optimizer: inline abstract union-split callsite

Currently the optimizer handles abstract callsite only when there is a
single dispatch candidate (in most cases), and so inlining and static-dispatch
are prohibited when the callsite is union-split (in other word, union-split
happens only when all the dispatch candidates are concrete).

However, there are certain patterns of code (most notably our Julia-level compiler code)
that inherently need to deal with abstract callsite.
The following example is taken from `Core.Compiler` utility:
```julia
julia> @inline isType(@nospecialize t) = isa(t, DataType) && t.name === Type.body.name
isType (generic function with 1 method)

julia> code_typed((Any,)) do x # abstract, but no union-split, successful inlining
           isType(x)
       end |> only
CodeInfo(
1 ─ %1 = (x isa Main.DataType)::Bool
└──      goto #3 if not %1
2 ─ %3 = π (x, DataType)
│   %4 = Base.getfield(%3, :name)::Core.TypeName
│   %5 = Base.getfield(Type{T}, :name)::Core.TypeName
│   %6 = (%4 === %5)::Bool
└──      goto #4
3 ─      goto #4
4 ┄ %9 = φ (#2 => %6, #3 => false)::Bool
└──      return %9
) => Bool

julia> code_typed((Union{Type,Nothing},)) do x # abstract, union-split, unsuccessful inlining
           isType(x)
       end |> only
CodeInfo(
1 ─ %1 = (isa)(x, Nothing)::Bool
└──      goto #3 if not %1
2 ─      goto #4
3 ─ %4 = Main.isType(x)::Bool
└──      goto #4
4 ┄ %6 = φ (#2 => false, #3 => %4)::Bool
└──      return %6
) => Bool
```
(note that this is a limitation of the inlining algorithm, and so any
user-provided hints like callsite inlining annotation doesn't help here)

This commit enables inlining and static dispatch for abstract union-split callsite.
The core idea here is that we can simulate our dispatch semantics by
generating `isa` checks in order of the specialities of dispatch candidates:
```julia
julia> code_typed((Union{Type,Nothing},)) do x # union-split, unsuccessful inlining
                  isType(x)
              end |> only
CodeInfo(
1 ─ %1  = (isa)(x, Nothing)::Bool
└──       goto #3 if not %1
2 ─       goto #9
3 ─ %4  = (isa)(x, Type)::Bool
└──       goto #8 if not %4
4 ─ %6  = π (x, Type)
│   %7  = (%6 isa Main.DataType)::Bool
└──       goto #6 if not %7
5 ─ %9  = π (%6, DataType)
│   %10 = Base.getfield(%9, :name)::Core.TypeName
│   %11 = Base.getfield(Type{T}, :name)::Core.TypeName
│   %12 = (%10 === %11)::Bool
└──       goto #7
6 ─       goto #7
7 ┄ %15 = φ (#5 => %12, #6 => false)::Bool
└──       goto #9
8 ─       Core.throw(ErrorException("fatal error in type inference (type bound)"))::Union{}
└──       unreachable
9 ┄ %19 = φ (#2 => false, #7 => %15)::Bool
└──       return %19
) => Bool
```

Inlining/static-dispatch of abstract union-split callsite will improve
the performance in such situations (and so this commit will improve the
latency of our JIT compilation). Especially, this commit helps us avoid
excessive specializations of `Core.Compiler` code by statically-resolving
`@nospecialize`d callsites, and as the result, the # of precompiled
statements is now reduced from  `1956` ([`master`](dc45d77)) to `1901` (this commit).

And also, as a side effect, the implementation of our inlining algorithm
gets much simplified now since we no longer need the previous special
handlings for abstract callsites.

One possible drawback would be increased code size.
This change seems to certainly increase the size of sysimage,
but I think these numbers are in an acceptable range:
> [`master`](dc45d77)
```
❯ du -sh usr/lib/julia/*
 17M    usr/lib/julia/corecompiler.ji
188M    usr/lib/julia/sys-o.a
164M    usr/lib/julia/sys.dylib
 23M    usr/lib/julia/sys.dylib.dSYM
101M    usr/lib/julia/sys.ji
```

> this commit
```
❯ du -sh usr/lib/julia/*
 17M    usr/lib/julia/corecompiler.ji
190M    usr/lib/julia/sys-o.a
166M    usr/lib/julia/sys.dylib
 23M    usr/lib/julia/sys.dylib.dSYM
102M    usr/lib/julia/sys.ji
```

Author: aviatesk

base/compiler/ssair/inlining.jl

@@ -241,7 +241,7 @@ function cfg_inline_unionsplit!(ir::IRCode, idx::Int,
         push!(from_bbs, length(state.new_cfg_blocks))
         # TODO: Right now we unconditionally generate a fallback block
         # in case of subtyping errors - This is probably unnecessary.
-        if i != length(cases) || (!fully_covered || (!params.trust_inference && isdispatchtuple(cases[i].sig)))
+        if i != length(cases) || (!fully_covered || (!params.trust_inference))
             # This block will have the next condition or the final else case
             push!(state.new_cfg_blocks, BasicBlock(StmtRange(idx, idx)))
             push!(state.new_cfg_blocks[cond_bb].succs, length(state.new_cfg_blocks))
@@ -313,7 +313,6 @@ function ir_inline_item!(compact::IncrementalCompact, idx::Int, argexprs::Vector
     spec = item.spec::ResolvedInliningSpec
     sparam_vals = item.mi.sparam_vals
     def = item.mi.def::Method
-    inline_cfg = spec.ir.cfg
     linetable_offset::Int32 = length(linetable)
     # Append the linetable of the inlined function to our line table
     inlined_at = Int(compact.result[idx][:line])
@@ -459,6 +458,47 @@ end
 
 const FATAL_TYPE_BOUND_ERROR = ErrorException("fatal error in type inference (type bound)")
 
+"""
+    ir_inline_unionsplit!
+
+The core idea of this function is to simulate the dispatch semantics by generating
+(flat) `isa`-checks corresponding to the signatures of union-split dispatch candidates,
+and then inline their bodies into each `isa`-conditional block.
+
+This `isa`-based virtual dispatch requires some pre-conditions to hold in order to simulate
+the actual semantics correctly.
+
+The first one is that these dispatch candidates need to be processed in order of their specificity,
+and the corresponding `isa`-checks should reflect the method specificities, since now their
+signatures are not necessarily concrete.
+Fortunately, `ml_matches` should already sorted them in that way, except cases when there is
+any ambiguity, from which we already bail out at this point.
+
+Another consideration is type equality constraint from type variables: the `isa`-checks are
+not enough to simulate the dispatch semantics in cases like:
+
+Given a definition:
+
+    f(x::T, y::T) where T<:Integer = ...
+
+Transform a callsite:
+
+    (x::Any, y::Any)
+
+Into the optimized form:
+
+    if isa(x, Integer) && isa(y, Integer)
+        f(x::Integer, y::Integer)
+    else
+        f(x::Integer, y::Integer)
+    end
+
+But again, we should already bail out from such cases at this point, essentially by
+excluding cases where `case.sig::UnionAll`.
+
+In short, here we can process the dispatch candidates in order, assuming we haven't changed
+their order somehow somewhere up to this point.
+"""
 function ir_inline_unionsplit!(compact::IncrementalCompact, idx::Int,
                                argexprs::Vector{Any}, linetable::Vector{LineInfoNode},
                                (; fully_covered, atype, cases, bbs)::UnionSplit,
@@ -468,17 +508,17 @@ function ir_inline_unionsplit!(compact::IncrementalCompact, idx::Int,
     join_bb = bbs[end]
     pn = PhiNode()
     local bb = compact.active_result_bb
-    @assert length(bbs) >= length(cases)
-    for i in 1:length(cases)
+    ncases = length(cases)
+    @assert length(bbs) >= ncases
+    for i = 1:ncases
         ithcase = cases[i]
         mtype = ithcase.sig::DataType # checked within `handle_cases!`
         case = ithcase.item
         next_cond_bb = bbs[i]
         cond = true
         nparams = fieldcount(atype)
         @assert nparams == fieldcount(mtype)
-        if i != length(cases) || !fully_covered ||
-                (!params.trust_inference && isdispatchtuple(cases[i].sig))
+        if i != ncases || !fully_covered || !params.trust_inference
             for i = 1:nparams
                 a, m = fieldtype(atype, i), fieldtype(mtype, i)
                 # If this is always true, we don't need to check for it
@@ -535,7 +575,7 @@ function ir_inline_unionsplit!(compact::IncrementalCompact, idx::Int,
     bb += 1
     # We're now in the fall through block, decide what to do
     if fully_covered
-        if !params.trust_inference && isdispatchtuple(cases[end].sig)
+        if !params.trust_inference
             e = Expr(:call, GlobalRef(Core, :throw), FATAL_TYPE_BOUND_ERROR)
             insert_node_here!(compact, NewInstruction(e, Union{}, line))
             insert_node_here!(compact, NewInstruction(ReturnNode(), Union{}, line))
@@ -558,7 +598,7 @@ function batch_inline!(todo::Vector{Pair{Int, Any}}, ir::IRCode, linetable::Vect
     state = CFGInliningState(ir)
     for (idx, item) in todo
         if isa(item, UnionSplit)
-            cfg_inline_unionsplit!(ir, idx, item::UnionSplit, state, params)
+            cfg_inline_unionsplit!(ir, idx, item, state, params)
         else
             item = item::InliningTodo
             spec = item.spec::ResolvedInliningSpec
@@ -1172,12 +1212,8 @@ function analyze_single_call!(
     sig::Signature, state::InliningState, todo::Vector{Pair{Int, Any}})
     argtypes = sig.argtypes
     cases = InliningCase[]
-    local only_method = nothing  # keep track of whether there is one matching method
-    local meth::MethodLookupResult
+    local any_fully_covered = false
     local handled_all_cases = true
-    local any_covers_full = false
-    local revisit_idx = nothing
-
     for i in 1:length(infos)
         meth = infos[i].results
         if meth.ambig
@@ -1188,66 +1224,20 @@ function analyze_single_call!(
             # No applicable methods; try next union split
             handled_all_cases = false
             continue
-        else
-            if length(meth) == 1 && only_method !== false
-                if only_method === nothing
-                    only_method = meth[1].method
-                elseif only_method !== meth[1].method
-                    only_method = false
-                end
-            else
-                only_method = false
-            end
         end
-        for (j, match) in enumerate(meth)
-            any_covers_full |= match.fully_covers
-            if !isdispatchtuple(match.spec_types)
-                if !match.fully_covers
-                    handled_all_cases = false
-                    continue
-                end
-                if revisit_idx === nothing
-                    revisit_idx = (i, j)
-                else
-                    handled_all_cases = false
-                    revisit_idx = nothing
-                end
-            else
-                handled_all_cases &= handle_match!(match, argtypes, flag, state, cases)
-            end
+        for match in meth
+            handled_all_cases &= handle_match!(match, argtypes, flag, state, cases, true)
+            any_fully_covered |= match.fully_covers
         end
     end
 
-    atype = argtypes_to_type(argtypes)
-    if handled_all_cases && revisit_idx !== nothing
-        # If there's only one case that's not a dispatchtuple, we can
-        # still unionsplit by visiting all the other cases first.
-        # This is useful for code like:
-        #   foo(x::Int) = 1
-        #   foo(@nospecialize(x::Any)) = 2
-        # where we where only a small number of specific dispatchable
-        # cases are split off from an ::Any typed fallback.
-        (i, j) = revisit_idx
-        match = infos[i].results[j]
-        handled_all_cases &= handle_match!(match, argtypes, flag, state, cases, true)
-    elseif length(cases) == 0 && only_method isa Method
-        # if the signature is fully covered and there is only one applicable method,
-        # we can try to inline it even if the signature is not a dispatch tuple.
-        # -- But don't try it if we already tried to handle the match in the revisit_idx
-        # case, because that'll (necessarily) be the same method.
-        if length(infos) > 1
-            (metharg, methsp) = ccall(:jl_type_intersection_with_env, Any, (Any, Any),
-                atype, only_method.sig)::SimpleVector
-            match = MethodMatch(metharg, methsp::SimpleVector, only_method, true)
-        else
-            @assert length(meth) == 1
-            match = meth[1]
-        end
-        handle_match!(match, argtypes, flag, state, cases, true) || return nothing
-        any_covers_full = handled_all_cases = match.fully_covers
+    if !handled_all_cases
+        # if we've not seen all candidates, union split is valid only for dispatch tuples
+        filter!(case::InliningCase->isdispatchtuple(case.sig), cases)
     end
 
-    handle_cases!(ir, idx, stmt, atype, cases, any_covers_full && handled_all_cases, todo, state.params)
+    handle_cases!(ir, idx, stmt, argtypes_to_type(argtypes), cases,
+        handled_all_cases & any_fully_covered, todo, state.params)
 end
 
 # similar to `analyze_single_call!`, but with constant results
@@ -1258,8 +1248,8 @@ function handle_const_call!(
     (; call, results) = cinfo
     infos = isa(call, MethodMatchInfo) ? MethodMatchInfo[call] : call.matches
     cases = InliningCase[]
+    local any_fully_covered = false
     local handled_all_cases = true
-    local any_covers_full = false
     local j = 0
     for i in 1:length(infos)
         meth = infos[i].results
@@ -1275,42 +1265,39 @@ function handle_const_call!(
         for match in meth
             j += 1
             result = results[j]
-            any_covers_full |= match.fully_covers
+            any_fully_covered |= match.fully_covers
             if isa(result, ConstResult)
                 case = const_result_item(result, state)
                 push!(cases, InliningCase(result.mi.specTypes, case))
             elseif isa(result, InferenceResult)
-                handled_all_cases &= handle_inf_result!(result, argtypes, flag, state, cases)
+                handled_all_cases &= handle_inf_result!(result, argtypes, flag, state, cases, true)
             else
                 @assert result === nothing
-                handled_all_cases &= handle_match!(match, argtypes, flag, state, cases)
+                handled_all_cases &= handle_match!(match, argtypes, flag, state, cases, true)
             end
         end
     end
 
-    # if the signature is fully covered and there is only one applicable method,
-    # we can try to inline it even if the signature is not a dispatch tuple
-    atype = argtypes_to_type(argtypes)
-    if length(cases) == 0
-        length(results) == 1 || return nothing
-        result = results[1]
-        isa(result, InferenceResult) || return nothing
-        handle_inf_result!(result, argtypes, flag, state, cases, true) || return nothing
-        spec_types = cases[1].sig
-        any_covers_full = handled_all_cases = atype <: spec_types
+    if !handled_all_cases
+        # if we've not seen all candidates, union split is valid only for dispatch tuples
+        filter!(case::InliningCase->isdispatchtuple(case.sig), cases)
     end
 
-    handle_cases!(ir, idx, stmt, atype, cases, any_covers_full && handled_all_cases, todo, state.params)
+    handle_cases!(ir, idx, stmt, argtypes_to_type(argtypes), cases,
+        handled_all_cases & any_fully_covered, todo, state.params)
 end
 
 function handle_match!(
     match::MethodMatch, argtypes::Vector{Any}, flag::UInt8, state::InliningState,
     cases::Vector{InliningCase}, allow_abstract::Bool = false)
     spec_types = match.spec_types
     allow_abstract || isdispatchtuple(spec_types) || return false
+    # we may see duplicated dispatch signatures here when a signature gets widened
+    # during abstract interpretation: for the purpose of inlining, we can just skip
+    # processing this dispatch candidate
+    _any(case->case.sig === spec_types, cases) && return true
     item = analyze_method!(match, argtypes, flag, state)
     item === nothing && return false
-    _any(case->case.sig === spec_types, cases) && return true
     push!(cases, InliningCase(spec_types, item))
     return true
 end
@@ -1346,7 +1333,9 @@ function handle_cases!(ir::IRCode, idx::Int, stmt::Expr, @nospecialize(atype),
         handle_single_case!(ir, idx, stmt, cases[1].item, todo, params)
     elseif length(cases) > 0
         isa(atype, DataType) || return nothing
-        all(case::InliningCase->isa(case.sig, DataType), cases) || return nothing
+        for case in cases
+            isa(case.sig, DataType) || return nothing
+        end
         push!(todo, idx=>UnionSplit(fully_covered, atype, cases))
     end
     return nothing
@@ -1442,7 +1431,8 @@ function assemble_inline_todo!(ir::IRCode, state::InliningState)
 
         analyze_single_call!(ir, idx, stmt, infos, flag, sig, state, todo)
     end
-    todo
+
+    return todo
 end
 
 function linear_inline_eligible(ir::IRCode)

base/sort.jl

@@ -5,7 +5,7 @@ module Sort
 import ..@__MODULE__, ..parentmodule
 const Base = parentmodule(@__MODULE__)
 using .Base.Order
-using .Base: copymutable, LinearIndices, length, (:),
+using .Base: copymutable, LinearIndices, length, (:), iterate,
     eachindex, axes, first, last, similar, zip, OrdinalRange,
     AbstractVector, @inbounds, AbstractRange, @eval, @inline, Vector, @noinline,
     AbstractMatrix, AbstractUnitRange, isless, identity, eltype, >, <, <=, >=, |, +, -, *, !,