Understanding STG

Question

The design of GHC is based on something called STG, which stands for \"spineless, tagless G-machine\".

Now G-machine is apparently short for \"graph reduction machine\",

Answer 1

The answer by migle is exactly what spinlessness and taglessness of the STGM mean. Today it does not worth trying to understand the names of the two features because the names stem from the history of graph reduction technologies: from G-machine, Spineless G-machine, and Spineless and Tagless G-machine.

The G-machine uses both spine and tags. A spine is a list of edges from the root node of a function application to the node of the function. For example, a function application of "f e1 e2 ... en" is represented as

root = AP left_n en
left_n = AP left_n-1 en-1 ...
left_2 = AP left_1 e1
left_1 = FUN f

in G-machine, and so a spine is a list of edges consisting of left_n -> left_n-1 -> ... -> left_2 -> left_1. It is literally a spine of the function application!

In the same function application, there are tags AP and FUN.

In the next advanced G-machine so called the Spineless G-machine, there is no such spine by representing such a function application in a contiguous block whose first slot points to f, the second slot points to e1, ..., and the n+1-th slot points to en. In this representation, we do not need a spine. But the block starts a special tag designating the number of slots and so on.

In the most advanced G-machine so called the Spineless Tagless G-machine, such a tag is replaced with a function pointer. To evaluate a function application is to jump to the code by the function pointer.

It is unfortunate to find that Simone Peyton Jones's STGM paper does not give compilation/evaluation rules in some abstract level, and so it is very natural for people not to be easy to understand the essence of the STGM.