MLton 20210117

# CallGraph

For easier visualization of profiling data, mlprof can create a call graph of the program in dot format, from which you can use the graphviz software package to create a PostScript or PNG graph. For example,

    mlprof -call-graph foo.dot foo mlmon.out

will create foo.dot with a complete call graph. For each source function, there will be one node in the graph that contains the function name (and source position with -show-line true), as well as the percentage of ticks. If you want to create a call graph for your program without any profiling data, you can simply call mlprof without any mlmon.out files, as in

    mlprof -call-graph foo.dot foo

Because SML has higher-order functions, the call graph is is dependent on MLton’s analysis of which functions call each other. This analysis depends on many implementation details and might display spurious edges that a human could conclude are impossible. However, in practice, the call graphs tend to be very accurate.

Because call graphs can get big, mlprof provides the -keep option to specify the nodes that you would like to see. This option also controls which functions appear in the table that mlprof prints. The argument to -keep is an expression describing a set of source functions (i.e. graph nodes). The expression e should be of the following form.

- all

- "s"

- (and e …)

- (from e)

- (not e)

- (or e)

- (pred e)

- (succ e)

- (thresh x)

- (thresh-gc x)

- (thresh-stack x)

- (to e)

In the grammar, all denotes the set of all nodes. "s" is a regular expression denoting the set of functions whose name (followed by a space and the source position) has a prefix matching the regexp. The and, not, and or expressions denote intersection, complement, and union, respectively. The pred and succ expressions add the set of immediate predecessors or successors to their argument, respectively. The from and to expressions denote the set of nodes that have paths from or to the set of nodes denoted by their arguments, respectively. Finally, thresh, thresh-gc, and thresh-stack denote the set of nodes whose percentage of ticks, gc ticks, or stack ticks, respectively, is greater than or equal to the real number x.

For example, if you want to see the entire call graph for a program, you can use -keep all (this is the default). If you want to see all nodes reachable from function foo in your program, you would use -keep '(from "foo")'. Or, if you want to see all the functions defined in subdirectory bar of your project that used at least 1% of the ticks, you would use

    -keep '(and ".*/bar/" (thresh 1.0))'

To see all functions with ticks above a threshold, you can also use -thresh x, which is an abbreviation for -keep '(thresh x)'. You can not use multiple -keep arguments or both -keep and -thresh. When you use -keep to display a subset of the functions, mlprof will add dashed edges to the call graph to indicate a path in the original call graph from one function to another.

When compiling with -profile-stack true, you can use mlprof -gray true to make the nodes darker or lighter depending on whether their stack percentage is higher or lower.

MLton’s optimizer may duplicate source functions for any of a number of reasons (functor duplication, monomorphisation, polyvariance, inlining). By default, all duplicates of a function are treated as one. If you would like to treat the duplicates separately, you can use mlprof -split regexp, which will cause all duplicates of functions whose name has a prefix matching the regular expression to be treated separately. This can be especially useful for higher-order utility functions like General.o.

## Caveats

Technically speaking, mlprof produces a call-stack graph rather than a call graph, because it describes the set of possible call stacks. The difference is in how tail calls are displayed. For example if f nontail calls g and g tail calls h, then the call-stack graph has edges from f to g and f to h, while the call graph has edges from f to g and g to h. That is, a tail call from g to h removes g from the call stack and replaces it with h.