rules.mak: speedup save-vars load-vars

Unnesting variables spends a lot of time parsing and executing foreach
and if functions.  Because actually very few variables have to be
saved and restored, a good strategy is to remember what has to be done
in load-vars, and only iterate the right variables in load-vars.
For save-vars, unroll the foreach loop to provide another small
improvement.

This speeds up a "noop" build from around 15.5 seconds on my laptop
to 11.7 (25% roughly).

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rules.mak

@@ -192,15 +192,15 @@ clean: clean-timestamp
 # save-vars
 # Usage: $(call save-vars, vars)
 # Save each variable $v in $vars as save-vars-$v, save their object's
-# variables, then clear $v.
+# variables, then clear $v.  saved-vars-$v contains the variables that
+# where saved for the objects, in order to speedup load-vars.
 define save-vars
     $(foreach v,$1,
         $(eval save-vars-$v := $(value $v))
-        $(foreach o,$($v),
-            $(foreach k,cflags libs objs,
-                $(if $($o-$k),
-                    $(eval save-vars-$o-$k := $($o-$k))
-                    $(eval $o-$k := ))))
+        $(eval saved-vars-$v := $(foreach o,$($v), \
+            $(if $($o-cflags), $o-cflags $(eval save-vars-$o-cflags := $($o-cflags))$(eval $o-cflags := )) \
+            $(if $($o-libs), $o-libs $(eval save-vars-$o-libs := $($o-libs))$(eval $o-libs := )) \
+            $(if $($o-objs), $o-objs $(eval save-vars-$o-objs := $($o-objs))$(eval $o-objs := ))))
         $(eval $v := ))
 endef
 
@@ -213,12 +213,10 @@ define load-vars
     $(eval $2-new-value := $(value $2))
     $(foreach v,$1,
         $(eval $v := $(value save-vars-$v))
-        $(foreach o,$($v),
-            $(foreach k,cflags libs objs,
-                $(if $(save-vars-$o-$k),
-                    $(eval $o-$k := $(save-vars-$o-$k))
-                    $(eval save-vars-$o-$k := ))))
-        $(eval save-vars-$v := ))
+        $(foreach o,$(saved-vars-$v),
+            $(eval $o := $(save-vars-$o)) $(eval save-vars-$o := ))
+        $(eval save-vars-$v := )
+        $(eval saved-vars-$v := ))
     $(eval $2 := $(value $2) $($2-new-value))
 endef