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-roret...@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(-)
