0 {
+		// In order to make it easier to port runtime assembly
+		// to the register ABI, we introduce a macro
+		// indicating the experiment is enabled.
+		//
+		// TODO(austin): Remove this once we commit to the
+		// register ABI (#40724).
+		args = append(args, "-D=GOEXPERIMENT_REGABI=1")
+	}
 
 	if cfg.Goarch == "mips" || cfg.Goarch == "mipsle" {
 		// Define GOMIPS_value from cfg.GOMIPS.