optimizer: eliminate isdefined in sroa_mutables! (JuliaLang#44708)

This commit implements a simple optimization within `sroa_mutables!` to
eliminate `isdefined` call by checking load-forwardability of the field.
This optimization may be especially useful to eliminate extra allocation
of `Core.Box` involved with a capturing closure, e.g.:
```julia
julia> callit(f, args...) = f(args...);

julia> function isdefined_elim()
           local arr::Vector{Any}
           callit() do
               arr = Any[]
           end
           return arr
       end;

julia> code_typed(isdefined_elim)
```
```diff
diff --git a/_master.jl b/_pr.jl
index 3aa40ba20e5..11eccf65f32 100644
--- a/_master.jl
+++ b/_pr.jl
@@ -1,15 +1,8 @@
 1-element Vector{Any}:
  CodeInfo(
-1 ─ %1  = Core.Box::Type{Core.Box}
-│   %2  = %new(%1)::Core.Box
-│   %3  = $(Expr(:foreigncall, :(:jl_alloc_array_1d), Vector{Any}, svec(Any, Int64), 0, :(:ccall), Vector{Any}, 0, 0))::Vector{Any}
-│         Core.setfield!(%2, :contents, %3)::Vector{Any}
-│   %5  = Core.isdefined(%2, :contents)::Bool
-└──       goto #3 if not %5
-2 ─       goto #4
-3 ─       $(Expr(:throw_undef_if_not, :arr, false))::Any
-4 ┄ %9  = Core.getfield(%2, :contents)::Any
-│         Core.typeassert(%9, Vector{Any})::Vector{Any}
-│   %11 = π (%9, Vector{Any})
-└──       return %11
+1 ─ %1 = $(Expr(:foreigncall, :(:jl_alloc_array_1d), Vector{Any}, svec(Any, Int64), 0, :(:ccall), Vector{Any}, 0, 0))::Vector{Any}
+└──      goto #3 if not true
+2 ─      goto #4
+3 ─      $(Expr(:throw_undef_if_not, :arr, false))::Any
+4 ┄      return %1
 ) => Vector{Any}
```

base/compiler/ssair/passes.jl

@@ -10,7 +10,7 @@ end
     du::SSADefUse
 
 This struct keeps track of all uses of some mutable struct allocated in the current function:
-- `du.uses::Vector{Int}` are all instances of `getfield` on the struct
+- `du.uses::Vector{Int}` are all instances of `getfield` / `isdefined` on the struct
 - `du.defs::Vector{Int}` are all instances of `setfield!` on the struct
 The terminology refers to the uses/defs of the "slot bundle" that the mutable struct represents.
 
@@ -27,7 +27,10 @@ struct SSADefUse
 end
 SSADefUse() = SSADefUse(Int[], Int[], Int[])
 
-compute_live_ins(cfg::CFG, du::SSADefUse) = compute_live_ins(cfg, du.defs, du.uses)
+function compute_live_ins(cfg::CFG, du::SSADefUse)
+    # filter out `isdefined` usages
+    return compute_live_ins(cfg, du.defs, filter(>(0), du.uses))
+end
 
 # assume `stmt == getfield(obj, field, ...)` or `stmt == setfield!(obj, field, val, ...)`
 try_compute_field_stmt(ir::Union{IncrementalCompact,IRCode}, stmt::Expr) =
@@ -725,7 +728,7 @@ function sroa_pass!(ir::IRCode)
     for ((_, idx), stmt) in compact
         # check whether this statement is `getfield` / `setfield!` (or other "interesting" statement)
         isa(stmt, Expr) || continue
-        is_setfield = false
+        is_setfield = is_isdefined = false
         field_ordering = :unspecified
         if is_known_call(stmt, setfield!, compact)
             4 <= length(stmt.args) <= 5 || continue
@@ -741,6 +744,13 @@ function sroa_pass!(ir::IRCode)
                 field_ordering = argextype(stmt.args[4], compact)
                 widenconst(field_ordering) === Bool && (field_ordering = :unspecified)
             end
+        elseif is_known_call(stmt, isdefined, compact)
+            3 <= length(stmt.args) <= 4 || continue
+            is_isdefined = true
+            if length(stmt.args) == 4
+                field_ordering = argextype(stmt.args[4], compact)
+                widenconst(field_ordering) === Bool && (field_ordering = :unspecified)
+            end
         elseif isexpr(stmt, :foreigncall)
             nccallargs = length(stmt.args[3]::SimpleVector)
             preserved = Int[]
@@ -795,13 +805,11 @@ function sroa_pass!(ir::IRCode)
                 lift_comparison!(===, compact, idx, stmt, lifting_cache)
             elseif is_known_call(stmt, isa, compact)
                 lift_comparison!(isa, compact, idx, stmt, lifting_cache)
-            elseif is_known_call(stmt, isdefined, compact)
-                lift_comparison!(isdefined, compact, idx, stmt, lifting_cache)
             end
             continue
         end
 
-        # analyze this `getfield` / `setfield!` call
+        # analyze this `getfield` / `isdefined` / `setfield!` call
 
         field = try_compute_field_stmt(compact, stmt)
         field === nothing && continue
@@ -812,10 +820,15 @@ function sroa_pass!(ir::IRCode)
         if isa(struct_typ, Union) && struct_typ <: Tuple
             struct_typ = unswitchtupleunion(struct_typ)
         end
+        if isa(struct_typ, Union) && is_isdefined
+            lift_comparison!(isdefined, compact, idx, stmt, lifting_cache)
+            continue
+        end
         isa(struct_typ, DataType) || continue
 
         struct_typ.name.atomicfields == C_NULL || continue # TODO: handle more
-        if !(field_ordering === :unspecified || (field_ordering isa Const && field_ordering.val === :not_atomic))
+        if !((field_ordering === :unspecified) ||
+             (field_ordering isa Const && field_ordering.val === :not_atomic))
             continue
         end
 
@@ -836,6 +849,8 @@ function sroa_pass!(ir::IRCode)
                 mid, defuse = get!(defuses, def.id, (SPCSet(), SSADefUse()))
                 if is_setfield
                     push!(defuse.defs, idx)
+                elseif is_isdefined
+                    push!(defuse.uses, -idx)
                 else
                     push!(defuse.uses, idx)
                 end
@@ -844,6 +859,8 @@ function sroa_pass!(ir::IRCode)
             continue
         elseif is_setfield
             continue # invalid `setfield!` call, but just ignore here
+        elseif is_isdefined
+            continue # TODO?
         end
 
         # perform SROA on immutable structs here on
@@ -927,9 +944,9 @@ function sroa_mutables!(ir::IRCode, defuses::IdDict{Int, Tuple{SPCSet, SSADefUse
         typ = typ::DataType
         # Partition defuses by field
         fielddefuse = SSADefUse[SSADefUse() for _ = 1:fieldcount(typ)]
-        all_forwarded = true
+        all_eliminated = all_forwarded = true
         for use in defuse.uses
-            stmt = ir[SSAValue(use)][:inst] # == `getfield` call
+            stmt = ir[SSAValue(abs(use))][:inst] # == `getfield`/`isdefined` call
             # We may have discovered above that this use is dead
             # after the getfield elim of immutables. In that case,
             # it would have been deleted. That's fine, just ignore
@@ -969,7 +986,15 @@ function sroa_mutables!(ir::IRCode, defuses::IdDict{Int, Tuple{SPCSet, SSADefUse
             blocks[fidx] = phiblocks, allblocks
             if fidx + 1 > length(defexpr.args)
                 for use in du.uses
-                    has_safe_def(ir, get_domtree(), allblocks, du, newidx, use) || @goto skip
+                    if use > 0 # == `getfield` use
+                        has_safe_def(ir, get_domtree(), allblocks, du, newidx, use) || @goto skip
+                    else # == `isdefined` use
+                        if has_safe_def(ir, get_domtree(), allblocks, du, newidx, -use)
+                            ir[SSAValue(-use)][:inst] = true
+                        else
+                            all_eliminated = false
+                        end
+                    end
                 end
             end
         end
@@ -991,8 +1016,12 @@ function sroa_mutables!(ir::IRCode, defuses::IdDict{Int, Tuple{SPCSet, SSADefUse
                         NewInstruction(PhiNode(), ftyp))
                 end
                 # Now go through all uses and rewrite them
-                for stmt in du.uses
-                    ir[SSAValue(stmt)][:inst] = compute_value_for_use(ir, domtree, allblocks, du, phinodes, fidx, stmt)
+                for use in du.uses
+                    if use > 0 # == `getfield` use
+                        ir[SSAValue(use)][:inst] = compute_value_for_use(ir, domtree, allblocks, du, phinodes, fidx, use)
+                    else # == `isdefined` use
+                        continue # already rewritten if possible
+                    end
                 end
                 if !isbitstype(ftyp)
                     if preserve_uses !== nothing
@@ -1010,6 +1039,10 @@ function sroa_mutables!(ir::IRCode, defuses::IdDict{Int, Tuple{SPCSet, SSADefUse
                     end
                 end
             end
+            all_eliminated || continue
+            # all "usages" (i.e. `getfield` and `isdefined` calls) are eliminated,
+            # now eliminate "definitions" (`setfield!`) calls
+            # (NOTE the allocation itself will be eliminated by DCE pass later)
             for stmt in du.defs
                 stmt == newidx && continue
                 ir[SSAValue(stmt)][:inst] = nothing

test/compiler/irpasses.jl

@@ -425,6 +425,44 @@ let src = code_typed1() do
     @test count(isnew, src.code) == 1
 end
 
+# isdefined elimination
+# ---------------------
+
+let src = code_typed1((Any,)) do a
+        r = Ref{Any}()
+        r[] = a
+        if isassigned(r)
+            return r[]
+        end
+        return nothing
+    end
+    @test is_scalar_replaced(src)
+end
+
+let src = code_typed1((Bool, Any,)) do cnd, a
+        r = Ref{Any}()
+        if cnd
+            r[] = a # this `setfield!` shouldn't be eliminated
+        end
+        return isassigned(r)
+    end
+    @test count(isnew, src.code) == 1
+    @test count(iscall((src, setfield!)), src.code) == 1
+end
+
+callit(f, args...) = f(args...)
+function isdefined_elim()
+    local arr::Vector{Any}
+    callit() do
+        arr = Any[]
+    end
+    return arr
+end
+let src = code_typed1(isdefined_elim)
+    @test is_scalar_replaced(src)
+end
+@test isdefined_elim() == Any[]
+
 # comparison lifting
 # ==================