optimizer: refactor SROA pass

- use `BitSet` instead of `IdSet{Int}`
- reduce # of dynamic allocations
- separate some computations into individual functions

Author: aviatesk

base/compiler/ssair/passes.jl

@@ -23,15 +23,15 @@ SSADefUse() = SSADefUse(Int[], Int[], Int[])
 
 compute_live_ins(cfg::CFG, du::SSADefUse) = compute_live_ins(cfg, du.defs, du.uses)
 
-function try_compute_field_stmt(compact::IncrementalCompact, stmt::Expr)
+function try_compute_field_stmt(ir::Union{IncrementalCompact,IRCode}, stmt::Expr)
     field = stmt.args[3]
     # fields are usually literals, handle them manually
     if isa(field, QuoteNode)
         field = field.value
     elseif isa(field, Int)
     # try to resolve other constants, e.g. global reference
     else
-        field = compact_exprtype(compact, field)
+        field = isa(ir, IncrementalCompact) ? compact_exprtype(ir, field) : argextype(field, ir)
         if isa(field, Const)
             field = field.val
         else
@@ -42,8 +42,8 @@ function try_compute_field_stmt(compact::IncrementalCompact, stmt::Expr)
     return field
 end
 
-function try_compute_fieldidx_stmt(compact::IncrementalCompact, stmt::Expr, typ::DataType)
-    field = try_compute_field_stmt(compact, stmt)
+function try_compute_fieldidx_stmt(ir::Union{IncrementalCompact,IRCode}, stmt::Expr, typ::DataType)
+    field = try_compute_field_stmt(ir, stmt)
     return try_compute_fieldidx(typ, field)
 end
 
@@ -112,6 +112,13 @@ function find_def_for_use(ir::IRCode, domtree::DomTree, allblocks::Vector{Int},
     return def, stmtblock, curblock
 end
 
+function collect_leaves(compact::IncrementalCompact, @nospecialize(val), @nospecialize(typeconstraint))
+    if isa(val, Union{OldSSAValue, SSAValue})
+        val, typeconstraint = simple_walk_constraint(compact, val, typeconstraint)
+    end
+    return walk_to_defs(compact, val, typeconstraint)
+end
+
 function simple_walk(compact::IncrementalCompact, @nospecialize(defssa#=::AnySSAValue=#),
                      callback = (@nospecialize(pi), @nospecialize(idx)) -> false)
     while true
@@ -152,7 +159,7 @@ function simple_walk(compact::IncrementalCompact, @nospecialize(defssa#=::AnySSA
 end
 
 function simple_walk_constraint(compact::IncrementalCompact, @nospecialize(defssa#=::AnySSAValue=#),
-                                @nospecialize(typeconstraint = types(compact)[defssa]))
+                                @nospecialize(typeconstraint))
     callback = function (@nospecialize(pi), @nospecialize(idx))
         if isa(pi, PiNode)
             typeconstraint = typeintersect(typeconstraint, widenconst(pi.typ))
@@ -164,20 +171,16 @@ function simple_walk_constraint(compact::IncrementalCompact, @nospecialize(defss
 end
 
 """
-    walk_to_defs(compact, val, intermediaries)
+    walk_to_defs(compact, val, typeconstraint)
 
-Starting at `val` walk use-def chains to get all the leaves feeding into
-this val (pruning those leaves rules out by path conditions).
+Starting at `val` walk use-def chains to get all the leaves feeding into this `val`
+(pruning those leaves rules out by path conditions).
 """
-function walk_to_defs(compact::IncrementalCompact, @nospecialize(defssa), @nospecialize(typeconstraint), visited_phinodes::Vector{AnySSAValue}=AnySSAValue[])
-    isa(defssa, AnySSAValue) || return Any[defssa]
+function walk_to_defs(compact::IncrementalCompact, @nospecialize(defssa), @nospecialize(typeconstraint))
+    visited_phinodes = AnySSAValue[]
+    isa(defssa, AnySSAValue) || return Any[defssa], visited_phinodes
     def = compact[defssa]
-    isa(def, PhiNode) || return Any[defssa]
-    # Step 2: Figure out what the struct is defined as
-    ## Track definitions through PiNode/PhiNode
-    found_def = false
-    ## Track which PhiNodes, SSAValue intermediaries
-    ## we forwarded through.
+    isa(def, PhiNode) || return Any[defssa], visited_phinodes
     visited_constraints = IdDict{AnySSAValue, Any}()
     worklist_defs = AnySSAValue[]
     worklist_constraints = Any[]
@@ -239,10 +242,10 @@ function walk_to_defs(compact::IncrementalCompact, @nospecialize(defssa), @nospe
             push!(leaves, defssa)
         end
     end
-    leaves
+    return leaves, visited_phinodes
 end
 
-function process_immutable_preserve(new_preserves::Vector{Any}, compact::IncrementalCompact, def::Expr)
+function process_immutable_preserve!(new_preserves::Vector{Any}, compact::IncrementalCompact, def::Expr)
     for arg in (isexpr(def, :new) ? def.args : def.args[2:end])
         if !isbitstype(widenconst(compact_exprtype(compact, arg)))
             push!(new_preserves, arg)
@@ -449,13 +452,10 @@ function lift_comparison!(compact::IncrementalCompact,
         return
     end
 
-    if isa(val, Union{OldSSAValue, SSAValue})
-        val, typeconstraint = simple_walk_constraint(compact, val, typeconstraint)
-    end
-
-    visited_phinodes = AnySSAValue[]
-    leaves = walk_to_defs(compact, val, typeconstraint, visited_phinodes)
+    valtyp = widenconst(compact_exprtype(compact, val))
+    isa(valtyp, Union) || return # bail out if there won't be a good chance for lifting
 
+    leaves, visited_phinodes = collect_leaves(compact, val, valtyp)
     length(leaves) ≤ 1 && return # bail out if we don't have multiple leaves
 
     # Let's check if we evaluate the comparison for each one of the leaves
@@ -596,17 +596,16 @@ a result of succeeding dead code elimination.
 """
 function sroa_pass!(ir::IRCode)
     compact = IncrementalCompact(ir)
-    defuses = IdDict{Int, Tuple{IdSet{Int}, SSADefUse}}()
+    defuses = nothing # will be initialized once we encounter mutability in order to reduce dynamic allocations
     lifting_cache = IdDict{Pair{AnySSAValue, Any}, AnySSAValue}()
     for ((_, idx), stmt) in compact
+        # check whether this statement is `getfield` / `setfield!` (or other "interesting" statement)
         isa(stmt, Expr) || continue
-        result_t = compact_exprtype(compact, SSAValue(idx))
         is_setfield = false
         field_ordering = :unspecified
-        # Step 1: Check whether the statement we're looking at is a getfield/setfield!
         if is_known_call(stmt, setfield!, compact)
-            is_setfield = true
             4 <= length(stmt.args) <= 5 || continue
+            is_setfield = true
             if length(stmt.args) == 5
                 field_ordering = compact_exprtype(compact, stmt.args[5])
             end
@@ -624,7 +623,7 @@ function sroa_pass!(ir::IRCode)
             old_preserves = stmt.args[(6+nccallargs):end]
             for (pidx, preserved_arg) in enumerate(old_preserves)
                 isa(preserved_arg, SSAValue) || continue
-                let intermediaries = IdSet{Int}()
+                let intermediaries = BitSet()
                     callback = function (@nospecialize(pi), @nospecialize(ssa))
                         push!(intermediaries, ssa.id)
                         return false
@@ -634,7 +633,7 @@ function sroa_pass!(ir::IRCode)
                     defidx = def.id
                     def = compact[defidx]
                     if is_tuple_call(compact, def)
-                        process_immutable_preserve(new_preserves, compact, def)
+                        process_immutable_preserve!(new_preserves, compact, def)
                         old_preserves[pidx] = nothing
                         continue
                     elseif isexpr(def, :new)
@@ -643,14 +642,17 @@ function sroa_pass!(ir::IRCode)
                             typ = unwrap_unionall(typ)
                         end
                         if typ isa DataType && !ismutabletype(typ)
-                            process_immutable_preserve(new_preserves, compact, def)
+                            process_immutable_preserve!(new_preserves, compact, def)
                             old_preserves[pidx] = nothing
                             continue
                         end
                     else
                         continue
                     end
-                    mid, defuse = get!(defuses, defidx, (IdSet{Int}(), SSADefUse()))
+                    if defuses === nothing
+                        defuses = IdDict{Int, Tuple{BitSet, SSADefUse}}()
+                    end
+                    mid, defuse = get!(defuses, defidx, (BitSet(), SSADefUse()))
                     push!(defuse.ccall_preserve_uses, idx)
                     union!(mid, intermediaries)
                 end
@@ -675,6 +677,9 @@ function sroa_pass!(ir::IRCode)
         else
             continue
         end
+
+        # analyze this `getfield` / `setfield!` call
+
         field = try_compute_field_stmt(compact, stmt)
         field === nothing && continue
 
@@ -689,19 +694,23 @@ function sroa_pass!(ir::IRCode)
             continue
         end
 
-        def, typeconstraint = stmt.args[2], struct_typ
+        val = stmt.args[2]
 
+        # analyze this mutable struct here for the later pass
         if ismutabletype(struct_typ)
-            isa(def, SSAValue) || continue
-            let intermediaries = IdSet{Int}()
+            isa(val, SSAValue) || continue
+            let intermediaries = BitSet()
                 callback = function (@nospecialize(pi), @nospecialize(ssa))
                     push!(intermediaries, ssa.id)
                     return false
                 end
-                def = simple_walk(compact, def, callback)
+                def = simple_walk(compact, val, callback)
                 # Mutable stuff here
                 isa(def, SSAValue) || continue
-                mid, defuse = get!(defuses, def.id, (IdSet{Int}(), SSADefUse()))
+                if defuses === nothing
+                    defuses = IdDict{Int, Tuple{BitSet, SSADefUse}}()
+                end
+                mid, defuse = get!(defuses, def.id, (BitSet(), SSADefUse()))
                 if is_setfield
                     push!(defuse.defs, idx)
                 else
@@ -711,26 +720,21 @@ function sroa_pass!(ir::IRCode)
             end
             continue
         elseif is_setfield
-            continue
+            continue # invalid `setfield!` call, but just ignore here
         end
 
         # perform SROA on immutable structs here on
 
-        if isa(def, Union{OldSSAValue, SSAValue})
-            def, typeconstraint = simple_walk_constraint(compact, def, typeconstraint)
-        end
-
-        visited_phinodes = AnySSAValue[]
-        leaves = walk_to_defs(compact, def, typeconstraint, visited_phinodes)
-
-        isempty(leaves) && continue
-
         field = try_compute_fieldidx(struct_typ, field)
         field === nothing && continue
 
-        r = lift_leaves(compact, result_t, field, leaves)
-        r === nothing && continue
-        lifted_leaves, any_undef = r
+        leaves, visited_phinodes = collect_leaves(compact, val, struct_typ)
+        isempty(leaves) && continue
+
+        result_t = compact_exprtype(compact, SSAValue(idx))
+        lifted_result = lift_leaves(compact, result_t, field, leaves)
+        lifted_result === nothing && continue
+        lifted_leaves, any_undef = lifted_result
 
         if any_undef
             result_t = make_MaybeUndef(result_t)
@@ -750,28 +754,32 @@ function sroa_pass!(ir::IRCode)
             @assert val !== nothing
         end
 
-        # global assertion_counter
-        # assertion_counter::Int += 1
-        # insert_node_here!(compact, Expr(:assert_egal, Symbol(string("assert_egal_", assertion_counter)), SSAValue(idx), val), nothing, 0, true)
-        # continue
         compact[idx] = val === nothing ? nothing : val.x
     end
 
     non_dce_finish!(compact)
-    # Copy the use count, `simple_dce!` may modify it and for our predicate
-    # below we need it consistent with the state of the IR here (after tracking
-    # phi node arguments, but before dce).
-    used_ssas = copy(compact.used_ssas)
-    simple_dce!(compact)
-    ir = complete(compact)
-
-    # Compute domtree, needed below, now that we have finished compacting the
-    # IR. This needs to be after we iterate through the IR with
-    # `IncrementalCompact` because removing dead blocks can invalidate the
-    # domtree.
+    if defuses !== nothing
+        # now go through analyzed mutable structs and see which ones we can eliminate
+        # NOTE copy the use count here, because `simple_dce!` may modify it and we need it
+        # consistent with the state of the IR here (after tracking `PhiNode` arguments,
+        # but before the DCE) for our predicate within `sroa_mutables!`
+        used_ssas = copy(compact.used_ssas)
+        simple_dce!(compact)
+        ir = complete(compact)
+        sroa_mutables!(ir, defuses, used_ssas)
+        return ir
+    else
+        simple_dce!(compact)
+        return complete(compact)
+    end
+end
+
+function sroa_mutables!(ir::IRCode, defuses::IdDict{Int, Tuple{BitSet, SSADefUse}}, used_ssas::Vector{Int})
+    # Compute domtree, needed below, now that we have finished compacting the IR.
+    # This needs to be after we iterate through the IR with `IncrementalCompact`
+    # because removing dead blocks can invalidate the domtree.
     @timeit "domtree 2" domtree = construct_domtree(ir.cfg.blocks)
 
-    # Now go through any mutable structs and see which ones we can eliminate
     for (idx, (intermediaries, defuse)) in defuses
         intermediaries = collect(intermediaries)
         # Check if there are any uses we did not account for. If so, the variable
@@ -806,12 +814,12 @@ function sroa_pass!(ir::IRCode)
             # it would have been deleted. That's fine, just ignore
             # the use in that case.
             stmt === nothing && continue
-            field = try_compute_fieldidx_stmt(compact, stmt::Expr, typ)
+            field = try_compute_fieldidx_stmt(ir, stmt::Expr, typ)
             field === nothing && @goto skip
             push!(fielddefuse[field].uses, use)
         end
         for use in defuse.defs
-            field = try_compute_fieldidx_stmt(compact, ir[SSAValue(use)]::Expr, typ)
+            field = try_compute_fieldidx_stmt(ir, ir[SSAValue(use)]::Expr, typ)
             field === nothing && @goto skip
             push!(fielddefuse[field].defs, use)
         end
@@ -846,8 +854,9 @@ function sroa_pass!(ir::IRCode)
                 end
             end
         end
-        preserve_uses = IdDict{Int, Vector{Any}}((idx=>Any[] for idx in IdSet{Int}(defuse.ccall_preserve_uses)))
         # Everything accounted for. Go field by field and perform idf
+        preserve_uses = isempty(defuse.ccall_preserve_uses) ? nothing :
+            IdDict{Int, Vector{Any}}((idx=>Any[] for idx in BitSet(defuse.ccall_preserve_uses)))
         for fidx in 1:ndefuse
             du = fielddefuse[fidx]
             ftyp = fieldtype(typ, fidx)
@@ -863,8 +872,10 @@ function sroa_pass!(ir::IRCode)
                     ir[SSAValue(stmt)] = compute_value_for_use(ir, domtree, allblocks, du, phinodes, fidx, stmt)
                 end
                 if !isbitstype(ftyp)
-                    for (use, list) in preserve_uses
-                        push!(list, compute_value_for_use(ir, domtree, allblocks, du, phinodes, fidx, use))
+                    if preserve_uses !== nothing
+                        for (use, list) in preserve_uses
+                            push!(list, compute_value_for_use(ir, domtree, allblocks, du, phinodes, fidx, use))
+                        end
                     end
                 end
                 for b in phiblocks
@@ -881,7 +892,7 @@ function sroa_pass!(ir::IRCode)
                 ir[SSAValue(stmt)] = nothing
             end
         end
-        isempty(defuse.ccall_preserve_uses) && continue
+        preserve_uses === nothing && continue
         push!(intermediaries, newidx)
         # Insert the new preserves
         for (use, new_preserves) in preserve_uses
@@ -897,10 +908,7 @@ function sroa_pass!(ir::IRCode)
 
         @label skip
     end
-
-    return ir
 end
-# assertion_counter = 0
 
 """
     canonicalize_typeassert!(compact::IncrementalCompact, idx::Int, stmt::Expr)