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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> Sent: Sunday, 7 April 2024 11.32
> 
> 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.

The RFC fails to mention why we need to replace VLAs with something else:

VLAs are C99, but not C++; VLAs were made optional in C11.

MSVC doesn't support VLAs, and is not going to:
https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support-arriving-in-msvc/#variable-length-arrays


I dislike alloca() too, and the notes section in the alloca(3) man page even discourages the use of alloca():
https://man7.org/linux/man-pages/man3/alloca.3.html

But I guess alloca() is the simplest replacement for VLAs.
This RFC patch series opens the discussion for alternatives in different use cases.

> 
> > 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(-)
> >