[Qemu-devel] [PATCH for-2.8?] rules.mak: speedup save-vars load-vars

[Paolo Bonzini]

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).

Signed-off-by: Paolo Bonzini <address@hidden>
---
        I'm wondering if this would be acceptable for 2.8.
        I also have sent patches to GNU make that save another
        20%, down to 9.8 seconds.

 rules.mak | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/rules.mak b/rules.mak
index 0333ae3..c0777d7 100644
--- a/rules.mak
+++ b/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 caches the list of variables
+# that were 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
 
-- 
2.7.4