diff --git a/libretro-build-common.sh b/libretro-build-common.sh
index 746542cb..43465b76 100755
--- a/libretro-build-common.sh
+++ b/libretro-build-common.sh
@@ -1,5 +1,7 @@
 # vim: set ts=3 sw=3 noet ft=sh : bash
 
+. ${BASE_DIR}/script-modules/fetch-rules.sh
+
 die() {
 	echo $1
 	#exit 1
@@ -44,6 +46,25 @@ build_summary_log() {
 	fi
 }
 
+build_should_skip() {
+	build_revision_file="$WORKDIR/build_revisions/$1"
+
+	[ -z "$SKIP_UNCHANGED" ] && return 1
+	[ ! -r "$build_revision_file" ] && return 1
+
+	read previous_revision < "$build_revision_file"
+	[ "$previous_revision" != "$(fetch_revision $2)" ] && return 1
+
+	return 0
+}
+
+build_save_revision() {
+	[ -z "$SKIP_UNCHANGED" ] && return
+	[ "$1" != "0" ] && return
+	echo $(fetch_revision) > "$WORKDIR/build_revisions/$2"
+}
+
+
 check_opengl() {
 	if [ "${BUILD_LIBRETRO_GL}" ]; then
 		if [ "${ENABLE_GLES}" ]; then
@@ -127,12 +148,14 @@ copy_core_to_dist() {
 	if [ "$FORMAT_COMPILER_TARGET" = "theos_ios" ]; then
 		echo "cp \"objs/obj/${1}_libretro${FORMAT}.$FORMAT_EXT\" \"$RARCH_DIST_DIR\""
 		cp "objs/obj/${1}_libretro${FORMAT}.$FORMAT_EXT" "$RARCH_DIST_DIR"
-		build_summary_log $? "$1"
 	else
 		echo "cp \"${1}_libretro${FORMAT}.$FORMAT_EXT\" \"$RARCH_DIST_DIR\""
 		cp "${1}_libretro${FORMAT}.$FORMAT_EXT" "$RARCH_DIST_DIR"
-		build_summary_log $? "$1"
 	fi
+
+	ret=$?
+	build_summary_log $ret "$1"
+	return $ret
 }
 
 build_libretro_generic() {
@@ -148,18 +171,25 @@ build_libretro_generic() {
 
 # build_libretro_generic_makefile
 #
-# $1	Display name of the core
+# $1	Name of the core
 # $2	Subdirectory of makefile (use "." for none)
 # $3	Name of makefile
 # $4	Either FORMAT_COMPILER_TARGET or an alternative
 # $5	Skip copying (for cores that don't produce exactly one core)
 build_libretro_generic_makefile() {
 	build_dir="$WORKDIR/libretro-$1"
+
+	if build_should_skip $1 "$build_dir"; then
+		echo "Core $1 is already built, skipping..."
+		return
+	fi
+
 	if [ -d "$build_dir" ]; then
 		echo "=== Building $1 ==="
 		build_libretro_generic $1 "$2" "$3" $4 "$build_dir"
 		if [ -z "$5" ]; then
 			copy_core_to_dist $1
+			build_save_revision $? $1
 		fi
 	else
 		echo "$1 not fetched, skipping ..."
@@ -168,10 +198,17 @@ build_libretro_generic_makefile() {
 
 build_retroarch_generic_makefile() {
 	build_dir="$WORKDIR/$1"
+
+	if build_should_skip $1 "$build_dir"; then
+		echo "Core $1 is already built, skipping..."
+		return
+	fi
+
 	if [ -d "$build_dir" ]; then
 		echo "=== Building $2 ==="
 		build_libretro_generic $1 "$2" "$3" $4 "$build_dir"
 		copy_core_to_dist $5
+		build_save_revision $? $1
 	else
 		echo "$1 not fetched, skipping ..."
 	fi
@@ -214,11 +251,20 @@ build_libretro_catsfc() {
 }
 
 build_libretro_emux() {
+	if build_should_skip emux "$WORKDIR/libretro-emux"; then
+		echo "Cores for emux are already built, skipping..."
+		return
+	fi
+
 	build_libretro_generic_makefile "emux" "libretro" "Makefile" $FORMAT_COMPILER_TARGET 1
+
 	copy_core_to_dist "emux_chip8"
 	copy_core_to_dist "emux_gb"
 	copy_core_to_dist "emux_nes"
 	copy_core_to_dist "emux_sms"
+
+	# TODO: Check for more than emux_sms here...
+	build_save_revision $? "emux"
 }
 
 build_libretro_test() {
@@ -406,6 +452,12 @@ build_libretro_ppsspp() {
 
 build_libretro_mame() {
 	build_dir="$WORKDIR/libretro-mame"
+
+	if build_should_skip mame "$build_dir"; then
+		echo "Core mame is already built, skipping..."
+		return
+	fi
+
 	if [ -d "$build_dir" ]; then
 		echo ''
 		echo '=== Building MAME ==='
@@ -441,6 +493,9 @@ build_libretro_mame() {
 		fi
 		echo "cp \"mame_libretro${FORMAT}.$FORMAT_EXT\" \"$RARCH_DIST_DIR\""
 		cp "mame_libretro${FORMAT}.$FORMAT_EXT" "$RARCH_DIST_DIR"
+		ret=$?
+		build_save_revision $ret mame
+		build_summary_log $ret "mame"
 	else
 		echo 'MAME not fetched, skipping ...'
 	fi
@@ -595,27 +650,53 @@ build_libretro_bsnes_modern() {
 		$MAKE -f Makefile platform="$FORMAT_COMPILER_TARGET" compiler="$CXX11" ui='target-libretro' profile="${3}" "-j$JOBS" || die "Failed to build $1 $3 core"
 		echo "cp -f \"out/${1}_libretro${FORMAT}.$FORMAT_EXT\" \"$RARCH_DIST_DIR/${1}_${3}_libretro${FORMAT}.$FORMAT_EXT\""
 		cp -f "out/${1}_libretro${FORMAT}.$FORMAT_EXT" "$RARCH_DIST_DIR/${1}_${3}_libretro${FORMAT}.$FORMAT_EXT"
-		build_summary_log $? "${1}_$3"
+		ret=$?
+		build_summary_log $ret "${1}_$3"
+
+		return $ret
 	else
 		echo "$1 $3 not fetched, skipping ..."
 	fi
 }
 
 build_libretro_bsnes() {
+	if build_should_skip bsnes "$WORKDIR/libretro-bsnes"; then
+		echo "Core bsnes is already built, skipping..."
+		return
+	fi
+
 	build_libretro_bsnes_modern "bsnes" "perf" "performance"
 	build_libretro_bsnes_modern "bsnes" "balanced" "balanced"
 	build_libretro_bsnes_modern "bsnes" "." "accuracy"
+
+	# TODO: Make this not depend on accuracy
+	build_save_revision $? bsnes
 }
 
 build_libretro_bsnes_mercury() {
+	if build_should_skip bsnes_mercury "$WORKDIR/libretro-bsnes"; then
+		echo "Core bsnes_mercury is already built, skipping..."
+		return
+	fi
+
+	set +x
 	build_libretro_bsnes_modern "bsnes_mercury" "perf" "performance"
 	build_libretro_bsnes_modern "bsnes_mercury" "balanced" "balanced"
 	build_libretro_bsnes_modern "bsnes_mercury" "." "accuracy"
+
+	# TODO: Make this not depend on accuracy
+	build_save_revision $? bsnes_mercury
 }
 
 build_libretro_bsnes_cplusplus98() {
 	CORENAME="bsnes_cplusplus98"
 	build_dir="$WORKDIR/libretro-$CORENAME"
+
+	if build_should_skip $CORENAME "$build_dir"; then
+		echo "Core $CORENAME is already built, skipping..."
+		return
+	fi
+
 	if [ -d "$build_dir" ]; then
 		echo "=== Building $CORENAME ==="
 		echo "cd \"$build_dir\""
@@ -629,7 +710,9 @@ build_libretro_bsnes_cplusplus98() {
 		$MAKE platform="$FORMAT_COMPILER_TARGET" CC="$CC" CXX="$CXX" "-j$JOBS"
 		echo "cp \"out/libretro.$FORMAT_EXT\" \"$RARCH_DIST_DIR/$CORENAME_libretro${FORMAT}.$FORMAT_EXT\""
 		cp "out/libretro.$FORMAT_EXT" "$RARCH_DIST_DIR/$CORENAME_libretro${FORMAT}.$FORMAT_EXT"
-		build_summary_log $? $CORENAME
+		ret=$?
+		build_summary_log $ret $CORENAME
+		build_save_revision $ret $CORENAME
 	else
 		echo "$CORENAME not fetched, skipping ..."
 	fi
@@ -637,6 +720,12 @@ build_libretro_bsnes_cplusplus98() {
 
 build_libretro_bnes() {
 	build_dir="$WORKDIR/libretro-bnes"
+
+	if build_should_skip bnes "$build_dir"; then
+		echo "Core bnes is already built, skipping..."
+		return
+	fi
+
 	if [ -d "$build_dir" ]; then
 		echo '=== Building bNES ==='
 		echo "cd \"$build_dir\""
@@ -651,7 +740,9 @@ build_libretro_bnes() {
 		$MAKE -f Makefile CC="$CC" CXX="$CXX" "-j$JOBS" compiler="${CXX11}" || die 'Failed to build bNES'
 		echo "cp \"libretro${FORMAT}.$FORMAT_EXT\" \"$RARCH_DIST_DIR/bnes_libretro${FORMAT}.$FORMAT_EXT\""
 		cp "libretro${FORMAT}.$FORMAT_EXT" "$RARCH_DIST_DIR/bnes_libretro${FORMAT}.$FORMAT_EXT"
-		build_summary_log $? "bnes"
+		ret=$?
+		build_summary_log $ret "bnes"
+		build_save_revision $ret "bnes"
 	else
 		echo 'bNES not fetched, skipping ...'
 	fi
@@ -660,8 +751,14 @@ build_libretro_bnes() {
 build_libretro_mupen64() {
 	check_opengl
 	build_dir="$WORKDIR/libretro-mupen64plus"
+
+	if build_should_skip mupen64plus "$build_dir"; then
+		echo "Core mupen64plus is already built, skipping..."
+		return
+	fi
+
 	if [ -d "$build_dir" ]; then
-		echo cd \"$build_dir\"
+		echo "cd \"$build_dir\""
 		cd "$build_dir"
 
 		mkdir -p obj
@@ -700,7 +797,9 @@ build_libretro_mupen64() {
 		fi
 		echo "cp \"mupen64plus_libretro${FORMAT}.$FORMAT_EXT\" \"$RARCH_DIST_DIR\""
 		cp "mupen64plus_libretro${FORMAT}.$FORMAT_EXT" "$RARCH_DIST_DIR"
-		build_summary_log $? "mupen64plus"
+		ret=$?
+		build_summary_log $ret "mupen64plus"
+		build_save_revision $ret "mupen64plus"
 	else
 		echo 'Mupen64 Plus not fetched, skipping ...'
 	fi
diff --git a/libretro-config.sh b/libretro-config.sh
index 2791c035..424c08a0 100755
--- a/libretro-config.sh
+++ b/libretro-config.sh
@@ -196,6 +196,20 @@ fi
 BUILD_SUMMARY="$WORKDIR/build-summary.log"
 
 
+# BUILD_REVISIONS
+# ===============
+#
+# libretro-super can save a revision string (e.g., the git sha hash) for any
+# core it has compiled.  If this feature is enabled, it will check if the
+# revison string has changed before it compiles the core.  This can speed up
+# the build process for end-users and buildbots, and it also results in nightly
+# build directories being smaller.  It is not enabled by default because it
+# cannot know about uncommitted changes in a working directory.
+
+# Set this to enable the feature
+#SKIP_UNCHANGED=1
+
+
 #USER DEFINES
 #------------
 #These options should be defined inside your own
diff --git a/script-modules/fetch-rules.sh b/script-modules/fetch-rules.sh
index a964f327..31a2ceb3 100644
--- a/script-modules/fetch-rules.sh
+++ b/script-modules/fetch-rules.sh
@@ -37,10 +37,19 @@ fetch_git() {
 	fi
 }
 
-# revision_git: # Output the hash of the last commit in a git repository
+# fetch_revision_git: Output the hash of the last commit in a git repository
 #
 # $1	Local directory to run git in
-revision_git() {
-	cd "$WORKDIR/$1"
+fetch_revision_git() {
+	[ -n "$1" ] && cd "$1"
 	git log -n 1 --pretty=format:%H
 }
+
+
+# fetch_revision: Output SCM-dependent revision string of a module
+#                 (currently just calls fetch_revision_git)
+#
+# $1	The directory of the module
+fetch_revision() {
+	   fetch_revision_git $1
+}