inlining: make union splitting account for uncovered call

#44421 changed the union-splitting to skip generating
unnecessary fallback dynamic dispatch call when there is any fully
covered call.
But it turned out that this is only valid when there is any fully
covered call in matches for all signatures that inference split, and it
is invalid if there is any union split signature against which any
uncovered call is found.

Consider the following example:
    # case 1
        # def
        nosplit(::Any) = [...]
        nosplit(::Int) = [...]
        # call
        nosplit(a::Any)
            split1: a::Any ┬ nosplit(a::Int)
                           └ nosplit(a::Any) # fully covers split1

    # case 2
        # def
        convert(::Type{T}, ::T) = T
        # call
        convert(::Type{Union{Bool,Tuple{Int,String}}}, a::Union{Bool,Tuple{Int,Any}})
            split1: a::Bool           ─ convert(::Type{Bool}, ::Bool)                           # fully covers split1
            split2: a::Tuple{Int,Any} ─ convert(::Type{Tuple{Int,String}}, ::Tuple{Int,String}) # NOT fully covers split2

#44421 allows us to optimize the the first case, but
handles the second case wrongly. This commit fixes it up while still
optimizing the first case.

fix #48397.

Author: aviatesk

base/compiler/ssair/inlining.jl

@@ -1335,14 +1335,13 @@ function compute_inlining_cases(@nospecialize(info::CallInfo), flag::UInt8, sig:
     nunion === nothing && return nothing
     cases = InliningCase[]
     argtypes = sig.argtypes
-    local any_fully_covered = false
     local handled_all_cases::Bool = true
     local revisit_idx = nothing
     local only_method = nothing
     local meth::MethodLookupResult
     local all_result_count = 0
     local joint_effects::Effects = EFFECTS_TOTAL
-    local nothrow::Bool = true
+    local fully_covered::Bool = true
     for i = 1:nunion
         meth = getsplit(info, i)
         if meth.ambig
@@ -1364,12 +1363,12 @@ function compute_inlining_cases(@nospecialize(info::CallInfo), flag::UInt8, sig:
                 only_method = false
             end
         end
+        local split_fully_covered::Bool = false
         for (j, match) in enumerate(meth)
             all_result_count += 1
             result = getresult(info, all_result_count)
             joint_effects = merge_effects(joint_effects, info_effects(result, match, state))
-            nothrow &= match.fully_covers
-            any_fully_covered |= match.fully_covers
+            split_fully_covered |= match.fully_covers
             if !validate_sparams(match.sparams)
                 if !match.fully_covers
                     handled_all_cases = false
@@ -1386,9 +1385,10 @@ function compute_inlining_cases(@nospecialize(info::CallInfo), flag::UInt8, sig:
                     result, match, argtypes, info, flag, state; allow_abstract=true, allow_typevars=false)
             end
         end
+        fully_covered &= split_fully_covered
     end
 
-    joint_effects = Effects(joint_effects; nothrow)
+    joint_effects = Effects(joint_effects; nothrow=fully_covered)
 
     if handled_all_cases && revisit_idx !== nothing
         # we handled everything except one match with unmatched sparams,
@@ -1415,13 +1415,13 @@ function compute_inlining_cases(@nospecialize(info::CallInfo), flag::UInt8, sig:
         end
         handle_any_const_result!(cases,
             result, match, argtypes, info, flag, state; allow_abstract=true, allow_typevars=true)
-        any_fully_covered = handled_all_cases = match.fully_covers
+        fully_covered = handled_all_cases = match.fully_covers
     elseif !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
 
-    return cases, (handled_all_cases & any_fully_covered), joint_effects
+    return cases, (handled_all_cases & fully_covered), joint_effects
 end
 
 function handle_call!(todo::Vector{Pair{Int,Any}},

test/compiler/inline.jl

@@ -1910,3 +1910,22 @@ function elim_full_ir(y)
 end
 
 @test fully_eliminated(elim_full_ir, Tuple{Int})
+
+# union splitting should account for uncovered call signature
+# https://github.com/JuliaLang/julia/issues/48397
+f48397(::Bool) = :ok
+f48397(::Tuple{String,String}) = :ok
+let src = code_typed1((Union{Bool,Tuple{String,Any}},)) do x
+        f48397(x)
+    end
+    @test any(iscall((src, f48397)), src.code)
+end
+g48397::Union{Bool,Tuple{String,Any}} = ("48397", 48397)
+@test_throws MethodError let
+    Base.Experimental.@force_compile
+    f48397(g48397)
+end
+@test_throws MethodError let
+    Base.Experimental.@force_compile
+    convert(Union{Bool,Tuple{String,String}}, g48397)
+end