> On Aug 20, 2024, at 05:58, Richard Biener <richard.guent...@gmail.com> wrote:
>
> On Tue, Aug 13, 2024 at 5:34 PM Qing Zhao <qing.z...@oracle.com> 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)?
Yes, that’s the initial request from Kees. -)
The title of PR116016 is: add __builtin_set_counted_by(P->FAM, COUNT) or
equivalent
After extensive discussion (Martin Uecker raised the initial idea in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116016#c24, more discussions
followed, till comments #31). we decided to provide
__builtin_get_counted_by(PTR) instead of __builtin_set_counted_by(PTR, VALUE)
due to the following two reasons:
1. __builtin_get_counted_by should be enough to provide the functionality, and
even simpler;
2. More flexible to be used by the programmer to be able to both WRITE and READ
the counted-by field.
>
> Note that __builtin_get_counted_by to me suggests it returns the
> value and not a pointer to the value.
The syntax of __builtin_get_counted_by is:
TYPE __builtin_get_counted_by (PTR)
The returned value is:
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.
> A more proper language extension might involve a keyword
> like __real, so __counted_by X would produce an lvalue, selecting
> the counted-by member.
Yes, if the returned value could be a LVALUE instead of a Pointer, that’s even
simpler and cleaner.
However, then as you mentioned below, another builtin
“__builtin_has_attribute(PTR, counted_by)” need
to be queried first to make sure the counted_by field exists.
We have discussed this approach, and I preferred this approach too.
However, the main reason we gave up on that direction is:
There is NO __builtin_has_attribute (PTR, counted_by) been supported by CLANG,
and not sure how difficult for CLANG to add this new builtin.
In order to provide consistent interface to Linux kernel from both CLANG and
GCC, I finally decided to provide the current interface in order to be
consistent with CLANG.
Bill, could you please provide a little bit more info on the possibility of a
new builtin __builtin_has_attribute() in CLANG?
> 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
Could you please explain this a little bit more? (Not quite understood, a small
example will be better, -:) thanks a lot.
> or __builtin_classify_type instead of overloading
> the value producing builtin might be more fit here?
You mean the following lines from the testing case:
+ if (__builtin_get_counted_by (__p->FAM)) \
+ *(__builtin_get_counted_by(__p->FAM)) = COUNT; \
How to improve it? (Thanks a lot for your suggestion).
Qing
>
> 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
