Add nothrow modeling for global assignment

Currently global assignment conservatively taints nothrow.
We can do better by looking at whether the global exists,
isconst, its type, etc. and determine whether there is
any possibility that the assignment will throw and taint
the effect accordingly.

base/compiler/abstractinterpretation.jl

@@ -2089,6 +2089,19 @@ function abstract_eval_global(M::Module, s::Symbol, frame::InferenceState)
     return ty
 end
 
+function abstract_eval_global_assignment(interp::AbstractInterpreter, frame::InferenceState, lhs::GlobalRef, @nospecialize(rhs))
+    M = lhs.mod
+    s = lhs.name
+    nothrow = false
+    if isdefined(M, s) && !isconst(M, s)
+        ty = ccall(:jl_binding_type, Any, (Any, Any), M, s)
+        nothrow = ty === nothing || rhs ⊑ ty
+    end
+    tristate_merge!(frame, Effects(EFFECTS_TOTAL,
+        effect_free=TRISTATE_UNKNOWN,
+        nothrow=nothrow ? ALWAYS_TRUE : TRISTATE_UNKNOWN))
+end
+
 abstract_eval_ssavalue(s::SSAValue, sv::InferenceState) = abstract_eval_ssavalue(s, sv.src)
 function abstract_eval_ssavalue(s::SSAValue, src::CodeInfo)
     typ = (src.ssavaluetypes::Vector{Any})[s.id]
@@ -2321,9 +2334,7 @@ function typeinf_local(interp::AbstractInterpreter, frame::InferenceState)
                 if isa(lhs, SlotNumber)
                     changes = StateUpdate(lhs, VarState(t, false), changes, false)
                 elseif isa(lhs, GlobalRef)
-                    tristate_merge!(frame, Effects(EFFECTS_TOTAL,
-                        effect_free=TRISTATE_UNKNOWN,
-                        nothrow=TRISTATE_UNKNOWN))
+                    abstract_eval_global_assignment(interp, frame, lhs, t)
                 elseif !isa(lhs, SSAValue)
                     tristate_merge!(frame, EFFECTS_UNKNOWN)
                 end

test/compiler/inference.jl

@@ -4133,3 +4133,11 @@ end |> !Core.Compiler.is_concrete_eval_eligible
 @test !fully_eliminated() do
     entry_to_be_invalidated('a')
 end
+
+# Nothrow for assignment to globals
+global glob_assign_int::Int = 0
+f_glob_assign_int() = global glob_assign_int += 1
+let effects = Base.infer_effects(f_glob_assign_int, ())
+    @test !Core.Compiler.is_effect_free(effects)
+    @test Core.Compiler.is_nothrow(effects)
+end