The Storage Allocation Scheme

This document describes the storage allocation system used by the LLDS code generator.

Forward liveness

Each goal has four sets of variables associated with it to give information about changes in liveness on forward execution. (Backward execution is a different matter; see a later part of this document.) These four sets are

The goal that contains the first value-giving occurrence of a variable on a particular computation path will have that variable in its pre-birth set; the goal that contains the last value-using occurrence of a variable on a particular computation path will have that variable in its post-death set.

The different arms of a disjunction or a switch are different computation paths. The condition and then parts of an if-then-else on the one hand and the else part of that if-then-else on the other hand are also different computation paths.

An occurrence is value-giving if it requires the code generator to associate some value with the variable. At the moment, the only value-giving occurrences are those that bind the variable. In the future, occurrences that don't bind the variable but give the address where it should later be put may also be considered value-giving occurrences.

An occurrence is value-using if it requires access to some value the code generator associates with the variable. At the moment we consider all occurrences to be value-using; this is a conservative approximation.

Mode correctness requires that all branches of a branched control structure define the same set of nonlocal variables; the exceptions are branches that cannot succeed, as indicated by the instmap at the end of the branch being unreachable. Such branches are considered by mode analysis to "produce" any variable they are required to produce by parallel branches. To make it easier to write code that tracks the liveness of variables, we implement this fiction by filling the post-birth sets of goals representing such non-succeed branches with the set of variables that must "magically" become live at the unreachable point at end of the branch in order to match the set of live variables at the ends of the other branches. (Variables that have become live in the ordinary way before the unreachable point will not be included.) The post-birth sets of all other goals will be empty.

This guarantees that the set of variables born in each branch of a branched control structure will be the same, modulo variables local to each branch.

We can optimize the treatment of variables that are live inside a branched control structure but not after, because it is possible for the variable to be used in one branch without also being used in some other branches. Each variable that is live before the branched structure, but not after, must die in the branched structure. Branches in which the variable is used will include the variable in the post-death set of one of their subgoals. As far as branches in which the variable is not used are concerned, the variable becomes dead to forward execution as soon as control enters the branch. In such circumstances, we therefore include the variable in the pre-death set of the goal representing the branch. (See below for the method we use for making sure that the values of such "dead" variables are still available to later branches into which we may backtrack and which may need them.)

This guarantees that the set of variables that die in each branch of a branched control structure will be the same, modulo variables local to each branch.

It is an invariant that in each goal_info, a variable will be included in zero, one or two of these four sets; and that if it is included in two sets, then these must be the pre-birth and post-death sets. (This latter will occur for singleton variables.)

Store maps

There are four kinds of situations in which the code generator must associate specific locations with every live variable, either to put those variables in those locations or to update its data structures to say that those variables are "magically" in those locations.

  1. At the ends of branched control structures, i.e. if-then-elses, switches and disjunctions. All branches of a branched structure must agree exactly on these locations.
  2. At the start and end of the procedure.
  3. At points at which execution may resume after a failure, i.e. at the start of the else parts of if-then-elses, at the start of the second and later disjuncts in disjunctions, and after negated goals.
  4. Just before and just after calls and higher-order calls (but not foreign_procs).

Ends of branched control structures

We handle these by including a store_map field in the goal_infos of if_then_else, switch and disj goals. This field, like most other goal_info fields we will talk about in the rest of this document, is a subfield of the code_gen_info field of the goal_info. Through most of the compilation process, the code_gen_info field contains no information; its individual subfields are filled in during the various pre-passes of the LLDS code generator. The store map subfield it is meaningful only from the follow_vars pass onwards.

The follow_vars pass fills this field of goals representing branched control structures with advisory information, saying where things that will be used in code following the branched structure should be. This advisory information may include duplicates (two variables mapped to the same location), it may miss some variables that are live at the end of the branched structure, and it may include variables that are not live at that point.

The store_map pass uses the advisory information left by the follow_vars pass to fill in these fields with definitive information. The definitive store maps guarantee that no two variables are allocated the same location, and they cover exactly the set of variables forward live at the end of the branched structure, plus the variables that are in the resume set of any enclosing resume point (see below).

The passes of the backend following store_map must not do anything to invalidate this invariant, which means that they must not rearrange the code or touch the field. The code generator will use these fields to know what variables to put where when flushing the expression cache at the end of each branch in a branched structure.

Starts and ends of procedures

We handle these using the mechanisms we use for the ends of branched structures, except the map of where things are at the start and where they should be at the end are computed by the code generator from the arg_info list.

Resumption points

We handle these through the resume_point subfield of the code_gen_info field in goal infos. During the liveness pass, we fill in this field for every goal that establishes a point at which execution may resume after backtracking. This means the conditions of if-then-elses (the resumption point is the start of the else part), every disjunct in a disjunction except the last (the resumption point is the start of the next disjunct), and goals inside negations (the resumption point is the start of the code following the negated goal). The value of this field will give the set of variables whose values may be needed when execution resumes at that point. Note that for the purposes of handling resumption points, it does not matter whether any part of an if-then-else, disjunction or negation can succeed more than once.

The resume_point field does not assign a location to these variables. The reason is that as an optimization, each conceptual resumption point is associated with either one or two labels, and if there are two labels, these will differ in where they expect these variables to be. The failure continuation stack entry created by the code generator that describes the resumption point will associate a resume map with each label, with each resume map assigning a location to each variable included in the resume vars set.

The usual case has two labels. The resume map of the first label maps each variable to its stack slot, while the resume map of the second label maps each variable to the location it was occupying on entry to the goal. The code emitted at the resumption point will have, in order, the first label, code that moves each variable from its location according to the first store map to its location according to the second store map, and the second label. The operation in the middle will be a null operation if the two maps agree on the location of a variable. The idea is that any failure that occurs while all these variables are guaranteed to still be in their original locations can be implemented as a jump directly to the second label, while failures at other points (including those from to the right of the disjunct itself, as well as failures from semidet or nondet calls inside the disjunct) will jump (directly or indirectly via a redo() or fail()) to the first label. The section on backward liveness below discusses how we make sure that at these points all the variables in the resume_point set are actually in their stack slots.

We can omit the first label and the code following it, up to but not including the second label, if we can guarantee that the first label will never be jumped to, directly or indirectly. We can give this guarantee for negated goals, conditions in if-then-elses, and disjuncts in disjunctions that cannot succeed more than once, if the goal concerned cannot flush any variable to the stack (which means it contains only inline builtins). We cannot give this guarantee for disjuncts in disjunctions that can succeed more than once even if the goal concerned contains only inline builtins, since in that case we may backtrack to the next disjunct after leaving the current disjunct.

We can omit the second label if we can guarantee that it will never be jumped to, directly or indirectly. We can give this guarantee if the goal concerned has no failure points before a construct (such as a call) that requires all the resumption point variables to be stored on the stack.

The resume_locs part of the resume_point field will say which labels will be needed.

It is an invariant that in a disjunction, the resume_point field of one disjunct must contain all the variables included in the resume_point fields of later disjuncts.

When one control structure that establishes a resumption point occurs inside another one, all the variables included in the relevant resume_point of the outer construct must appear in *all* the resume_point fields associated with the inner construct. This is necessary to make sure that in establishing the inner resumption point, we do not destroy the values of the variables needed to restart forward execution at the resumption point established by the outer construct. (See the section on resumption liveness below.)

When one control structure which establishes a resumption point occurs after but not inside another one, there is no such requirement; see the section on backward liveness below.

Calls and higher order calls

We handle these by flushing all variables that are live after the call except those produced by the call. This is equivalent to the set of variables that are live immediately after the call, minus the pre-birth and post-birth sets of the call, which in turn is equivalent to the set of variables live before the call minus the pre-death and post-death sets of the call.

The stack allocation code and the code generator figure out the set of variables that need to be flushed at each call independently, but based on the same algorithm. Not attaching the set of variables to be saved to each call reduces the space requirement of the compiler.

The same applies to higher order calls.

Backward liveness

There are three kinds of goals that can introduce nondeterminism: nondet disjunctions, nondet calls and nondet higher order calls. All code that executes after one of these constructs must take care not to destroy the variables that are needed to resume in those constructs. (We are *not* talking here about preserving variables needed for later disjuncts; that is discussed in the next section.)

The variables needed to resume after nondet calls and higher order calls are the variables saved across the call in the normal fashion. The variables needed to resume after nondet disjunctions are the variables included in any of the resume_point sets associated with the disjuncts of the disjunction.

The achievement of this objective is in two parts. First, the code generator makes sure that each of these variables is flushed to its stack slot before control leaves the construct that introduces nondeterminism. For calls and higher order calls this is done as part of the call mechanism. For nondet disjunctions, the code generator emits code at the end of every disjunct to copy every variable in the resume_point set for that disjunct into its stack slot, if it isn't there already. (The mechanism whereby these variables survive to this point is discussed in the next section.)

Second, the stack slot allocation pass makes sure that each of the variables needed to resume in a construct that introduces nondeterminism is allocated a stack slot that is not reused in any following code from which one can backtrack to that construct. Normally, this is all following code, but if the construct that introduced the nondeterminism is inside a cut (a scope that changes determinism), then it means only the following code inside the cut.

Resumption liveness

Variables whose values are needed when execution resumes at a resumption point may become dead in the goal that establishes the resumption point. Some points of failure that may cause backtracking to the resumption point may occur after some of these variables have become dead wrt forward liveness. However, when generating the failure code the code generator must know the current locations of these variables so it can pick the correct label to branch to (and possibly generate some code to shuffle the variables to the locations expected at the picked label).

When entering a goal that establishes a resumption point, the code generator pushes the set of variables that are needed at that resumption point onto a resumption point variables stack inside code_info. When we make a variable dead, we consult the top entry on this stack. If the variable being made dead is in that set, we do not forget about it; we just insert it into a set of zombie variables.

To allow a test of membership in the top element of this stack to function as a test of membership of *any* element of this stack, we enforce the invariant that each entry on this stack includes all the other entries below it as subsets.

At the end of the goal that established the resumption point, after popping the resumption point stack, the code generator will attempt to kill all the zombie variables again (after saving them on the stack if we can backtrack to the resumption point from the following code, which is possible only for nondet disjunctions). Any zombie variables that occur in the next entry of the resumption point stack will stay zombies; any that don't occur there will finally die (i.e. the code generator will forget about them, and release the space they occupy.)

The sets of zombie variables and forward live variables are always disjoint, since a variable is not made a zombie until it is no longer forward live.

It is an invariant that at any point in the code generator, the code generator's "set of known variables" is the union of "set of zombie variables" maintained by the code generator, and the set of forward live variables as defined in the forward liveness section above.

Follow vars

When the code generator emits code to materialize the value of a variable, it ought to put it directly into the location where it is required to be next.

The code generator maintains a field in the code_info structure that records advisory information about this. The information comes from the follow_vars pass, which fills in the follow_vars field in the goal info structure of some goals. Whenever the code generator starts processing a goal, it sets the field in the code_info structure from the field of the goal info structure of that goal, if that field is filled in.

The follow_vars pass will fill in this field for the following goals:

The semantics of a filled in follow_vars field:

The follow_vars field should map a variable to a real location if the following code will require that variable to be in exactly that location. For example, if the variable is an input argument of a call, it will need to be in the register holding that argument; if the variable is not an input argument but will need to be saved across the call, it will need to be in its stack slot.

The follow_vars field should map a variable to register r(-1) if the variable is an input to a builtin that does not require its inputs to be anywhere in particular. In that case, we would prefer that the variable be in a register, since this should make the code generated for the builtin somewhat faster.

When the code generator materializes a variable in way that requires several accesses to the materialized location (e.g. filling in the fields of a structure), it should put the variable into a register even if the follow_vars field says otherwise.

Since there may be many variables that should be in their stack slots, and we don't want to represent all of these explicitly, the follow_vars field may omit any mention of these variables. This also makes it easier to merge follow_vars fields at the starts of branched control structures. If some branches want a variable in a register, their wishes should take precedence over the wishes of the branches that wish the variable to be in its stack slot, or in which the variable is not used at all.

When the code generator picks a random free register, it should try to avoid registers that are needed for variables in the follow_vars map.

When a variable that is currently in its stack slot is supposed to be put in any currently free register for speed of future access, the code generator should refuse to use any virtual machine registers that are not real machine registers. Instead, it should keep the variable in its stack slot.