Re: [PATCH 0/4] RFC samples converting VLA to alloca

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2024-04-04 19:15, Tyler Retzlaff wrote:
> This series is not intended for merge.  It insteat provides examples of
> converting use of VLAs to alloca() would look like.
> 
> what's the advantages of VLA over alloca()?
> 
> * sizeof(array) works as expected.
> 
> * multi-dimensional arrays are still arrays instead of pointers to
>    dynamically allocated space. this means multiple subscript syntax
>    works (unlike on a pointer) and calculation of addresses into allocated
>    space in ascending order is performed by the compiler instead of manually.
> 

alloca() is a pretty obscure mechanism, and also not a part of the C 
standard. VLAs are C99, and well-known and understood, and very efficient.

> what's the disadvantage of VLA over alloca()?
> 
> * VLA generation is subtl/implicit, there do appear to be places where
>    a VLA is being used where it perhaps was not intended but it is hard
>    to spot. e.g. hotpath rte_mbuf *array[burst_size]; where burst_size
>    is not a constant expression, e.g. unintended in other syntax positions
>    that are not intuitive, see patchwork link.
> 
>    https://patchwork.dpdk.org/project/dpdk/patch/1699896038-28106-1-git-send-email-roretzla@linux.microsoft.com/
> 
> for the above reasons i'd recommend only converting to alloca() where
> necessary (msvc has to compile it) and for the other instances leave
> them as they are.
> 
> Tyler Retzlaff (4):
>    latencystats: use alloca instead of vla trivial
>    hash: use alloca instead of vla trivial
>    vhost: use alloca instead of vla sizeof
>    dispatcher: use alloca instead of vla multi dimensional
> 
>   lib/dispatcher/rte_dispatcher.c     | 6 +++---
>   lib/hash/rte_thash.c                | 2 +-
>   lib/latencystats/rte_latencystats.c | 2 +-
>   lib/vhost/socket.c                  | 5 +++--
>   4 files changed, 8 insertions(+), 7 deletions(-)
>