[PATCH 5/5] eventdev: fix compilation with clang C++ builds

[Stephen Hemminger]

On Fri, 11 Mar 2022 20:05:23 +0000
Bruce Richardson <bruce.richardson at intel.com> wrote:

> When compiling on FreeBSD with clang and include checking enabled,
> errors are emitted due to differences in how empty structs/unions are
> handled in C and C++, as C++ structs cannot have zero size.
> 
> ../lib/eventdev/rte_eventdev.h:992:2: error: union has size 0 in C, non-zero size in C++
> 
> Since the contents of the union are all themselves of zero size,
> the actual union wrapper is unnecessary. We therefore remove it for C++
> builds - though keep it for C builds for safety and clarity of
> understanding the code. The alignment constraint on the union is
> unnecessary in the case where the whole struct is aligned on a 16-byte
> boundary, so we add that constraint to the overall structure to ensure
> it applies for C++ code as well as C.
> 
> Fixes: 1cc44d409271 ("eventdev: introduce event vector capability")
> Cc: pbhagavatula at marvell.com
> 
> Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
> ---
>  lib/eventdev/rte_eventdev.h | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/eventdev/rte_eventdev.h b/lib/eventdev/rte_eventdev.h
> index 67c4a5e036..42a5660169 100644
> --- a/lib/eventdev/rte_eventdev.h
> +++ b/lib/eventdev/rte_eventdev.h
> @@ -984,21 +984,31 @@ struct rte_event_vector {
>  	};
>  	/**< Union to hold common attributes of the vector array. */
>  	uint64_t impl_opaque;
> +
> +/* empty structures do not have zero size in C++ leading to compilation errors
> + * with clang about structure having different sizes in C and C++.
> + * Since these are all zero-sized arrays, we can omit the "union" wrapper for
> + * C++ builds, removing the warning.
> + */
> +#ifndef __cplusplus
>  	/**< Implementation specific opaque value.
>  	 * An implementation may use this field to hold implementation specific
>  	 * value to share between dequeue and enqueue operation.
>  	 * The application should not modify this field.
>  	 */
>  	union {
> +#endif
>  		struct rte_mbuf *mbufs[0];
>  		void *ptrs[0];
>  		uint64_t *u64s[0];
> +#ifndef __cplusplus
>  	} __rte_aligned(16);
> +#endif
>  	/**< Start of the vector array union. Depending upon the event type the
>  	 * vector array can be an array of mbufs or pointers or opaque u64
>  	 * values.
>  	 */
> -};
> +} __rte_aligned(16);
>  
>  /* Scheduler type definitions */
>  #define RTE_SCHED_TYPE_ORDERED          0

Zero size arrays should be replaced by flexible arrays.
Linux has this coccinelle script for that:

// SPDX-License-Identifier: GPL-2.0-only
///
/// Zero-length and one-element arrays are deprecated, see
/// Documentation/process/deprecated.rst
/// Flexible-array members should be used instead.
///
//
// Confidence: High
// Copyright: (C) 2020 Denis Efremov ISPRAS.
// Comments:
// Options: --no-includes --include-headers

virtual context
virtual report
virtual org
virtual patch

@initialize:python@
@@
def relevant(positions):
    for p in positions:
        if "uapi" in p.file:
             return False
    return True

@r depends on !patch@
identifier name, array;
type T;
position p : script:python() { relevant(p) };
@@

(
  struct name {
    ...
*   T array at p[\(0\|1\)];
  };
|
  struct {
    ...
*   T array at p[\(0\|1\)];
  };
|
  union name {
    ...
*   T array at p[\(0\|1\)];
  };
|
  union {
    ...
*   T array at p[\(0\|1\)];
  };
)

@only_field depends on patch@
identifier name, array;
type T;
position q;
@@

(
  struct name {@q
    T array[0];
  };
|
  struct {@q
    T array[0];
  };
)

@depends on patch@
identifier name, array;
type T;
position p : script:python() { relevant(p) };
// position @q with rule "only_field" simplifies
// handling of bitfields, arrays, etc.
position q != only_field.q;
@@

(
  struct name {@q
    ...
    T array at p[
-       0
    ];
  };
|
  struct {@q
    ...
    T array at p[
-       0
    ];
  };
)

@script: python depends on report@
p << r.p;
@@

msg = "WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)"
coccilib.report.print_report(p[0], msg)

@script: python depends on org@
p << r.p;
@@

msg = "WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)"
coccilib.org.print_todo(p[0], msg)