Stefan Monnier wrote:

> Thanks.  AFAICT the only solution is to use the GCALIGNED_UNION trick in
> each and every "real Lisp_Object struct" rather than once and forall in
> vectorlike_header.

The trick does need to move out of union vectorlike_header. However, the trick 
is not needed for most of those structs, since they're allocated only by the GC 
and are therefore already GC-aligned. The trick is needed only for structs that 
C might allocate statically or on the stack, and whose addresses are tagged as 
Lisp pointers. Just a few types do that, and I've noted them in the first 
attached patch.

Although the first attached patch shrinks sizeof (struct Lisp_Maker) from 32 to 
24 bytes on x86 as requested, allocate_pseudovector still *allocates* 32 bytes 
for the struct, as it rounds the size up to the next multiple of alignof 
(max_align_t), which is 16 on x86. It's not hard to change that to 8 (please see 
2nd attached patch) but this causes a 20% CPU performance hit (!) to 'make 
compile-always' on my platform (AMD Phenom II X4 910e circa 2010, Fedora 28 
x86-64, gcc -m32 -march=native), so I didn't install and can't recommend the 2nd 
attached patch.