Use MD5 checksums for recompilation checking (fixes #1372, #1959)

This is a much more robust way to do recompilation checking.  The idea
is to create a fingerprint of the ABI of an interface, and track
dependencies by recording the fingerprints of ABIs that a module
depends on.  If any of those ABIs have changed, then we need to
recompile.

In bug #1372 we weren't recording dependencies on package modules,
this patch fixes that by recording fingerprints of package modules
that we depend on.  Within a package there is still fine-grained
recompilation avoidance as before.

We currently use MD5 for fingerprints, being a good compromise between
efficiency and security.  We're not worried about attackers, but we
are worried about accidental collisions.

All the MD5 sums do make interface files a bit bigger, but compile
times on the whole are about the same as before.  Recompilation
avoidance should be a bit more accurate than in 6.8.2 due to fixing
#1959, especially when using -O.

1 files changed, 24 insertions, 0 deletions

diff --git a/compiler/utils/md5.h b/compiler/utils/md5.h
new file mode 100644
index 0000000000..8d375df268
--- /dev/null
+++ b/compiler/utils/md5.h
@@ -0,0 +1,24 @@
+/* MD5 message digest */
+#ifndef _MD5_H
+#define _MD5_H
+
+#include "HsFFI.h"
+
+typedef HsWord32 word32;
+typedef HsWord8  byte;
+
+struct MD5Context {
+	word32 buf[4];
+	word32 bytes[2];
+	word32 in[16];
+};
+
+void MD5Init(struct MD5Context *context);
+void MD5Update(struct MD5Context *context, byte const *buf, int len);
+void MD5Final(byte digest[16], struct MD5Context *context);
+void MD5Transform(word32 buf[4], word32 const in[16]);
+
+#endif /* _MD5_H */
+
+
+