On Tue, Aug 13, 2024 at 5:34 PM Qing Zhao <[email protected]> wrote:
>
> With the addition of the 'counted_by' attribute and its wide roll-out
> within the Linux kernel, a use case has been found that would be very
> nice to have for object allocators: being able to set the counted_by
> counter variable without knowing its name.
>
> For example, given:
>
> struct foo {
> ...
> int counter;
> ...
> struct bar array[] __attribute__((counted_by (counter)));
> } *p;
>
> The existing Linux object allocators are roughly:
>
> #define alloc(P, FAM, COUNT) ({ \
> size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT; \
> kmalloc(__size, GFP); \
> })
>
> Right now, any addition of a counted_by annotation must also
> include an open-coded assignment of the counter variable after
> the allocation:
>
> p = alloc(p, array, how_many);
> p->counter = how_many;
>
> In order to avoid the tedious and error-prone work of manually adding
> the open-coded counted-by intializations everywhere in the Linux
> kernel, a new GCC builtin __builtin_get_counted_by will be very useful
> to be added to help the adoption of the counted-by attribute.
>
> -- Built-in Function: TYPE __builtin_get_counted_by (PTR)
> The built-in function '__builtin_get_counted_by' checks whether the
> array object pointed by the pointer PTR has another object
> associated with it that represents the number of elements in the
> array object through the 'counted_by' attribute (i.e. the
> counted-by object). If so, returns a pointer to the corresponding
> counted-by object. If such counted-by object does not exist,
> returns a NULL pointer.
>
> This built-in function is only available in C for now.
>
> The argument PTR must be a pointer to an array. The TYPE of the
> returned value must be a pointer type pointing to the corresponding
> type of the counted-by object or a pointer type pointing to the
> SIZE_T in case of a NULL pointer being returned.
>
> With this new builtin, the central allocator could be updated to:
>
> #define alloc(P, FAM, COUNT) ({ \
> typeof(P) __p; \
> size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT; \
> __p = kmalloc(__size, GFP); \
> if (__builtin_get_counted_by (__p->FAM)) \
> *(__builtin_get_counted_by(__p->FAM)) = COUNT; \
> __p; \
> })
>
> And then structs can gain the counted_by attribute without needing
> additional open-coded counter assignments for each struct, and
> unannotated structs could still use the same allocator.
Did you consider a __builtin_set_counted_by (PTR, VALUE)?
Note that __builtin_get_counted_by to me suggests it returns the
value and not a pointer to the value.
A more proper language extension might involve a keyword
like __real, so __counted_by X would produce an lvalue, selecting
the counted-by member. You'd need another way to query whether
'X' has a counted-by member - the apparent runtime test in your
example is odd for a statically typed language, type selection
like with generics or __builtin_classify_type instead of overloading
the value producing builtin might be more fit here?
No objection to the patch but I wanted to share my thoughts here.
Richard.
> Bootstrapped and regression tested on both X86 and aarch64, no issue.
>
> Okay for trunk?
>
> thanks.
>
> Qing.
>
>
> PR c/116016
>
> gcc/c-family/ChangeLog:
>
> * c-common.cc: Add new __builtin_get_counted_by.
> * c-common.h (enum rid): Add RID_BUILTIN_GET_COUNTED_BY.
>
> gcc/c/ChangeLog:
>
> * c-decl.cc (names_builtin_p): Add RID_BUILTIN_GET_COUNTED_BY.
> * c-parser.cc (has_counted_by_object): New routine.
> (get_counted_by_ref): New routine.
> (c_parser_postfix_expression): Handle New RID_BUILTIN_GET_COUNTED_BY.
>
> gcc/ChangeLog:
>
> * doc/extend.texi: Add documentation for __builtin_get_counted_by.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/builtin-get-counted-by-1.c: New test.
> * gcc.dg/builtin-get-counted-by.c: New test.
> ---
> gcc/c-family/c-common.cc | 1 +
> gcc/c-family/c-common.h | 1 +
> gcc/c/c-decl.cc | 1 +
> gcc/c/c-parser.cc | 72 +++++++++++++++
> gcc/doc/extend.texi | 55 +++++++++++
> .../gcc.dg/builtin-get-counted-by-1.c | 91 +++++++++++++++++++
> gcc/testsuite/gcc.dg/builtin-get-counted-by.c | 54 +++++++++++
> 7 files changed, 275 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/builtin-get-counted-by-1.c
> create mode 100644 gcc/testsuite/gcc.dg/builtin-get-counted-by.c
>
> diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
> index e7e371fd26f..4b27c6bfeeb 100644
> --- a/gcc/c-family/c-common.cc
> +++ b/gcc/c-family/c-common.cc
> @@ -430,6 +430,7 @@ const struct c_common_resword c_common_reswords[] =
> { "__builtin_choose_expr", RID_CHOOSE_EXPR, D_CONLY },
> { "__builtin_complex", RID_BUILTIN_COMPLEX, D_CONLY },
> { "__builtin_convertvector", RID_BUILTIN_CONVERTVECTOR, 0 },
> + { "__builtin_get_counted_by", RID_BUILTIN_GET_COUNTED_BY, D_CONLY },
> { "__builtin_has_attribute", RID_BUILTIN_HAS_ATTRIBUTE, 0 },
> { "__builtin_launder", RID_BUILTIN_LAUNDER, D_CXXONLY },
> { "__builtin_shuffle", RID_BUILTIN_SHUFFLE, 0 },
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index 2510ee4dbc9..5d5a297012f 100644
> --- a/gcc/c-family/c-common.h
> +++ b/gcc/c-family/c-common.h
> @@ -110,6 +110,7 @@ enum rid
> RID_TYPES_COMPATIBLE_P, RID_BUILTIN_COMPLEX,
> RID_BUILTIN_SHUFFLE,
> RID_BUILTIN_SHUFFLEVECTOR, RID_BUILTIN_CONVERTVECTOR,
> RID_BUILTIN_TGMATH,
> RID_BUILTIN_HAS_ATTRIBUTE, RID_BUILTIN_ASSOC_BARRIER, RID_BUILTIN_STDC,
> + RID_BUILTIN_GET_COUNTED_BY,
> RID_DFLOAT32, RID_DFLOAT64, RID_DFLOAT128,
>
> /* TS 18661-3 keywords, in the same sequence as the TI_* values. */
> diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
> index 8cef8f2c289..40bc808878b 100644
> --- a/gcc/c/c-decl.cc
> +++ b/gcc/c/c-decl.cc
> @@ -11739,6 +11739,7 @@ names_builtin_p (const char *name)
> case RID_BUILTIN_SHUFFLE:
> case RID_BUILTIN_SHUFFLEVECTOR:
> case RID_BUILTIN_STDC:
> + case RID_BUILTIN_GET_COUNTED_BY:
> case RID_CHOOSE_EXPR:
> case RID_OFFSETOF:
> case RID_TYPES_COMPATIBLE_P:
> diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
> index 9b9284b1ba4..7b179449fb0 100644
> --- a/gcc/c/c-parser.cc
> +++ b/gcc/c/c-parser.cc
> @@ -10646,6 +10646,35 @@ c_parser_predefined_identifier (c_parser *parser)
> return expr;
> }
>
> +/* Check whether the ARRAY_REF has an counted-by object associated with it
> + through the "counted_by" attribute. */
> +static bool
> +has_counted_by_object (tree array_ref)
> +{
> + /* Currently, only when the array_ref is an indirect_ref to a call to the
> + .ACCESS_WITH_SIZE, return true.
> + More cases can be included later when the counted_by attribute is
> + extended to other situations. */
> + if ((TREE_CODE (array_ref) == INDIRECT_REF)
> + && is_access_with_size_p (TREE_OPERAND (array_ref, 0)))
> + return true;
> + return false;
> +}
> +
> +/* Get the reference to the counted-by object associated with the ARRAY_REF.
> */
> +static tree
> +get_counted_by_ref (tree array_ref)
> +{
> + /* Currently, only when the array_ref is an indirect_ref to a call to the
> + .ACCESS_WITH_SIZE, get the corresponding counted_by ref.
> + More cases can be included later when the counted_by attribute is
> + extended to other situations. */
> + if ((TREE_CODE (array_ref) == INDIRECT_REF)
> + && is_access_with_size_p (TREE_OPERAND (array_ref, 0)))
> + return CALL_EXPR_ARG (TREE_OPERAND (array_ref, 0), 1);
> + return NULL_TREE;
> +}
> +
> /* Parse a postfix expression (C90 6.3.1-6.3.2, C99 6.5.1-6.5.2,
> C11 6.5.1-6.5.2). Compound literals aren't handled here; callers have to
> call c_parser_postfix_expression_after_paren_type on encountering them.
> @@ -11740,6 +11769,49 @@ c_parser_postfix_expression (c_parser *parser)
> set_c_expr_source_range (&expr, loc, close_paren_loc);
> break;
> }
> + case RID_BUILTIN_GET_COUNTED_BY:
> + {
> + vec<c_expr_t, va_gc> *cexpr_list;
> + c_expr_t *e_p;
> + location_t close_paren_loc;
> +
> + c_parser_consume_token (parser);
> + if (!c_parser_get_builtin_args (parser,
> + "__builtin_get_counted_by",
> + &cexpr_list, false,
> + &close_paren_loc))
> + {
> + expr.set_error ();
> + break;
> + }
> + if (vec_safe_length (cexpr_list) != 1)
> + {
> + error_at (loc, "wrong number of arguments to "
> + "%<__builtin_get_counted_by%>");
> + expr.set_error ();
> + break;
> + }
> +
> + e_p = &(*cexpr_list)[0];
> +
> + if (TREE_CODE (TREE_TYPE (e_p->value)) != ARRAY_TYPE)
> + {
> + error_at (loc, "the argument must be an array"
> + "%<__builtin_get_counted_by%>");
> + expr.set_error ();
> + break;
> + }
> +
> + else if (has_counted_by_object ((*cexpr_list)[0].value))
> + expr.value
> + = get_counted_by_ref ((*cexpr_list)[0].value);
> + else
> + expr.value
> + = build_int_cst (build_pointer_type (size_type_node), 0);
> +
> + set_c_expr_source_range (&expr, loc, close_paren_loc);
> + break;
> + }
> case RID_BUILTIN_SHUFFLE:
> {
> vec<c_expr_t, va_gc> *cexpr_list;
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 48b27ff9f39..485f6f8dd8d 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -15198,6 +15198,61 @@ initializers of variables usable in constant
> expressions. For more details
> refer to the latest revision of the C++ standard.
> @enddefbuiltin
>
> +@defbuiltin{@var{type} __builtin_get_counted_by (@var{ptr})}
> +The built-in function @code{__builtin_get_counted_by} checks whether the
> array
> +object pointed by the pointer @var{ptr} has another object associated with it
> +that represents the number of elements in the array object through the
> +@code{counted_by} attribute (i.e. the counted-by object). If so, returns a
> +pointer to the corresponding counted-by object.
> +If such counted-by object does not exist, returns a NULL pointer.
> +
> +This built-in function is only available in C for now.
> +
> +The argument @var{ptr} must be a pointer to an array.
> +The @var{type} of the returned value must be a pointer type pointing to the
> +corresponding type of the counted-by object or a pointer type pointing to
> +the @var{size_t} in case of a NULL pointer being returned.
> +
> +For example:
> +
> +@smallexample
> +struct foo1 @{
> + int counter;
> + struct bar1 array[] __attribute__((counted_by (counter)));
> +@} *p;
> +
> +struct foo2 @{
> + int other;
> + struct bar2 array[];
> +@} *q;
> +@end smallexample
> +
> +@noindent
> +the following call to the built-in
> +
> +@smallexample
> +__builtin_get_counted_by (p->array)
> +@end smallexample
> +
> +@noindent
> +returns:
> +
> +@smallexample
> +&p->counter with type @code{int *}
> +@end smallexample
> +
> +@noindent
> +However, the following call to the built-in
> +
> +@smallexample
> +__builtin_get_counted_by (q->array)
> +@end smallexample
> +
> +@noindent
> +returns a NULL pointer with type @code{size_t *}.
> +
> +@enddefbuiltin
> +
> @defbuiltin{void __builtin_clear_padding (@var{ptr})}
> The built-in function @code{__builtin_clear_padding} function clears
> padding bits inside of the object representation of object pointed by
> diff --git a/gcc/testsuite/gcc.dg/builtin-get-counted-by-1.c
> b/gcc/testsuite/gcc.dg/builtin-get-counted-by-1.c
> new file mode 100644
> index 00000000000..fb195f03970
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/builtin-get-counted-by-1.c
> @@ -0,0 +1,91 @@
> +/* Test the code generation for the new __builtin_get_counted_by. */
> +/* { dg-do run } */
> +/* { dg-options "-O2" } */
> +#include <stdio.h>
> +
> +struct annotated {
> + char b;
> + int c[] __attribute ((counted_by (b)));
> +} *array_annotated;
> +
> +struct flex {
> + short b;
> + int c[];
> +} *array_flex;
> +
> +struct nested_annotated {
> + struct {
> + union {
> + int b;
> + float f;
> + };
> + int n;
> + };
> + char c[] __attribute__ ((counted_by (b)));
> +} *array_nested_annotated;
> +
> +struct nested_flex {
> + struct {
> + union {
> + unsigned int b;
> + float f;
> + };
> + int n;
> + };
> + char c[];
> +} *array_nested_flex;
> +
> +#define MY_ALLOC(P, FAM, COUNT) ({ \
> + typeof(P) __p; \
> + size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT; \
> + __p = (typeof(P)) __builtin_malloc(__size); \
> + __builtin_memset(__p, 0, __size); \
> + if (__builtin_get_counted_by (__p->FAM)) \
> + *(__builtin_get_counted_by(__p->FAM)) = COUNT; \
> + P = __p; \
> +})
> +
> +int count;
> +
> +int main(int argc, char *argv[])
> +{
> + MY_ALLOC(array_annotated, c, 10);
> + if (array_annotated->b != 10)
> + __builtin_abort ();
> +
> + MY_ALLOC(array_flex, c, 20);
> + if (array_flex->b == 20)
> + __builtin_abort ();
> +
> + MY_ALLOC(array_nested_annotated, c, 30);
> + if (array_nested_annotated->b != 30)
> + __builtin_abort ();
> +
> + MY_ALLOC(array_nested_flex, c, 40);
> + if (array_nested_flex->b == 40)
> + __builtin_abort ();
> +
> + count = array_annotated->b * 2 + array_nested_annotated->b * 3;
> + struct annotated * annotated_p;
> + struct flex * flex_p;
> + struct nested_annotated * nested_annotated_p;
> + struct nested_flex * nested_flex_p;
> +
> + MY_ALLOC(annotated_p, c, count);
> + if (annotated_p->b != count)
> + __builtin_abort ();
> +
> + MY_ALLOC(flex_p, c, count * 2);
> + if (flex_p->b == count * 2)
> + __builtin_abort ();
> +
> + MY_ALLOC(nested_annotated_p, c, count * 3);
> + if (nested_annotated_p->b != count * 3)
> + __builtin_abort ();
> +
> + MY_ALLOC(nested_flex_p, c, count * 4);
> + if (nested_flex_p->b == count * 4)
> + __builtin_abort ();
> +
> + return 0;
> +}
> diff --git a/gcc/testsuite/gcc.dg/builtin-get-counted-by.c
> b/gcc/testsuite/gcc.dg/builtin-get-counted-by.c
> new file mode 100644
> index 00000000000..5eca12bc992
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/builtin-get-counted-by.c
> @@ -0,0 +1,54 @@
> +/* Testing the correct usage of the new __builtin_get_counted_by. */
> +/* { dg-do compile } */
> +/* { dg-options "-O" } */
> +
> +#include <stdio.h>
> +
> +struct annotated {
> + size_t b;
> + int other;
> + int c[] __attribute ((counted_by (b)));
> +} *array_annotated;
> +
> +struct flex {
> + size_t b;
> + int other;
> + int c[];
> +} *array_flex;
> +
> +#define MY_ALLOC(P, FAM, COUNT) ({ \
> + typeof(P) __p; \
> + size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT; \
> + __p = (typeof(P)) __builtin_malloc(__size); \
> + if (__builtin_get_counted_by (__p->FAM)) \
> + *(__builtin_get_counted_by(__p->FAM)) = COUNT; \
> + __p; \
> +})
> +
> +extern char c_count;
> +extern short s_count;
> +extern int i_count;
> +extern long l_count;
> +extern float f_count;
> +
> +extern int * foo ();
> +
> +int main(int argc, char *argv[])
> +{
> + /* The good usages. */
> + MY_ALLOC(array_annotated, c, 10);
> + MY_ALLOC(array_flex, c, 20);
> + MY_ALLOC(array_annotated, c, c_count);
> + MY_ALLOC(array_flex, c, i_count);
> + MY_ALLOC(array_annotated, c, l_count);
> + MY_ALLOC(array_flex, c, c_count * 3);
> + MY_ALLOC(array_annotated, c, l_count * i_count);
> +
> + /* The bad usages, issue errors. */
> + __builtin_get_counted_by (); /* { dg-error "wrong number of arguments to"
> } */
> + __builtin_get_counted_by (array_annotated->c, 10); /* { dg-error "wrong
> number of arguments to" } */
> + __builtin_get_counted_by (array_annotated->other); /* { dg-error "the
> argument must be an array" } */
> + __builtin_get_counted_by (foo()); /* { dg-error "the argument must be an
> array" } */
> +
> + return 0;
> +}
> --
> 2.31.1
>
