[PATCH v2 11/11] fast-export: use local array to store anonymized oid

From: Jeff King <peff@peff.net>
Date: Jun 25, 2020, 19:48 UTC
Message-ID: <20200625194837.GK4029374@coredump.intra.peff.net>
In-Reply-To: <20200625194802.GA4028913@coredump.intra.peff.net>

Some older versions of gcc complain about this line:

  builtin/fast-export.c:412:2: error: dereferencing type-punned pointer
       will break strict-aliasing rules [-Werror=strict-aliasing]
    put_be32(oid.hash + hashsz - 4, counter++);
    ^

This seems to be a false positive, as there's no type-punning at all here. oid.hash is an array of unsigned char; when we pass it to a function it decays to a pointer to unsigned char. We do take a void pointer in put_be32(), but it's immediately aliased with another pointer to unsigned char (and clearly the compiler is looking inside the inlined put_be32(), since the warning doesn't happen with -O0).

This happens on gcc 4.8 and 4.9, but not later versions (I tested gcc 6, 7, 8, and 9).

We can work around it by using a local array instead of an object_id struct. This is a little more intimate with the details of object_id, but for whatever reason doesn't seem to trigger the compiler warning. We can revert this patch once we decide that those gcc versions are too old to care about for a warning like this (gcc 4.8 is the default compiler for Ubuntu Trusty, which is out-of-support but not fully end-of-life'd until April 2022).

Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/fast-export.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index c6ecf404d7..9f37895d4c 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -405,12 +405,12 @@ static char *generate_fake_oid(void *data)
 {
 	static uint32_t counter = 1; /* avoid null oid */
 	const unsigned hashsz = the_hash_algo->rawsz;
-	struct object_id oid;
+	unsigned char out[GIT_MAX_RAWSZ];
 	char *hex = xmallocz(GIT_MAX_HEXSZ);
 
-	oidclr(&oid);
-	put_be32(oid.hash + hashsz - 4, counter++);
-	return oid_to_hex_r(hex, &oid);
+	hashclr(out);
+	put_be32(out + hashsz - 4, counter++);
+	return hash_to_hex_algop_r(hex, out, the_hash_algo);
 }
 
 static const char *anonymize_oid(const char *oid_hex)

-- 
2.27.0.593.gb3082a2aaf