Hi, On 2020-04-05 19:54:19 -0400, Alvaro Herrera wrote: > Isn't it the case that you can create an inner block with a constant > whose size is determined by a containing block's variable? I mean as in > the attached, which refuses to compile because of our -Werror=vla -- but > if I remove it, it compiles fine and works in my system.
IIRC msvc doesn't support VLAs. And there's generally a slow push towards deprecating them (they've e.g. been moved to optional in C11). They don't tend to make a lot of sense for sizes that aren't tightly bound. In contrast to palloc etc, there's no good way to catch allocation errors. Most of the time you'll just get a SIGBUS or such, but sometimes you'll just end up overwriting data (if the allocation is large enough to not touch the guard pages). Both alloca/vlas also add some per-call overhead. Allocating the common size on-stack, and the uncommon ones on heap should be cheaper, and handles the cases of large allocations much better. Greetings, Andres Freund
