On Tue, Aug 21, 2018 at 11:03:44PM -0400, Jeff King wrote:
> So I wonder if there's some other way to tell the compiler that we'll
> only have a few values. An enum comes to mind, though I don't think the
> enum rules are strict enough to make this guarantee (after all, it's OK
> to bitwise-OR enums, so they clearly don't specify all possible values).

I was thinking about this:

diff --git a/cache.h b/cache.h
index 1398b2a4e4..1f5c6e9319 100644
--- a/cache.h
+++ b/cache.h
@@ -1033,7 +1033,14 @@ extern const struct object_id null_oid;
 
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
-	return memcmp(sha1, sha2, the_hash_algo->rawsz);
+	switch (the_hash_algo->rawsz) {
+		case 20:
+			return memcmp(sha1, sha2, 20);
+		case 32:
+			return memcmp(sha1, sha2, 32);
+		default:
+			assert(0);
+	}
 }
 
 static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)

That would make it obvious that there are at most two options.
Unfortunately, gcc for me determines that the buffer in walker.c is 20
bytes in size and steadfastly refuses to compile because it doesn't know
that the value will never be 32 in our codebase currently.  I'd need to
send in more patches before it would compile.

I don't know if something like this is an improvement or now, but this
seems to at least compile:

diff --git a/cache.h b/cache.h
index 1398b2a4e4..3207f74771 100644
--- a/cache.h
+++ b/cache.h
@@ -1033,7 +1033,13 @@ extern const struct object_id null_oid;
 
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
-	return memcmp(sha1, sha2, the_hash_algo->rawsz);
+	switch (the_hash_algo->rawsz) {
+		case 20:
+		case 32:
+			return memcmp(sha1, sha2, the_hash_algo->rawsz);
+		default:
+			assert(0);
+	}
 }
 
 static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)

I won't have time to sit down and test this out until tomorrow afternoon
at the earliest.  If you want to send in something in the mean time,
even if that limits things to just 20 for now, that's fine.
-- 
brian m. carlson: Houston, Texas, US
OpenPGP: https://keybase.io/bk2204