Re: [PATCH] blame.c: replace instance of !oidcmp for oideq

From: Jeff King <peff@peff.net>
Date: Sep 9, 2020, 19:13 UTC
Message-ID: <20200909191345.GA2511547@coredump.intra.peff.net>
In-Reply-To: <CAOc6etZS3mGxsPPh25XFi2-qR0TNzq0Gx1NrydgQwmHbsjxejA@mail.gmail.com>

On Wed, Sep 09, 2020 at 08:00:57AM -0600, Edmundo Carmona Antoranz wrote:

Show 17 quoted lines

> On Wed, Sep 9, 2020 at 3:11 AM Jeff King <peff@peff.net> wrote:
> >
> > Yeah, it looks obviously correct. I am puzzled why "make coccicheck"
> > doesn't find this, though. +cc René, as my favorite target for
> > coccinelle nerd-snipes. :)
> >
> 
> I added this to contrib/coccinelle/object_id.cocci in v2.27.0
> 
> @@
> identifier f != oideq;
> expression E1, E2;
> @@
> - !oidcmp(E1, E2)
> + oideq(E1, E2)
> 
> And it found it:

Interesting. The existing rule is:

  struct object_id *OIDPTR1;
  struct object_id *OIDPTR2;
  @@
  - oidcmp(OIDPTR1, OIDPTR2) == 0
  + oideq(OIDPTR1, OIDPTR2)

The "== 0" part looks like it might be significant, but it's not. Coccinelle knows that "!foo" is the same as "foo == 0" (and you can confirm by tweaking it).

The addition of "identifer f != oideq" here isn't necessary (we don't even define an "f" in the semantic patch part). And anyway, we use hasheq() inside oideq(), so no need to override the rule there.

So the relevant part is probably that our existing rule specifies the exact type, whereas your rule allows any expression.

And indeed, if I do this, it works:

diff --git a/contrib/coccinelle/object_id.cocci b/contrib/coccinelle/object_id.cocci
index ddf4f22bd7..62a6cee0eb 100644
--- a/contrib/coccinelle/object_id.cocci
+++ b/contrib/coccinelle/object_id.cocci
@@ -55,8 +55,8 @@ struct object_id OID;
 + oidcmp(&OID, OIDPTR)
 
 @@
-struct object_id *OIDPTR1;
-struct object_id *OIDPTR2;
+expression OIDPTR1;
+expression OIDPTR2;
 @@
 - oidcmp(OIDPTR1, OIDPTR2) == 0
 + oideq(OIDPTR1, OIDPTR2)

Which really _seems_ like a bug in coccinelle, unless I am missing
something. Because both of those parameters look like object_id pointers
(and the compiler would be complaining if it were not the case).  But I
also wonder if giving the specific types in the coccinelle rule is
buying us anything. If you passed two void pointers or ints or whatever
to !oidcmp(), we'd still want to rewrite it as oideq().

-Peff