optimizer: SROA mutable(immutable(...)) case correctly

Our SROA should be able to handle `mutable(immutable(...))` case even
without "proper" alias analysis, since we eliminate `immutable` first
and then process `mutable` within our iterative approach.

It turns out it hasn't worked at all because after the immutable 
handling
we keep the reference count _before_ DCE, but didn't update dead 
reference
count. This commit fixes it and now we should be able to eliminate more
allocations, e.g.:
```julia
julia> simple_sroa(s) = broadcast(identity, Ref(s))
simple_sroa (generic function with 1 method)

julia> simple_sroa("compile"); @allocated(simple_sroa("julia"))
0
```

base/compiler/ssair/ir.jl

@@ -1311,7 +1311,9 @@ function iterate(compact::IncrementalCompact, (idx, active_bb)::Tuple{Int, Int}=
         compact.result[old_result_idx][:inst]), (compact.idx, active_bb)
 end
 
-function maybe_erase_unused!(extra_worklist::Vector{Int}, compact::IncrementalCompact, idx::Int, callback = x::SSAValue->nothing)
+function maybe_erase_unused!(
+    extra_worklist::Vector{Int}, compact::IncrementalCompact, idx::Int,
+    callback = null_dce_callback)
     stmt = compact.result[idx][:inst]
     stmt === nothing && return false
     if compact_exprtype(compact, SSAValue(idx)) === Bottom
@@ -1404,19 +1406,21 @@ function just_fixup!(compact::IncrementalCompact)
     end
 end
 
-function simple_dce!(compact::IncrementalCompact)
+function simple_dce!(compact::IncrementalCompact, callback = null_dce_callback)
     # Perform simple DCE for unused values
     extra_worklist = Int[]
     for (idx, nused) in Iterators.enumerate(compact.used_ssas)
         idx >= compact.result_idx && break
         nused == 0 || continue
-        maybe_erase_unused!(extra_worklist, compact, idx)
+        maybe_erase_unused!(extra_worklist, compact, idx, callback)
     end
     while !isempty(extra_worklist)
-        maybe_erase_unused!(extra_worklist, compact, pop!(extra_worklist))
+        maybe_erase_unused!(extra_worklist, compact, pop!(extra_worklist), callback)
     end
 end
 
+null_dce_callback(x::SSAValue) = return
+
 function non_dce_finish!(compact::IncrementalCompact)
     result_idx = compact.result_idx
     resize!(compact.result, result_idx - 1)

base/compiler/ssair/passes.jl

@@ -740,7 +740,7 @@ function sroa_pass!(ir::IRCode)
             result_t = make_MaybeUndef(result_t)
         end
 
-        val = perform_lifting!(compact, visited_phinodes, field, lifting_cache, result_t, lifted_leaves, stmt.args[2])
+        val = perform_lifting!(compact, visited_phinodes, field, lifting_cache, result_t, lifted_leaves, val)
 
         # Insert the undef check if necessary
         if any_undef
@@ -762,9 +762,10 @@ function sroa_pass!(ir::IRCode)
         # 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!`
+        # but before the DCE) for our predicate within `sroa_mutables!`, but we also
+        # try an extra effort using a callback so that reference counts are updated
         used_ssas = copy(compact.used_ssas)
-        simple_dce!(compact)
+        simple_dce!(compact, (x::SSAValue) -> used_ssas[x.id] -= 1)
         ir = complete(compact)
         sroa_mutables!(ir, defuses, used_ssas)
         return ir
@@ -834,10 +835,7 @@ function sroa_mutables!(ir::IRCode, defuses::IdDict{Int, Tuple{SPCSet, SSADefUse
             isempty(du.uses) && continue
             push!(du.defs, newidx)
             ldu = compute_live_ins(ir.cfg, du)
-            phiblocks = Int[]
-            if !isempty(ldu.live_in_bbs)
-                phiblocks = iterated_dominance_frontier(ir.cfg, ldu, domtree)
-            end
+            phiblocks = isempty(ldu.live_in_bbs) ? Int[] : iterated_dominance_frontier(ir.cfg, ldu, domtree)
             allblocks = sort(vcat(phiblocks, ldu.def_bbs))
             blocks[fidx] = phiblocks, allblocks
             if fidx + 1 > length(defexpr.args)

test/compiler/irpasses.jl

@@ -199,21 +199,49 @@ let src = code_typed1((Any,Any,Any)) do x, y, z
         x.args[2:end] == Any[#=x=# Core.Argument(2), #=y=# Core.Argument(3), #=y=# Core.Argument(4)]
     end
 end
-# FIXME our analysis isn't yet so powerful at this moment, e.g. it can't handle nested mutable objects
+
+# FIXME our analysis isn't yet so powerful at this moment: may be unable to handle nested objects well
+# OK: mutable(immutable(...)) case
+let src = code_typed1((Any,Any,Any)) do x, y, z
+        xyz = MutableXYZ(x, y, z)
+        t   = (xyz,)
+        v = t[1].x
+        v, v, v
+    end
+    @test !any(isnew, src.code)
+end
 let src = code_typed1((Any,Any,Any)) do x, y, z
         xyz = MutableXYZ(x, y, z)
         outer = ImmutableOuter(xyz, xyz, xyz)
         outer.x.x, outer.y.y, outer.z.z
     end
-    @test_broken !any(isnew, src.code)
+    @test !any(isnew, src.code)
+    @test any(src.code) do @nospecialize x
+        iscall((src, tuple), x) &&
+        x.args[2:end] == Any[#=x=# Core.Argument(2), #=y=# Core.Argument(3), #=y=# Core.Argument(4)]
+    end
+end
+let
+    simple_sroa(s) = broadcast(identity, Ref(s))
+    @allocated(simple_sroa("julia"))
+    @test @allocated(simple_sroa("julia")) == 0
 end
+# FIXME: immutable(mutable(...)) case
 let src = code_typed1((Any,Any,Any)) do x, y, z
         xyz = ImmutableXYZ(x, y, z)
         outer = MutableOuter(xyz, xyz, xyz)
         outer.x.x, outer.y.y, outer.z.z
     end
     @test_broken !any(isnew, src.code)
 end
+# FIXME: mutable(mutable(...)) case
+let src = code_typed1((Any,Any,Any)) do x, y, z
+        xyz = MutableXYZ(x, y, z)
+        outer = MutableOuter(xyz, xyz, xyz)
+        outer.x.x, outer.y.y, outer.z.z
+    end
+    @test_broken !any(isnew, src.code)
+end
 
 # should work nicely with inlining to optimize away a complicated case
 # adapted from http://wiki.luajit.org/Allocation-Sinking-Optimization#implementation%5B