Date: Sun, 11 Oct 2009 23:42:33 -0700
From: Josh Triplett <josh@joshtriplett.org>
To: Christopher Li <sparse@chrisli.org>
Cc: linux-sparse@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] New attribute designated_init: mark a struct as
 requiring designated init
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On Sun, Oct 11, 2009 at 10:18:24PM -0700, Christopher Li wrote:
> On Sat, Oct 10, 2009 at 9:03 AM, Josh Triplett <josh@joshtriplett.org> wrote:
> > After getting this patch reviewed, I intended to add many such
> > annotations myself. ?I have patches sitting in my local Linux tree.
> > (However, I don't plan to try to get the patches accepted into Linux
> > until this attribute shows up in a Sparse release, since I don't want to
> > make Linux require the latest Sparse from the git tree.)
> 
> I apply your patch so you can have a fair chance to pitch to the
> kernel. If nobody use it, I can remove it later. I don't think you need
> to wait for the patch to go into sparse release to collect feed back
> from lkml.

Oh, definitely.  I just don't think the patches should actually get
accepted into Linux before the next Sparse release.  I certainly plan to
*submit* them. :)

Thanks for accepting the patch.

> > You *hope* that positional initialization will fail. :) ?Some of these
> > types of structures contain many fields with the same or similar types,
> > and initializers don't inherently contain a lot of distinguishing type
> > information that helps cause type errors rather than silent failures.
> 
> I am very interested to know some precedence from the kernel commit history.
> If we can find such a commit which changes the structure and breaks
> other initialization due to the position initialization, it will make your
> case stronger as well.

I don't know of any cases in the commit history off the top of my head.
However, I do recall seeing this come up during LKML review comments on
patch, where the patch used positional initializers and the comments
suggested designated initializers.

> > Even if the positional initialization does fail, this failure will occur
> > at the time of attempting to change the structure, and many such
> > failures may occur all over the tree, making such changes significantly
> > more difficult. ?With the designated_init attribute, sparse will warn
> > when attempting to create the structure initializer.
> 
> Without sparse, you can temporary  insert some unused strange type
> into the beginning of the struct. That should cause any position initializer
> fail to compile. You can then find call the call site.

Interesting idea!

However, again, this doesn't trigger the warning as soon as someone adds
a positional initializer; the person who wants to change the structure
will have to do this kind of work.  Warnings help ensure that those
types of changes get made by the person submitting potentially
problematic code, rather than the person who will trip over that code
later.  Warnings also reduce the load on patch reviewers, by making
certain types of issues less likely to make it to patch submission time,
and by letting reviewers suggest tools like sparse rather than having to
point out each new issue.

> > With the designated_init attribute, someone making changes to a
> > structure can check that Sparse produces no warnings about positional
> > initializers and then know that certain types of changes can occur
> > without having to check every initializer. ?For instance, removing a
> > field will only require checking any initializer that initializes that
> > field, and the compiler will find all of those. ?Adding a field that can
> > safely remain NULL does not require checking any initializers.
> 
> I think the initializer is just a small part of the work if you even change
> the structure. You need to make sure the *code* use these C struct
> actually works as expected.

Very true.  However, the types of structs for which the designated_init
attribute will prove useful often already follow semantics that avoid
the need to check the code.  For instance, as long as you know that a
field can safely remain 0 except in code that wants to use it, you can
add a new field without checking every designated initializer.

> > (As an aside, though: any fundamental reason why we use that error-prone
> > approach rather than allocating one extra byte and NUL-terminating
> > idents? ?Then we could drop the (larger) len field, and remove the
> > annoying restriction on calling show_ident multiple times in the same
> > statement.)
> 
> When sparse does the hash table look up, if there is collision on the
> hash table entry, sparse will compare the size first before the memcmp().
> That will reduce the number of the memcmp call it makes. I would like to
> keep the size of struct ident. If any thing, we can NUL terminate the string
> in ident to make printing easy.

Fair enough on the hash comparisons, though I do wonder how much value
we get from that compared to a highly optimized memcmp implementation;
if the strings differ in the first 4 or 8 bytes, I'd expect memcmp to
notice after one comparison.  But even if we keep the length, requiring
NUL termination seems highly useful.

- Josh Triplett
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