inference: forward Conditional inter-procedurally (JuliaLang#42529)

The PR JuliaLang#38905 only "back-propagates" conditional constraint
(from callee to caller), but currently we don't "forward" it
(caller to callee), and so inter-procedural constraint propagation
won't happen for e.g.:
```julia
ifelselike(cnd, x, y) = cnd ? x : y
@test Base.return_types((Any,Int,)) do x, y
    ifelselike(isa(x, Int), x, y)
end |> only == Int
```

This commit complements JuliaLang#38905 and enables further inter-procedural
conditional constraint propagation by forwarding `Conditional` to
callees when it imposes a constraint on any other argument,
during constant propagation.

We also improve constant-prop' heuristics in these ways:

- remove `const_prop_rettype_heuristic` since it handles rare cases,
  where const-prop' doens't seem to be worthwhile, e.g. it won't be
  so useful to try to propagate `Const(Tuple{DataType,DataType})` for
  `Const(convert)(::Const(Tuple{DataType,DataType}), ::Tuple{DataType,DataType} -> Tuple{DataType,DataType}`
- rename `is_allconst` to `is_all_overridden`
- also minor refactors and improvements added

base/compiler/inferenceresult.jl

@@ -3,20 +3,57 @@
 function is_argtype_match(@nospecialize(given_argtype),
                           @nospecialize(cache_argtype),
                           overridden_by_const::Bool)
-    if isa(given_argtype, Const) || isa(given_argtype, PartialStruct) || isa(given_argtype, PartialOpaque)
+    if is_forwardable_argtype(given_argtype)
         return is_lattice_equal(given_argtype, cache_argtype)
     end
     return !overridden_by_const
 end
 
+function is_forwardable_argtype(@nospecialize x)
+    return isa(x, Const) ||
+           isa(x, Conditional) ||
+           isa(x, PartialStruct) ||
+           isa(x, PartialOpaque)
+end
+
 # In theory, there could be a `cache` containing a matching `InferenceResult`
 # for the provided `linfo` and `given_argtypes`. The purpose of this function is
 # to return a valid value for `cache_lookup(linfo, argtypes, cache).argtypes`,
 # so that we can construct cache-correct `InferenceResult`s in the first place.
-function matching_cache_argtypes(linfo::MethodInstance, given_argtypes::Vector, va_override::Bool)
+function matching_cache_argtypes(
+    linfo::MethodInstance, (; fargs, argtypes)::ArgInfo, va_override::Bool)
     @assert isa(linfo.def, Method) # ensure the next line works
     nargs::Int = linfo.def.nargs
-    given_argtypes = anymap(widenconditional, given_argtypes)
+    cache_argtypes, overridden_by_const = matching_cache_argtypes(linfo, nothing, va_override)
+    given_argtypes = Vector{Any}(undef, length(argtypes))
+    local condargs = nothing
+    for i in 1:length(argtypes)
+        argtype = argtypes[i]
+        # forward `Conditional` if it conveys a constraint on any other argument
+        if isa(argtype, Conditional) && fargs !== nothing
+            cnd = argtype
+            slotid = find_constrained_arg(cnd, fargs)
+            if slotid !== nothing
+                # using union-split signature, we may be able to narrow down `Conditional`
+                sigt = widenconst(slotid > nargs ? argtypes[slotid] : cache_argtypes[slotid])
+                vtype = tmeet(cnd.vtype, sigt)
+                elsetype = tmeet(cnd.elsetype, sigt)
+                if vtype === Bottom && elsetype === Bottom
+                    # we accidentally proved this method match is impossible
+                    # TODO bail out here immediately rather than just propagating Bottom ?
+                    given_argtypes[i] = Bottom
+                else
+                    if condargs === nothing
+                        condargs = Tuple{Int,Int}[]
+                    end
+                    push!(condargs, (slotid, i))
+                    given_argtypes[i] = Conditional(SlotNumber(slotid), vtype, elsetype)
+                end
+                continue
+            end
+        end
+        given_argtypes[i] = widenconditional(argtype)
+    end
     isva = va_override || linfo.def.isva
     if isva || isvarargtype(given_argtypes[end])
         isva_given_argtypes = Vector{Any}(undef, nargs)
@@ -30,15 +67,22 @@ function matching_cache_argtypes(linfo::MethodInstance, given_argtypes::Vector,
                 last = nargs
             end
             isva_given_argtypes[nargs] = tuple_tfunc(given_argtypes[last:end])
+            # invalidate `Conditional` imposed on varargs
+            if condargs !== nothing
+                for (slotid, i) in condargs
+                    if slotid ≥ last
+                        isva_given_argtypes[i] = widenconditional(isva_given_argtypes[i])
+                    end
+                end
+            end
         end
         given_argtypes = isva_given_argtypes
     end
     @assert length(given_argtypes) == nargs
-    cache_argtypes, overridden_by_const = matching_cache_argtypes(linfo, nothing, va_override)
     for i in 1:nargs
         given_argtype = given_argtypes[i]
         cache_argtype = cache_argtypes[i]
-        if !is_argtype_match(given_argtype, cache_argtype, overridden_by_const[i])
+        if !is_argtype_match(given_argtype, cache_argtype, false)
             # prefer the argtype we were given over the one computed from `linfo`
             cache_argtypes[i] = given_argtype
             overridden_by_const[i] = true

base/compiler/tfuncs.jl

@@ -964,7 +964,7 @@ function abstract_modifyfield!(interp::AbstractInterpreter, argtypes::Vector{Any
         v = unwrapva(argtypes[5])
         TF = getfield_tfunc(o, f)
         push!(sv.ssavalue_uses[sv.currpc], sv.currpc) # temporarily disable `call_result_unused` check for this call
-        callinfo = abstract_call(interp, nothing, Any[op, TF, v], sv, #=max_methods=# 1)
+        callinfo = abstract_call(interp, ArgInfo(nothing, Any[op, TF, v]), sv, #=max_methods=# 1)
         pop!(sv.ssavalue_uses[sv.currpc], sv.currpc)
         TF2 = tmeet(callinfo.rt, widenconst(TF))
         if TF2 === Bottom
@@ -1747,7 +1747,7 @@ function return_type_tfunc(interp::AbstractInterpreter, argtypes::Vector{Any}, s
                     if contains_is(argtypes_vec, Union{})
                         return CallMeta(Const(Union{}), false)
                     end
-                    call = abstract_call(interp, nothing, argtypes_vec, sv, -1)
+                    call = abstract_call(interp, ArgInfo(nothing, argtypes_vec), sv, -1)
                     info = verbose_stmt_info(interp) ? ReturnTypeCallInfo(call.info) : false
                     rt = widenconditional(call.rt)
                     if isa(rt, Const)

base/compiler/types.jl

@@ -17,6 +17,11 @@ If `interp` is an `AbstractInterpreter`, it is expected to provide at least the
 """
 abstract type AbstractInterpreter end
 
+struct ArgInfo
+    fargs::Union{Nothing,Vector{Any}}
+    argtypes::Vector{Any}
+end
+
 """
     InferenceResult
 
@@ -29,8 +34,10 @@ mutable struct InferenceResult
     result # ::Type, or InferenceState if WIP
     src #::Union{CodeInfo, OptimizationState, Nothing} # if inferred copy is available
     valid_worlds::WorldRange # if inference and optimization is finished
-    function InferenceResult(linfo::MethodInstance, given_argtypes = nothing, va_override=false)
-        argtypes, overridden_by_const = matching_cache_argtypes(linfo, given_argtypes, va_override)
+    function InferenceResult(linfo::MethodInstance,
+                             arginfo::Union{Nothing,ArgInfo} = nothing,
+                             va_override::Bool = false)
+        argtypes, overridden_by_const = matching_cache_argtypes(linfo, arginfo, va_override)
         return new(linfo, argtypes, overridden_by_const, Any, nothing, WorldRange())
     end
 end

base/compiler/typeutils.jl

@@ -259,15 +259,6 @@ unioncomplexity(u::UnionAll) = max(unioncomplexity(u.body)::Int, unioncomplexity
 unioncomplexity(t::TypeofVararg) = isdefined(t, :T) ? unioncomplexity(t.T)::Int : 0
 unioncomplexity(@nospecialize(x)) = 0
 
-function improvable_via_constant_propagation(@nospecialize(t))
-    if isconcretetype(t) && t <: Tuple
-        for p in t.parameters
-            p === DataType && return true
-        end
-    end
-    return false
-end
-
 # convert a Union of Tuple types to a Tuple of Unions
 function unswitchtupleunion(u::Union)
     ts = uniontypes(u)

test/compiler/inference.jl

@@ -2000,6 +2000,61 @@ function _g_ifelse_isa_()
 end
 @test Base.return_types(_g_ifelse_isa_, ()) == [Int]
 
+@testset "Conditional forwarding" begin
+    # forward `Conditional` if it conveys a constraint on any other argument
+    ifelselike(cnd, x, y) = cnd ? x : y
+
+    @test Base.return_types((Any,Int,)) do x, y
+        ifelselike(isa(x, Int), x, y)
+    end |> only == Int
+
+    # should work nicely with union-split
+    @test Base.return_types((Union{Int,Nothing},)) do x
+        ifelselike(isa(x, Int), x, 0)
+    end |> only == Int
+
+    @test Base.return_types((Any,Int)) do x, y
+        ifelselike(!isa(x, Int), y, x)
+    end |> only == Int
+
+    @test Base.return_types((Any,Int)) do x, y
+        a = ifelselike(x === 0, x, 0) # ::Const(0)
+        if a == 0
+            return y
+        else
+            return nothing # dead branch
+        end
+    end |> only == Int
+
+    # pick up the first if there are multiple constrained arguments
+    @test Base.return_types((Any,)) do x
+        ifelselike(isa(x, Int), x, x)
+    end |> only == Any
+
+    # just propagate multiple constraints
+    ifelselike2(cnd1, cnd2, x, y, z) = cnd1 ? x : cnd2 ? y : z
+    @test Base.return_types((Any,Any)) do x, y
+        ifelselike2(isa(x, Int), isa(y, Int), x, y, 0)
+    end |> only == Int
+
+    # work with `invoke`
+    @test Base.return_types((Any,Any)) do x, y
+        Base.@invoke ifelselike(isa(x, Int), x, y::Int)
+    end |> only == Int
+
+    # don't be confused with vararg method
+    vacond(cnd, va...) = cnd ? va : 0
+    @test Base.return_types((Any,)) do x
+        # at runtime we will see `va::Tuple{Tuple{Int,Int}, Tuple{Int,Int}}`
+        vacond(isa(x, Tuple{Int,Int}), x, x)
+    end |> only == Union{Int,Tuple{Any,Any}}
+
+    # demonstrate extra constraint propagation for Base.ifelse
+    @test Base.return_types((Any,Int,)) do x, y
+        ifelse(isa(x, Int), x, y)
+    end |> only == Int
+end
+
 # Equivalence of Const(T.instance) and T for singleton types
 @test Const(nothing) ⊑ Nothing && Nothing ⊑ Const(nothing)