Re: Reducing the chunk header sizes on all memory context types

David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> On Tue, 30 Aug 2022 at 03:16, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> ../../../../src/include/utils/memutils_memorychunk.h:186:18: error:
>> comparison of constant 7 with expression of type
>> 'MemoryContextMethodID' (aka 'enum MemoryContextMethodID') is always
>> true [-Werror,-Wtautological-constant-out-of-range-compare]
>> Assert(methodid <= MEMORY_CONTEXT_METHODID_MASK);
>> ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

> I think the Assert is useful as if we were ever to add an enum member
> with the value of 8 and forgot to adjust MEMORY_CONTEXT_METHODID_BITS
> then bad things would happen inside MemoryChunkSetHdrMask() and
> MemoryChunkSetHdrMaskExternal(). I think it's unlikely we'll ever get
> that many MemoryContext types, but I don't know for sure and would
> rather the person who adds the 9th one get alerted to the lack of bit
> space in MemoryChunk as soon as possible.

I think that's a weak argument, so I don't mind dropping this Assert.
What would be far more useful is a comment inside the
MemoryContextMethodID enum pointing out that we can support at most
8 values because XYZ.

However, I'm still wondering why Robert sees this when the rest of us
don't.

regards, tom lane