On Tue, Nov 8, 2022 at 8:57 AM David Rowley <dgrowle...@gmail.com> wrote:
> On Tue, 8 Nov 2022 at 05:24, Andres Freund <and...@anarazel.de> wrote: > > I doubtthere's ever a need to realloc such a pointer? Perhaps we could just > > elog(ERROR)? > > Are you maybe locked into just thinking about your current planned use > case that we want to allocate BLCKSZ bytes in each case? It does not > seem impossible to me that someone will want something more than an > 8-byte alignment and also might want to enlarge the allocation at some > point. I thought it might be more dangerous not to implement repalloc. > It might not be clear to someone using palloc_aligned() that there's > no code path that can call repalloc on the returned pointer. I can imagine a use case for arrays of cacheline-sized objects. > TYPEALIGN() will not work correctly unless the alignment is a power of > 2. We could modify it to, but that would require doing some modular > maths instead of bitmasking. That seems pretty horrible when the macro > is given a value that's not constant at compile time as we'd end up > with a (slow) divide in the code path. I think the restriction is a > good idea. I imagine there will never be any need to align to anything > that's not a power of 2. +1 > > Should we handle the case where we get a suitably aligned pointer from > > MemoryContextAllocExtended() differently? > > Maybe it would be worth the extra check. I'm trying to imagine future > use cases. Maybe if someone wanted to ensure that we're aligned to > CPU cache line boundaries then the chances of the pointer already > being aligned to 64 bytes is decent enough. The problem is it that > it's too late to save any memory, it just saves a bit of boxing and > unboxing of the redirect headers. To my mind the main point of detecting this case is to save memory, so if that's not possible/convenient, special-casing doesn't seem worth it. - Assert((char *) chunk > (char *) block); + Assert((char *) chunk >= (char *) block); Is this related or independent? -- John Naylor EDB: http://www.enterprisedb.com
