Hello, in the trunk's documentation
http://gcc.gnu.org/onlinedocs/gccint/GTY-Options.html I am reading:
> variable_size
> The type machinery expects the types to be of constant size.
> When this is not true, for example, with structs that have
> array fields or unions, the type machinery cannot tell how
> many bytes need to be allocated at each allocation. The
> variable_size is used to mark such types. The type machinery
> then provides allocators that take a parameter indicating an
> exact size of object being allocated.
>
> For example,
>
> struct GTY((variable_size)) sorted_fields_type {
> int len;
> tree GTY((length ("%h.len"))) elts[1];
> };
>
>
> Then the objects of struct sorted_fields_type are allocated in
> GC memory as follows:
>
> field_vec = ggc_alloc_sorted_fields_type (size);
I find the last paragraph and example very misleading, but perhaps I am not a
native english speaker and am understanding wrongly.
I would much prefer a sentence like
The type machinery then provides allocators that take a parameter indicating
an exact size of object being allocated (like malloc does)
and the exemple should better be
Then the objects of struct sorted_fields_type are allocated in GC memory as
follows, assuming n is the number of elements:
field_vec = ggc_alloc_sorted_fields_type (sizeof(sorted_fields_type) +
(n-1) * sizeof(tree));
The factor is n-1 (not n) since the struct sorted_fields_type is declared with
an array elts[1]
By the way, I would very much like that GCC code would assume that
variable-sized struct-ures are permitted, and declare instead
struct GTY((variable_size)) sorted_fields_type {
int len;
tree GTY((length ("%h.len"))) elts[];
};
Perhaps such a transition is needed, or at least very useful, even before any
transition to C++ ?
Cheers.
P.S. I am not yet proposing a gty.texi patch, because I have still a one-line
patch to it awaiting approval
http://gcc.gnu.org//ml/gcc-patches/2010-07/msg00744.html and I usually don't
work on patching files for which I have patches awaiting review.
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Basile STARYNKEVITCH http://starynkevitch.net/Basile/
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