3 files changed, 53 insertions, 0 deletions
diff --git a/contrib/README b/contrib/README
index 5885aa9..d145490 100644
--- a/contrib/README
+++ b/contrib/README
@@ -5,6 +5,13 @@ By default, re-synthesis of a project (eg, after making a change to any HDL
 files) will attempt to use the previous place-and-route netlist as a starting
 point via the "smartguide" mechanism. 'make mostlyclean' will prevent this.
 
+By default, a git checkout will update timestamps on all files under version
+control. There are good reasons for this, but it can be particularly
+frustrating when there are large coregen cores in a project which must be
+rebuilt from scratch, even is the .xco files were not touched. The
+'untouchcores' target will try to cleverly reset timestamps to prevent this;
+run it after a git pull or checkout.
+
 Changes to Makefiles will *not* necessarily result in a full rebuild; the
 makefiles themselves are not a prerequisite of any targets. It is *strongly*
 recommended that you 'make clean' (not just 'mostlyclean') after any
@@ -20,6 +27,8 @@ make lint           - runs lint program on synthesizable Verilog files
 make mostlyclean    - cleans most sim and synth files
 make clean          - cleans all sim and synth files, incl. coregen'd
 
+make untouchcores   - reset timestamps on .xco files to last change
+
 make isim/<name>_tb   - compiles sim files, then launches simulator GUI
 make resim/<name>_tb  - recompiles sim files w/o launching GUI
 make test/<name>_tb   - runs a single unit test
diff --git a/contrib/git_untouch.sh b/contrib/git_untouch.sh
new file mode 100755
index 0000000..c9d3ede
--- /dev/null
+++ b/contrib/git_untouch.sh
@@ -0,0 +1,40 @@
+#!/bin/bash -e
+
+# From http://stackoverflow.com/questions/1964470/whats-the-equivalent-of-use-commit-times-for-git
+# Modified to only run on files passed on command line
+
+OS=${OS:-`uname`}
+
+get_file_rev() {
+    git rev-list -n 1 HEAD "$1"
+}    
+
+if [ "$OS" = 'FreeBSD' ]
+then
+    update_file_timestamp() {
+        file_time=`date -r "$(git show --pretty=format:%at --abbrev-commit "$(get_file_rev "$1")" | head -n 1)" '+%Y%m%d%H%M.%S'`
+        touch -h -t "$file_time" "$1"
+    }    
+else    
+    update_file_timestamp() {
+        file_time=`git show --pretty=format:%ai --abbrev-commit "$(get_file_rev "$1")" | head -n 1`
+        touch -d "$file_time" "$1"
+    }    
+fi    
+
+OLD_IFS=$IFS
+IFS=$'\n'
+
+for file in $@
+do
+    if [ ! `git status --ignored --porcelain $file` ]
+    then
+        update_file_timestamp "$file"
+    fi
+done
+
+IFS=$OLD_IFS
+
+git update-index --refresh &> /dev/null || true
+
+exit 0
diff --git a/contrib/xilinx.mk b/contrib/xilinx.mk
index 7c16cd7..2d5c0a5 100644
--- a/contrib/xilinx.mk
+++ b/contrib/xilinx.mk
@@ -132,6 +132,10 @@ $(coregen_work_dir)/$(project).cgp: contrib/template.cgp
 	@echo "SET speedgrade = $(speedgrade)" >> $@
 	@echo "SET workingdirectory = ./tmp/" >> $@
 
+untouchcores:
+	@echo "Resetting .xco timestamps so that cores won't be rebuilt"
+	./contrib/git_untouch.sh $(xilinx_cores)
+
 %.ngc %.v: %.xco $(coregen_work_dir)/$(project).cgp
 	@echo "=== rebuilding $@"
 	@bash -c "$(xil_env); \