Re: [PATCH 7/7] doc: BPF CO-RE documentation

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Aug 4, 2021 at 8:01 PM David Faust via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Document the new command line options (-mcore and -mno-core), the new
> BPF target builtin (__builtin_preserve_access_index), and the new BPF
> target attribute (preserve_access_index) introduced with BPF CO-RE.

OK.

> gcc/ChangeLog:
>
>         * doc/extend.texi (BPF Type Attributes) New node.
>         Document new preserve_access_index attribute.
>         Document new preserve_access_index builtin.
> ---
>  gcc/doc/extend.texi | 16 ++++++++++++++++
>  gcc/doc/invoke.texi | 13 ++++++++++++-
>  2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index b83cd4919bb..bb5fc921907 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -8194,6 +8194,7 @@ attributes.
>  * Common Type Attributes::
>  * ARC Type Attributes::
>  * ARM Type Attributes::
> +* BPF Type Attributes::
>  * MeP Type Attributes::
>  * PowerPC Type Attributes::
>  * x86 Type Attributes::
> @@ -8757,6 +8758,17 @@ virtual table for @code{C} is not exported.  (You can use
>  @code{__attribute__} instead of @code{__declspec} if you prefer, but
>  most Symbian OS code uses @code{__declspec}.)
>
> +@node BPF Type Attributes
> +@subsection BPF Type Attributes
> +
> +@cindex @code{preserve_access_index} type attribute, BPF
> +BPF Compile Once - Run Everywhere (CO-RE) support. When attached to a
> +@code{struct} or @code{union} type definition, indicates that CO-RE
> +relocation information should be generated for any access to a variable
> +of that type. The behavior is equivalent to the programmer manually
> +wrapping every such access with @code{__builtin_preserve_access_index}.
> +
> +
>  @node MeP Type Attributes
>  @subsection MeP Type Attributes
>
> @@ -15388,6 +15400,10 @@ Load 16-bits from the @code{struct sk_buff} packet data pointed by the register
>  Load 32-bits from the @code{struct sk_buff} packet data pointed by the register @code{%r6} and return it.
>  @end deftypefn
>
> +@deftypefn {Built-in Function} void * __builtin_preserve_access_index (@var{expr})
> +BPF Compile Once-Run Everywhere (CO-RE) support. Instruct GCC to generate CO-RE relocation records for any accesses to aggregate data structures (struct, union, array types) in @var{expr}. This builtin is otherwise transparent, the return value is whatever @var{expr} evaluates to. It is also overloaded: @var{expr} may be of any type (not necessarily a pointer), the return type is the same. Has no effect if @code{-mcore} is not in effect (either specified or implied).
> +@end deftypefn
> +
>  @node FR-V Built-in Functions
>  @subsection FR-V Built-in Functions
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 32697e6117c..915bbc4ee65 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -903,7 +903,7 @@ Objective-C and Objective-C++ Dialects}.
>
>  @emph{eBPF Options}
>  @gccoptlist{-mbig-endian -mlittle-endian -mkernel=@var{version}
> --mframe-limit=@var{bytes} -mxbpf}
> +-mframe-limit=@var{bytes} -mxbpf -mcore -mno-core}
>
>  @emph{FR30 Options}
>  @gccoptlist{-msmall-model  -mno-lsim}
> @@ -22520,6 +22520,17 @@ Generate code for a big-endian target.
>  @opindex mlittle-endian
>  Generate code for a little-endian target.  This is the default.
>
> +@item -mcore
> +@opindex mcore
> +Enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and
> +is implied by @option{-gbtf}.
> +
> +@item -mno-core
> +@opindex mno-core
> +Disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE
> +support is enabled by default when generating BTF debug information for
> +the BPF target.
> +
>  @item -mxbpf
>  Generate code for an expanded version of BPF, which relaxes some of
>  the restrictions imposed by the BPF architecture:
> --
> 2.32.0
>