inference: form PartialStruct for extra type information propagation (JuliaLang#42831)

* inference: form `PartialStruct` for extra type information propagation

This commit forms `PartialStruct` whenever there is any type-level
refinement available about a field, even if it's not "constant" information.

In Julia "definitions" are allowed to be abstract whereas "usages"
(i.e. callsites) are often concrete. The basic idea is to allow inference
to make more use of such precise callsite type information by encoding it
as `PartialStruct`.

This may increase optimization possibilities of "unidiomatic" Julia code,
which may contain poorly-typed definitions, like this very contrived example:
```julia
struct Problem
    n; s; c; t
end

function main(args...)
    prob = Problem(args...)
    s = 0
    for i in 1:prob.n
        m = mod(i, 3)
        s += m == 0 ? sin(prob.s) : m == 1 ? cos(prob.c) : tan(prob.t)
    end
    return prob, s
end

main(10000, 1, 2, 3)
```

One of the obvious limitation is that this extra type information can be
propagated inter-procedurally only as a const-propagation.
I'm not sure this kind of "just a type-level" refinement can often make
constant-prop' successful (i.e. shape-up a method body and allow it to
be inlined, encoding the extra type information into the generated code),
thus I didn't not modify any part of const-prop' heuristics.

So the improvements from this change might not be very useful for general
inter-procedural analysis currently, but they should definitely improve the
accuracy of local analysis and very simple inter-procedural analysis.

Author: aviatesk

base/compiler/abstractinterpretation.jl

@@ -345,9 +345,9 @@ function from_interconditional(@nospecialize(typ), (; fargs, argtypes)::ArgInfo,
             else
                 elsetype = tmeet(elsetype, widenconst(new_elsetype))
             end
-            if (slot > 0 || condval !== false) && !(old ⊑ vtype) # essentially vtype ⋤ old
+            if (slot > 0 || condval !== false) && vtype ⋤ old
                 slot = id
-            elseif (slot > 0 || condval !== true) && !(old ⊑ elsetype) # essentially elsetype ⋤ old
+            elseif (slot > 0 || condval !== true) && elsetype ⋤ old
                 slot = id
             else # reset: no new useful information for this slot
                 vtype = elsetype = Any
@@ -1598,36 +1598,35 @@ function abstract_eval_statement(interp::AbstractInterpreter, @nospecialize(e),
     elseif ehead === :new
         t = instanceof_tfunc(abstract_eval_value(interp, e.args[1], vtypes, sv))[1]
         if isconcretetype(t) && !ismutabletype(t)
-            args = Vector{Any}(undef, length(e.args)-1)
-            ats = Vector{Any}(undef, length(e.args)-1)
-            anyconst = false
-            allconst = true
+            nargs = length(e.args) - 1
+            ats = Vector{Any}(undef, nargs)
+            local anyrefine = false
+            local allconst = true
             for i = 2:length(e.args)
                 at = widenconditional(abstract_eval_value(interp, e.args[i], vtypes, sv))
-                if !anyconst
-                    anyconst = has_nontrivial_const_info(at)
-                end
-                ats[i-1] = at
+                ft = fieldtype(t, i-1)
+                at = tmeet(at, ft)
                 if at === Bottom
                     t = Bottom
-                    allconst = anyconst = false
-                    break
-                elseif at isa Const
-                    if !(at.val isa fieldtype(t, i - 1))
-                        t = Bottom
-                        allconst = anyconst = false
-                        break
-                    end
-                    args[i-1] = at.val
-                else
+                    @goto t_computed
+                elseif !isa(at, Const)
                     allconst = false
                 end
+                if !anyrefine
+                    anyrefine = has_nontrivial_const_info(at) || # constant information
+                                at ⋤ ft                          # just a type-level information, but more precise than the declared type
+                end
+                ats[i-1] = at
             end
             # For now, don't allow partially initialized Const/PartialStruct
-            if t !== Bottom && fieldcount(t) == length(ats)
+            if fieldcount(t) == nargs
                 if allconst
-                    t = Const(ccall(:jl_new_structv, Any, (Any, Ptr{Cvoid}, UInt32), t, args, length(args)))
-                elseif anyconst
+                    argvals = Vector{Any}(undef, nargs)
+                    for j in 1:nargs
+                        argvals[j] = (ats[j]::Const).val
+                    end
+                    t = Const(ccall(:jl_new_structv, Any, (Any, Ptr{Cvoid}, UInt32), t, argvals, nargs))
+                elseif anyrefine
                     t = PartialStruct(t, ats)
                 end
             end
@@ -1638,7 +1637,7 @@ function abstract_eval_statement(interp::AbstractInterpreter, @nospecialize(e),
             at = abstract_eval_value(interp, e.args[2], vtypes, sv)
             n = fieldcount(t)
             if isa(at, Const) && isa(at.val, Tuple) && n == length(at.val::Tuple) &&
-                let t = t; _all(i->getfield(at.val::Tuple, i) isa fieldtype(t, i), 1:n); end
+                let t = t, at = at; _all(i->getfield(at.val::Tuple, i) isa fieldtype(t, i), 1:n); end
                 t = Const(ccall(:jl_new_structt, Any, (Any, Any), t, at.val))
             elseif isa(at, PartialStruct) && at ⊑ Tuple && n == length(at.fields::Vector{Any}) &&
                 let t = t, at = at; _all(i->(at.fields::Vector{Any})[i] ⊑ fieldtype(t, i), 1:n); end
@@ -1718,6 +1717,7 @@ function abstract_eval_statement(interp::AbstractInterpreter, @nospecialize(e),
     else
         t = abstract_eval_value_expr(interp, e, vtypes, sv)
     end
+    @label t_computed
     @assert !isa(t, TypeVar) "unhandled TypeVar"
     if isa(t, DataType) && isdefined(t, :instance)
         # replace singleton types with their equivalent Const object
@@ -1801,17 +1801,18 @@ function widenreturn(@nospecialize(rt), @nospecialize(bestguess), nslots::Int, s
     isa(rt, Type) && return rt
     if isa(rt, PartialStruct)
         fields = copy(rt.fields)
-        haveconst = false
+        local anyrefine = false
         for i in 1:length(fields)
             a = fields[i]
             a = isvarargtype(a) ? a : widenreturn(a, bestguess, nslots, slottypes, changes)
-            if !haveconst && has_const_info(a)
+            if !anyrefine
                 # TODO: consider adding && const_prop_profitable(a) here?
-                haveconst = true
+                anyrefine = has_const_info(a) ||
+                            a ⊏ fieldtype(rt.typ, i)
             end
             fields[i] = a
         end
-        haveconst && return PartialStruct(rt.typ, fields)
+        anyrefine && return PartialStruct(rt.typ, fields)
     end
     if isa(rt, PartialOpaque)
         return rt # XXX: this case was missed in #39512

base/compiler/typelattice.jl

@@ -140,7 +140,12 @@ function maybe_extract_const_bool(c::AnyConditional)
 end
 maybe_extract_const_bool(@nospecialize c) = nothing
 
-function ⊑(@nospecialize(a), @nospecialize(b))
+"""
+    a ⊑ b -> Bool
+
+The non-strict partial order over the type inference lattice.
+"""
+@nospecialize(a) ⊑ @nospecialize(b) = begin
     if isa(b, LimitedAccuracy)
         if !isa(a, LimitedAccuracy)
             return false
@@ -232,6 +237,22 @@ function ⊑(@nospecialize(a), @nospecialize(b))
     end
 end
 
+"""
+    a ⊏ b -> Bool
+
+The strict partial order over the type inference lattice.
+This is defined as the irreflexive kernel of `⊑`.
+"""
+@nospecialize(a) ⊏ @nospecialize(b) = a ⊑ b && !⊑(b, a)
+
+"""
+    a ⋤ b -> Bool
+
+This order could be used as a slightly more efficient version of the strict order `⊏`,
+where we can safely assume `a ⊑ b` holds.
+"""
+@nospecialize(a) ⋤ @nospecialize(b) = !⊑(b, a)
+
 # Check if two lattice elements are partial order equivalent. This is basically
 # `a ⊑ b && b ⊑ a` but with extra performance optimizations.
 function is_lattice_equal(@nospecialize(a), @nospecialize(b))

test/compiler/inference.jl

@@ -3669,3 +3669,26 @@ end
 
 # issue #42646
 @test only(Base.return_types(getindex, (Array{undef}, Int))) >: Union{} # check that it does not throw
+
+# form PartialStruct for extra type information propagation
+struct FieldTypeRefinement{S,T}
+    s::S
+    t::T
+end
+@test Base.return_types((Int,)) do s
+    o = FieldTypeRefinement{Any,Int}(s, s)
+    o.s
+end |> only == Int
+@test Base.return_types((Int,)) do s
+    o = FieldTypeRefinement{Int,Any}(s, s)
+    o.t
+end |> only == Int
+@test Base.return_types((Int,)) do s
+    o = FieldTypeRefinement{Any,Any}(s, s)
+    o.s, o.t
+end |> only == Tuple{Int,Int}
+@test Base.return_types((Int,)) do a
+    s1 = Some{Any}(a)
+    s2 = Some{Any}(s1)
+    s2.value.value
+end |> only == Int

test/compiler/irpasses.jl

@@ -426,31 +426,22 @@ let # `getfield_elim_pass!` should work with constant globals
     end
 end
 
-let # `typeassert_elim_pass!`
+let
+    # `typeassert` elimination after SROA
+    # NOTE we can remove this optimization once inference is able to reason about memory-effects
     src = @eval Module() begin
-        struct Foo; x; end
+        mutable struct Foo; x; end
 
         code_typed((Int,)) do a
             x1 = Foo(a)
             x2 = Foo(x1)
-            x3 = Foo(x2)
-
-            r1 = (x2.x::Foo).x
-            r2 = (x2.x::Foo).x::Int
-            r3 = (x2.x::Foo).x::Integer
-            r4 = ((x3.x::Foo).x::Foo).x
-
-            return r1, r2, r3, r4
+            return typeassert(x2.x, Foo).x
         end |> only |> first
     end
-    # eliminate `typeassert(f2.a, Foo)`
-    @test all(src.code) do @nospecialize(stmt)
+    # eliminate `typeassert(x2.x, Foo)`
+    @test all(src.code) do @nospecialize stmt
         Meta.isexpr(stmt, :call) || return true
         ft = Core.Compiler.argextype(stmt.args[1], src, Any[], src.slottypes)
         return Core.Compiler.widenconst(ft) !== typeof(typeassert)
     end
-    # succeeding simple DCE will eliminate `Foo(a)`
-    @test all(src.code) do @nospecialize(stmt)
-        return !Meta.isexpr(stmt, :new)
-    end
 end