Am 11.03.2017 um 00:33 schrieb Junio C Hamano:
> René Scharfe <[email protected]> writes:
>
>> @ depends on r @
>> expression E;
>> @@
>> - *&
>> E
>
> I guess my source of the confusion is that the tool that understands
> the semantics of the C language still needs to be told about that.
Understanding that E and *&E have the same type does not imply (for a
machine) that E alone is preferable.
> I was hoping that something that understands C only needs to be told
> only a single rule:
>
> type T
> T src, dst
>
> -memcpy(&dst, &src, sizeof(dst));
> +dst = src;
>
> and then can apply that rule to this code in four ways:
>
> struct foo A, *Bp;
>
> memcpy(Bp, &A, sizeof(*Bp));
> memcpy(Bp, &A, sizeof(A));
> memcpy(&src, dstp, sizeof(A));
> memcpy(&src, dstp, sizeof(*Bp));
I guess src is A and dstp is Bp?
Coccinelle does not know that the sizeof expressions are equivalent,
but you could normalize them with an additional rule and then use a
single memcpy transformation like this:
@ normalize_sizeof @
type T;
T var;
@@
- sizeof(var)
+ sizeof(T)
@ r @
type T;
T *dst;
T *src;
@@
- memcpy(dst, src, sizeof(T));
+ *dst = *src;
@ depends on r @
expression E;
@@
- *&
E
That would give you what you expected here:
> to obtain its rewrite:
>
> struct foo A, *Bp;
>
> *Bp = A;
> *Bp = A;
> A = *Bp;
> A = *Bp;
But that normalization rule would be applied everywhere because it's so
broad, and that's probably not what we want. You could replace it with
an isomorphism like this one:
Expression
@@
type T;
T E;
@@
sizeof(T) => sizeof E
In your example it allows you to specify sizeof(struct foo) (or a
generic sizeof(T) as in rule r above) in the semantic patch and
Coccinelle would let that match sizeof(A) and sizeof(*Bp) in the C
file as well.
Isomorphisms are kept in a separate file, the default one is
/usr/lib/coccinelle/standard.iso on my machine and you can chose a
different one with --iso-file. Perhaps we want to have our own for
git, but we'd need to synchronize it with upstream from time to time.
There is already a very similar isomorphism rule in the default file,
but I don't think I understand it:
Expression
@ sizeof_type_expr @
pure type T; // pure because we drop a metavar
T E;
@@
sizeof(T) => sizeof E
In particular I'm a bit worried about the lack of documentation for
"pure", and I don't understand the comment. There's another comment
at the top of another rule in the same file:
// pure means that either the whole field reference expression is dropped,
// or E is context code and has no attached + code
// not really... pure means matches a unitary unplussed metavariable
// but this rule doesn't work anyway
Hmm.
Here's an isomorphism that allows you to use "sizeof *src" (the third
part for T is not strictly needed for your example):
Expression
@ sizeof_equiv @
type T;
T E1, E2;
@@
sizeof E1 <=> sizeof E2 <=> sizeof(T)
That's the kind of bidirectional equivalence you expected to be part
of Coccinelle's standard set of rules, right? With that rule in place
the following semantic patch matches your four cases:
@ r @
type T;
T *dst;
T *src;
@@
- memcpy(dst, src, sizeof *dst);
+ *dst = *src;
@ depends on r @
expression E;
@@
- *&
E
But I'd be careful with that as long as "pure" is present in that
standard rule and not fully understood. The isomorphism sizeof_equiv
doesn't seem to cause any trouble with our existing semantic patches
and the code in master, though.
René
