x/tools/go/ssa/ssautil: deprecate AllFunctions

[adonovan]

I would like to deprecate the AllFunctions helper function, because it's a poorly specified mess (my bad).
As its doc comment says:

// I think we should deprecate AllFunctions function in favor of two
// clearly defined ones:
//
//  1. The first would efficiently compute CHA reachability from a set
//     of main packages, making it suitable for a whole-program
//     analysis context with InstantiateGenerics, in conjunction with
//     Program.Build.
//
//  2. The second would return only the set of functions corresponding
//     to source Func{Decl,Lit} syntax, like SrcFunctions in
//     go/analysis/passes/buildssa; this is suitable for
//     package-at-a-time (or handful of packages) context.
//     ssa.Package could easily expose it as a field.

Before we can consider doing that (which will require a proposal), we should stop using it ourselves in x/tools and x/vuln. Its existing uses are:

// These test could use a simpler ad hoc algorithm, or an efficient CHA:
go/ssa/stdlib_test.go
go/ssa/builder_test.go

// both CHA and static do essentially the same reachability fixed point
// iteration and could do (the good bits of) AllFunctions themselves:
go/callgraph/cha/cha.go -- AllFunctions is not even correct here! see #66429
go/callgraph/static/static.go

--

// VTA uses AllFunctions to produce a set of entry points. 
// I suspect it is both a massive overapproximation and yet still not always correct (see #66251)
// What exactly does vta.CallGraph need?
cmd/callgraph/main.go
go/callgraph/vta/vta_test.go
go/callgraph/vta/graph_test.go
go/callgraph/callgraph_test.go
x/vuln/internal/vulncheck/utils.go

// just for debugging; could easily be deleted.
go/callgraph/vta/helpers_test.go 

I will tackle the first half. @zpavlinovic perhaps you could look at the VTA-related ones? What we need at this stage is just crisp descriptions of the exact requirements of each of these algorithms.

[gopherbot]

Change https://go.dev/cl/609280 mentions this issue: go/callgraph/static: avoid ssautil.AllFunctions

[gopherbot]

Change https://go.dev/cl/609281 mentions this issue: go/callgraph/cha: make CHA, VTA faster and more precise

[gopherbot]

Change https://go.dev/cl/611216 mentions this issue: internal/vulncheck: remove use of ssautil.AllFunctions

[gabyhelp]

Related Issues and Documentation

• proposal: x/tools/go/ssa/ssautil: Reachable: conservative approximation to reachable functions, runtime types #69291

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[zpavlinovic]

Regarding VTA:

• the _test files need some set of functions for the test programs. We currently use ssautil.AllFunctions but we could in principle write a simple helper that collects functions for the test programs. These programs are rather simple. Also, we could use x/tools/go/ssa/ssautil: Reachable: conservative approximation to reachable functions, runtime types #69291.
• CL/611216 removes the use of ssautil.AllFunctions for some unnecessary pruning in vulncheck. However, we will have to reintroduce the use of this function soon for a different purpose, i.e., when we try to take advantage of some improvements in VTA. The set of functions passed to VTA will be a forward slice of roots computed using the (copy of) golang.org/x/tools/go/callgraph/internal/chautil.LazyCallees. This function in turn expects a set of program functions. We have to use ssautil.AllFunctions for the time being and deprecate it when x/tools/go/ssa/ssautil: Reachable: conservative approximation to reachable functions, runtime types #69291 is done.

[gopherbot]

Change https://go.dev/cl/611535 mentions this issue: internal/callgraph/chautil: add Reachable function

[gopherbot]

Change https://go.dev/cl/611536 mentions this issue: go/ssa: stop using ssautil.AllFunctions

[adonovan]

With my https://go.dev/cl/609281 stack (the last CL needs some thought), there are no remaining uses of AllFunctions in x/tools. (Not even tests of it, alarmingly.)

I notice that AllFunctions returns uninstantiated generics whereas the new chautil.Reachable does not, and for this reason it's probably not safe for us ever to make AllFunctions a wrapper around Reachable; we'll just have to leave it there with a doc explaining what it does and why there are better solutions for most tasks, and what they are.

[StevenACoffman]

@adonovan Reading through the current source code, I don't believe the second part of AllFunctions doc comment is available yet.

I was hoping for something like this:

// Funcs returns the package-level functions.
func (p *Package) Funcs() ([]*Function) {
	var funcs []*ssa.Function
	for _, f := range p.Members {
	  if f, ok = f.(*Function); ok {
	    funcs = append(funcs, f)
	  }
	}
	return funcs
}

This would be very convenient for some linters to have, so they don't need to waste time calling SrcFunctions in go/analysis/passes/buildssa if AllFunctions is no longer available.