On Sun, 7 Apr 2024 13:07:06 +0200 Morten Brørup <m...@smartsharesystems.com> wrote:
> > From: Mattias Rönnblom [mailto:hof...@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. > The other issue with VLA's is that if the number is something that can be externally input, then it can be a source of stack overflow bugs. That is why the Linux kernel has stopped using them; for security reasons. DPDK has much less of a security trust domain. Mostly need to make sure that no data from network is being used to compute VLA size.
