From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Mattias Rönnblom" <hofors@lysator.liu.se>,
"Tyler Retzlaff" <roretzla@linux.microsoft.com>,
dev@dpdk.org
Cc: "Bruce Richardson" <bruce.richardson@intel.com>,
"Stephen Hemminger" <stephen@networkplumber.org>,
"Thomas Monjalon" <thomas@monjalon.net>
Subject: RE: [PATCH 0/4] RFC samples converting VLA to alloca
Date: Sun, 7 Apr 2024 13:07:06 +0200 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9F379@smartserver.smartshare.dk> (raw)
In-Reply-To: <adb81b30-bece-416b-a233-b08873d196f6@lysator.liu.se>
> 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(-)
> >
next prev parent reply other threads:[~2024-04-07 11:07 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-07 19:32 RFC acceptable handling of VLAs across toolchains Tyler Retzlaff
2023-11-08 2:31 ` Stephen Hemminger
2023-11-08 3:25 ` Tyler Retzlaff
2023-11-08 8:19 ` Morten Brørup
2023-11-08 16:51 ` Stephen Hemminger
2023-11-08 17:48 ` Morten Brørup
2023-11-09 10:25 ` RFC: default burst sizes in rte_config Morten Brørup
2023-11-09 20:26 ` RFC acceptable handling of VLAs across toolchains Tyler Retzlaff
2024-03-21 0:12 ` Tyler Retzlaff
2024-04-04 17:15 ` [PATCH 0/4] RFC samples converting VLA to alloca Tyler Retzlaff
2024-04-04 17:15 ` [PATCH 1/4] latencystats: use alloca instead of vla trivial Tyler Retzlaff
2024-04-06 15:28 ` Morten Brørup
2024-04-07 9:36 ` Mattias Rönnblom
2024-04-07 17:00 ` Stephen Hemminger
2024-04-04 17:15 ` [PATCH 2/4] hash: " Tyler Retzlaff
2024-04-06 16:01 ` Morten Brørup
2024-04-04 17:15 ` [PATCH 3/4] vhost: use alloca instead of vla sizeof Tyler Retzlaff
2024-04-06 22:30 ` Morten Brørup
2024-04-04 17:15 ` [PATCH 4/4] dispatcher: use alloca instead of vla multi dimensional Tyler Retzlaff
2024-04-06 15:49 ` Morten Brørup
2024-04-07 9:31 ` [PATCH 0/4] RFC samples converting VLA to alloca Mattias Rönnblom
2024-04-07 11:07 ` Morten Brørup [this message]
2024-04-07 17:03 ` Stephen Hemminger
2024-04-08 15:27 ` Tyler Retzlaff
2024-04-08 15:53 ` Morten Brørup
2024-04-09 8:28 ` Konstantin Ananyev
2024-04-09 15:08 ` Tyler Retzlaff
2024-04-10 9:58 ` Konstantin Ananyev
2024-04-10 17:03 ` Tyler Retzlaff
2024-04-10 7:32 ` Mattias Rönnblom
2024-04-10 7:52 ` Morten Brørup
2024-04-10 17:04 ` Tyler Retzlaff
2024-04-10 7:27 ` Mattias Rönnblom
2024-04-10 17:10 ` Tyler Retzlaff
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