Re: [PATCH 03/10] fast-export: store anonymized oids as hex strings

From: Jeff King <peff@peff.net>
To: "SZEDER Gábor" <szeder.dev@gmail.com>
Cc: git@vger.kernel.org, Eric Sunshine <sunshine@sunshineco.com>,
	Junio C Hamano <gitster@pobox.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>
Subject: Re: [PATCH 03/10] fast-export: store anonymized oids as hex strings
Date: Wed, 24 Jun 2020 11:54:20 -0400	[thread overview]
Message-ID: <20200624155420.GC2088459@coredump.intra.peff.net> (raw)
In-Reply-To: <20200624114313.GJ2898@szeder.dev>

On Wed, Jun 24, 2020 at 01:43:13PM +0200, SZEDER Gábor wrote:

> >  	static uint32_t counter = 1; /* avoid null oid */
> >  	const unsigned hashsz = the_hash_algo->rawsz;
> > -	unsigned char *out = xcalloc(hashsz, 1);
> > -	put_be32(out + hashsz - 4, counter++);
> > -	return out;
> > +	struct object_id oid;
> > +	char *hex = xmallocz(GIT_MAX_HEXSZ);
> > +
> > +	oidclr(&oid);
> > +	put_be32(oid.hash + hashsz - 4, counter++);
> > +	return oid_to_hex_r(hex, &oid);
> >  }
> 
> GCC 4.8 complains about this change in our CI job:
> 
>   $ make CC=gcc-4.8 builtin/fast-export.o
>       CC builtin/fast-export.o
>   builtin/fast-export.c: In function ‘generate_fake_oid’:
>   builtin/fast-export.c:394:2: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
>     put_be32(oid.hash + hashsz - 4, counter++);
>     ^
>   cc1: all warnings being treated as errors

Interesting. The only change on this line is swapping out the local
"unsigned char *" for an object_id, which also stores an "unsigned
char" (though as an array). And while put_be32() takes a void pointer,
it's inlined and treats it the argument an "unsigned char *" internally.
So I'm not sure I see that any type punning is going on at all.

I'll see if I can appease the compiler, though.

-Peff