RFC acceptable handling of VLAs across toolchains

[Stephen Hemminger]

On Tue, 7 Nov 2023 11:32:20 -0800
Tyler Retzlaff <roretzla at linux.microsoft.com> wrote:

> hi folks,
> 
> i'm seeking advice. we have use of VLAs which are now optional in
> standard C. some toolchains provide a conformant implementation and msvc
> does not (and never will).
> 
> we seem to have a few options, just curious about what people would
> prefer.
> 
> * use alloca
> 
> * use dynamically allocated storage
> 
> * conditional compiled code where the msvc leg uses one of the previous
>   two options
> 
> i'll leave it simple for now, i'd like to hear input rather than provide
> a recommendation for now.
> 
> thanks!

As an experiment did a build of current DPDK with -Wvla option.

Lots of errors, some have obvious solutions like:

../drivers/net/failsafe/failsafe_intr.c: In function ‘fs_rx_event_proxy_service_install’:
../drivers/net/failsafe/failsafe_intr.c:142:17: warning: ISO C90 forbids variable length array ‘service_core_list’ [-Wvla]
  142 |                 uint32_t service_core_list[num_service_cores];
      |                 ^~~~~~~~

This could just be RTE_MAX_LCORES.

others like rte_metrics should just use malloc() as is used already in
that function.

../lib/metrics/rte_metrics_telemetry.c: In function ‘rte_metrics_tel_update_metrics_ethdev’:
../lib/metrics/rte_metrics_telemetry.c:140:9: warning: ISO C90 forbids variable length array ‘xstats_values’ [-Wvla]
  140 |         uint64_t xstats_values[num_xstats];
      |         ^~~~~~~~
../lib/metrics/rte_metrics_telemetry.c: In function ‘rte_metrics_tel_extract_data’:
../lib/metrics/rte_metrics_telemetry.c:384:9: warning: ISO C90 forbids variable length array ‘stat_names’ [-Wvla]
  384 |         const char *stat_names[num_stat_names];
|         ^~~~~

Others already have an implicit upper bound.
Example is in rte_cuckoo_hash where some fields us RTE_HASH_LOOKUP_BULK_MAX
and some use VLA.

[170/2868] Compiling C object lib/librte_hash.a.p/hash_rte_cuckoo_hash.c.o
../lib/hash/rte_cuckoo_hash.c: In function ‘rte_hash_lookup_bulk_data’:
../lib/hash/rte_cuckoo_hash.c:2355:9: warning: ISO C90 forbids variable length array ‘positions’ [-Wvla]
 2355 |         int32_t positions[num_keys];
      |         ^~~~~~~
../lib/hash/rte_cuckoo_hash.c: In function ‘rte_hash_lookup_with_hash_bulk_data’:
../lib/hash/rte_cuckoo_hash.c:2471:9: warning: ISO C90 forbids variable length array ‘positions’ [-Wvla]
 2471 |         int32_t positions[num_keys];
      |         ^~~~~~~

Would it make sense to have an rte_config.h value for maximum burst size?
Lots of code is using nb_pkts.

There is also some confusing ones like:
../lib/mempool/rte_mempool.c: In function ‘mempool_cache_init’:
../lib/mempool/rte_mempool.c:751:50: warning: ISO C90 forbids array whose size cannot be evaluated [-Wvla]
  751 |                          RTE_SIZEOF_FIELD(struct rte_mempool_cache, objs[0]));
      |                                                  ^~~~~~~~~~~~~~~~~
../lib/eal/include/rte_common.h:498:65: note: in definition of macro ‘RTE_BUILD_BUG_ON’
  498 | #define RTE_BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
      |                                                                 ^~~~~~~~~
../lib/mempool/rte_mempool.c:751:26: note: in expansion of macro ‘RTE_SIZEOF_FIELD’
  751 |                          RTE_SIZEOF_FIELD(struct rte_mempool_cache, objs[0]));