Memory footprints: OCaml vs HLVM

Someone recently published a blog article about the sizes of values in the Ruby and OCaml programming languages.

This is also interesting in the context of HLVM because our high performance goals also require an efficient memory representation but our solution is quite different to OCaml's because HLVM is designed for numerical performance whereas OCaml was designed for symbolic performance.

The following table lists the sizes of values of several different types in OCaml and HLVM on 32-bit architectures:

Type	OCaml	HLVM
unit	32 bits	32 bits
bool	32 bits	8 bits
int31	32 bits	None
int32	96 bits	32 bits
int64	128 bits	64 bits
float32	None	32 bits
float64	128 bits	64 bits
float * float	320 bits	128 bits
Enumeration	32 bits	96 bits
float64 array	64+64n bits	96+64n bits

Note how HLVM uses the most memory efficient representations possible for ints, floating point numbers and tuples but uses less efficient representations for reference types such as arrays and variant types. A side-effect is that heaps in HLVM contain far fewer pointers than heaps in OCaml and this greatly reduces the stress on the garbage collector.