RFC: Change lowering of destructuring to avoid const prop dependence

I'm currently doing some work with inference passes that have const
prop (temporarily) disabled and I noticed we actually rely on it
quite a bit for basic things. That's not terrible - const prop works
pretty well after all, but it still imposes a cost and while I want
to support it in my AD use case also, it makes destructuring quite
expensive, because everything needs to be inferred twice. This PR
is an experiment in changing the lowering to avoid having to const
prop the index. Rather than lowering `(a,b,c) = foo()` as:

```
it = foo()
a, s = indexed_iterate(it, 1)
b, s = indexed_iterate(it, 2)
c, s = indexed_iterate(it, 3)
```

we lower as:

```
it = foo()
iterate, index = index_and_itereate(it)
x = iterate(it)
a = index(x, 1)
y = iterate(it, y)
b = index(y, 2)
z = iterate(it, z)
c = index(z, 3)
```

For tuples `iterate` would simply return the first argument and
`index` would be `getfield`. That way, there is no const prop,
since `getfield` is called directly and inference can directly
use its tfunc. For the fallback case `iterate` is basically
just `Base.iterate`, with just a slight tweak to give an intelligent
error for short iterables.

On simple functions, there isn't much of a difference in execution
time, but benchmarking something more complicated like:
```
function g()
  a, = getfield(((1,),(2.0,3),("x",),(:x,)), Base.inferencebarrier(1))
  nothing
end
```

shows about a 20% improvement in end-to-end inference/optimize time,
which is substantial.

base/missing.jl

@@ -69,6 +69,7 @@ convert(::Type{T}, x::T) where {T>:Union{Missing, Nothing}} = x
 convert(::Type{T}, x) where {T>:Missing} = convert(nonmissingtype_checked(T), x)
 convert(::Type{T}, x) where {T>:Union{Missing, Nothing}} = convert(nonmissingtype_checked(nonnothingtype_checked(T)), x)
 
+index_and_iterate(::Missing) = throw(MethodError(iterate, (missing,)))
 
 # Comparison operators
 ==(::Missing, ::Missing) = missing

base/namedtuple.jl

@@ -112,10 +112,10 @@ firstindex(t::NamedTuple) = 1
 lastindex(t::NamedTuple) = nfields(t)
 getindex(t::NamedTuple, i::Int) = getfield(t, i)
 getindex(t::NamedTuple, i::Symbol) = getfield(t, i)
-indexed_iterate(t::NamedTuple, i::Int, state=1) = (getfield(t, i), i+1)
 isempty(::NamedTuple{()}) = true
 isempty(::NamedTuple) = false
 empty(::NamedTuple) = NamedTuple()
+index_and_iterate(t::NamedTuple) = (arg1, getfield)
 
 convert(::Type{NamedTuple{names,T}}, nt::NamedTuple{names,T}) where {names,T<:Tuple} = nt
 convert(::Type{NamedTuple{names}}, nt::NamedTuple{names}) where {names} = nt

base/pair.jl

@@ -47,7 +47,7 @@ Pair, =>
 
 eltype(p::Type{Pair{A, B}}) where {A, B} = Union{A, B}
 iterate(p::Pair, i=1) = i > 2 ? nothing : (getfield(p, i), i + 1)
-indexed_iterate(p::Pair, i::Int, state=1) = (getfield(p, i), i + 1)
+index_and_iterate(p::Pair) = (arg1, getfield)
 
 hash(p::Pair, h::UInt) = hash(p.second, hash(p.first, h))
 

base/tuple.jl

@@ -81,21 +81,41 @@ function _maxlength(t::Tuple, t2::Tuple, t3::Tuple...)
     max(length(t), _maxlength(t2, t3...))
 end
 
-# this allows partial evaluation of bounded sequences of next() calls on tuples,
-# while reducing to plain next() for arbitrary iterables.
-indexed_iterate(t::Tuple, i::Int, state=1) = (@_inline_meta; (getfield(t, i), i+1))
-indexed_iterate(a::Array, i::Int, state=1) = (@_inline_meta; (a[i], i+1))
-function indexed_iterate(I, i)
-    x = iterate(I)
-    x === nothing && throw(BoundsError(I, i))
-    x
-end
-function indexed_iterate(I, i, state)
-    x = iterate(I, state)
-    x === nothing && throw(BoundsError(I, i))
-    x
+# this allows partial evaluation of bounded sequences of iterate() calls on tuples,
+# while reducing to plain iterate() for arbitrary iterables.
+
+arg1(a) = a
+arg1(a, b) = a
+index_and_iterate(t::Tuple) = (arg1, getfield)
+index_and_iterate(t::Array) = (arg1, getindex)
+
+struct BadSlurp
+    a
+end
+
+function slurp_iterate(a)
+    @_inline_meta
+    s = iterate(a)
+    s === nothing && return BadSlurp(a)
+    s
 end
 
+function slurp_iterate(a, b)
+    @_inline_meta
+    s = iterate(a, getfield(b, 2))
+    s === nothing && return BadSlurp(a)
+    s
+end
+
+select_first(a::BadSlurp, i) = throw(BoundsError(a.a, i))
+select_first(a, i) = getfield(a, 1)
+
+index_and_iterate(x) = (slurp_iterate, select_first)
+
+# Nothing is often union'ed into other things. Kill that as quickly as possible
+# to make inference's life easier.
+index_and_iterate(::Nothing) = throw(MethodError(iterate, (nothing,)))
+
 # Use dispatch to avoid a branch in first
 first(::Tuple{}) = throw(ArgumentError("tuple must be non-empty"))
 first(t::Tuple) = t[1]

src/common_symbols1.inc

@@ -33,7 +33,7 @@ jl_symbol("*"),
 jl_symbol("bitcast"),
 jl_symbol("slt_int"),
 jl_symbol("isempty"),
-jl_symbol("indexed_iterate"),
+jl_symbol("index_and_iterate"),
 jl_symbol("size"),
 jl_symbol("!"),
 jl_symbol("nothing"),

src/julia-syntax.scm

@@ -2049,17 +2049,26 @@
                                 x (make-ssavalue)))
                        (ini (if (eq? x xx) '() (list (sink-assignment xx (expand-forms x)))))
                        (n   (length lhss))
+                       (funcs (make-ssavalue))
+                       (iterate (make-ssavalue))
+                       (index (make-ssavalue))
                        (st  (gensy)))
                   `(block
                     ,@ini
+                    ,(lower-tuple-assignment
+                      (list iterate index)
+                      `(call (top index_and_iterate) ,xx))
                     ,.(map (lambda (i lhs)
-                             (expand-forms
-                              (lower-tuple-assignment
-                               (if (= i (- n 1))
-                                   (list lhs)
-                                   (list lhs st))
-                               `(call (top indexed_iterate)
-                                      ,xx ,(+ i 1) ,.(if (eq? i 0) '() `(,st))))))
+                            (expand-forms
+                              `(block
+                                (= ,st (call ,iterate
+                                    ,xx ,.(if (eq? i 0) '() `(,st))))
+                               ,(if (eventually-call? lhs)
+                                  (let ((val (gensy)))
+                                    `(block
+                                      (= ,val (call ,index ,st ,(+ i 1)))
+                                      (= ,lhs ,val)))
+                                  `(= ,lhs (call ,index ,st ,(+ i 1)))))))
                            (iota n)
                            lhss)
                     (unnecessary ,xx))))))

stdlib/Serialization/src/Serialization.jl

@@ -66,7 +66,7 @@ const TAGS = Any[
     :(=), :(==), :(===), :gotoifnot, :A, :B, :C, :M, :N, :T, :S, :X, :Y, :a, :b, :c, :d, :e, :f,
     :g, :h, :i, :j, :k, :l, :m, :n, :o, :p, :q, :r, :s, :t, :u, :v, :w, :x, :y, :z, :add_int,
     :sub_int, :mul_int, :add_float, :sub_float, :new, :mul_float, :bitcast, :start, :done, :next,
-    :indexed_iterate, :getfield, :meta, :eq_int, :slt_int, :sle_int, :ne_int, :push_loc, :pop_loc,
+    :index_and_iterate, :getfield, :meta, :eq_int, :slt_int, :sle_int, :ne_int, :push_loc, :pop_loc,
     :pop, :arrayset, :arrayref, :apply_type, :inbounds, :getindex, :setindex!, :Core, :!, :+,
     :Base, :static_parameter, :convert, :colon, Symbol("#self#"), Symbol("#temp#"), :tuple, Symbol(""),
 
@@ -78,7 +78,7 @@ const TAGS = Any[
 
 @assert length(TAGS) == 255
 
-const ser_version = 11 # do not make changes without bumping the version #!
+const ser_version = 12 # do not make changes without bumping the version #!
 
 const NTAGS = length(TAGS)
 

test/compiler/inference.jl

@@ -562,10 +562,7 @@ end
 
 function g19348(x)
     a, b = x
-    g = 1
-    g = 2
-    c = Base.indexed_iterate(x, g, g)
-    return a + b + c[1]
+    return a + b
 end
 
 for (codetype, all_ssa) in Any[

test/core.jl

@@ -557,6 +557,14 @@ let (f(), x) = (1, 2)
     @test x == 2
 end
 
+foo23091_cnt = 0
+struct Foo23091; end
+Base.iterate(::Foo23091, state...) = (global foo23091_cnt += 1; (1, nothing))
+(g23091(), h23091()) = Foo23091()
+@test foo23091_cnt == 2
+g23091(); h23091()
+@test foo23091_cnt == 2
+
 # issue #21900
 f21900_cnt = 0
 function f21900()

test/dict.jl

@@ -11,8 +11,6 @@ using Random
     @test iterate(p, iterate(p, iterate(p)[2])[2]) == nothing
     @test firstindex(p) == 1
     @test lastindex(p) == length(p) == 2
-    @test Base.indexed_iterate(p, 1, nothing) == (10,2)
-    @test Base.indexed_iterate(p, 2, nothing) == (20,3)
     @test (1=>2) < (2=>3)
     @test (2=>2) < (2=>3)
     @test !((2=>3) < (2=>3))