Control flow: use domtree analysis (#98)

Fixes #96

This vendors Julia 1.10's Core.Compiler's domtree code for earlier
Julia versions.

Project.toml

@@ -1,7 +1,7 @@
 name = "LoweredCodeUtils"
 uuid = "6f1432cf-f94c-5a45-995e-cdbf5db27b0b"
 authors = ["Tim Holy <tim.holy@gmail.com>"]
-version = "2.4.3"
+version = "2.4.4"
 
 [deps]
 JuliaInterpreter = "aa1ae85d-cabe-5617-a682-6adf51b2e16a"

src/codeedges.jl

@@ -609,7 +609,8 @@ function lines_required!(isrequired::AbstractVector{Bool}, objs, src::CodeInfo,
 
     # Compute basic blocks, which we'll use to make sure we mark necessary control-flow
     cfg = Core.Compiler.compute_basic_blocks(src.code)  # needed for control-flow analysis
-    paths = enumerate_paths(cfg)
+    domtree = construct_domtree(cfg.blocks)
+    postdomtree = construct_postdomtree(cfg.blocks)
 
     # We'll mostly use generic graph traversal to discover all the lines we need,
     # but structs are in a bit of a different category (especially on Julia 1.5+).
@@ -629,7 +630,7 @@ function lines_required!(isrequired::AbstractVector{Bool}, objs, src::CodeInfo,
 
         # Add control-flow
         changed |= add_loops!(isrequired, cfg)
-        changed |= add_control_flow!(isrequired, cfg, paths)
+        changed |= add_control_flow!(isrequired, cfg, domtree, postdomtree)
 
         # So far, everything is generic graph traversal. Now we add some domain-specific information
         changed |= add_typedefs!(isrequired, src, edges, typedefs, norequire)
@@ -706,17 +707,6 @@ end
 
 ## Add control-flow
 
-struct Path
-    path::Vector{Int}
-    visited::BitSet
-end
-Path() = Path(Int[], BitSet())
-Path(i::Int) = Path([i], BitSet([i]))
-Path(path::Path) = copy(path)
-Base.copy(path::Path) = Path(copy(path.path), copy(path.visited))
-Base.in(node::Int, path::Path) = node ∈ path.visited
-Base.push!(path::Path, node::Int) = (push!(path.path, node); push!(path.visited, node); return path)
-
 # Mark loops that contain evaluated statements
 function add_loops!(isrequired, cfg)
     changed = false
@@ -741,26 +731,7 @@ function add_loops!(isrequired, cfg)
     return changed
 end
 
-enumerate_paths(cfg) = enumerate_paths!(Path[], cfg, Path(1))
-function enumerate_paths!(paths, cfg, path)
-    bb = cfg.blocks[path.path[end]]
-    if isempty(bb.succs)
-        push!(paths, copy(path))
-        return paths
-    end
-    for ibbs in bb.succs
-        if ibbs ∈ path
-            push!(paths, push!(copy(path), ibbs))   # close the loop
-            continue
-        end
-        enumerate_paths!(paths, cfg, push!(copy(path), ibbs))
-    end
-    return paths
-end
-
-# Mark exits of blocks that bifurcate execution paths in ways that matter for required statements
-function add_control_flow!(isrequired, cfg, paths::AbstractVector{Path})
-    withnode, withoutnode, shared = BitSet(), BitSet(), BitSet()
+function add_control_flow!(isrequired, cfg, domtree, postdomtree)
     changed, _changed = false, true
     blocks = cfg.blocks
     nblocks = length(blocks)
@@ -769,28 +740,33 @@ function add_control_flow!(isrequired, cfg, paths::AbstractVector{Path})
         for (ibb, bb) in enumerate(blocks)
             r = rng(bb)
             if any(view(isrequired, r))
-                # Check if the exit of this block is a GotoNode or `return`
-                if length(bb.succs) < 2 && ibb < nblocks
-                    idxlast = r[end]
-                    _changed |= !isrequired[idxlast]
-                    isrequired[idxlast] = true
-                end
-                empty!(withnode)
-                empty!(withoutnode)
-                for path in paths
-                    union!(ibb ∈ path ? withnode : withoutnode, path.visited)
+                # Walk up the dominators
+                jbb = ibb
+                while jbb != 1
+                    jdbb = domtree.idoms_bb[jbb]
+                    dbb = blocks[jdbb]
+                    # Check the successors; if jbb doesn't post-dominate, mark the last statement
+                    for s in dbb.succs
+                        if !postdominates(postdomtree, jbb, s)
+                            idxlast = rng(dbb)[end]
+                            _changed |= !isrequired[idxlast]
+                            isrequired[idxlast] = true
+                            break
+                        end
+                    end
+                    jbb = jdbb
                 end
-                empty!(shared)
-                union!(shared, withnode)
-                intersect!(shared, withoutnode)
-                for icfbb in shared
-                    cfbb = blocks[icfbb]
-                    if any(∉(shared), cfbb.succs)
-                        rcfbb = rng(blocks[icfbb])
-                        idxlast = rcfbb[end]
+                # Walk down the post-dominators, including self
+                jbb = ibb
+                while jbb != 0 && jbb < nblocks
+                    pdbb = blocks[jbb]
+                    # Check if the exit of this block is a GotoNode or `return`
+                    if length(pdbb.succs) < 2
+                        idxlast = rng(pdbb)[end]
                         _changed |= !isrequired[idxlast]
                         isrequired[idxlast] = true
                     end
+                    jbb = postdomtree.idoms_bb[jbb]
                 end
             end
         end