optimize: Delete incoming unreachable edges from PhiNode (#53877)

Incoming IR very rarely contains PhiNodes, but we do allow it to make
things easier on downstream packages like Diffractor that want to
generate the same code structures in both typed and untyped mode.
However, this does of course mean that once inference is finished, any
PhiNodes in the original source must be adjusted to maintain IRCode
invariants. One particular important invariant here is that the edges
list in a PhiNode must match the predecessor list, so in particular if a
predecessor becomes unreachable during inference, we must filter that
edge before passing it on to the optimizer.

---------

Co-authored-by: Shuhei Kadowaki <[email protected]>

base/compiler/optimize.jl

@@ -1104,6 +1104,20 @@ function convert_to_ircode(ci::CodeInfo, sv::OptimizationState)
                         code[i] = nothing
                     end
                 end
+            elseif isa(expr, PhiNode)
+                new_edges = Int32[]
+                new_vals = Any[]
+                for j = 1:length(expr.edges)
+                    edge = expr.edges[j]
+                    (edge in sv.unreachable || (ssavaluetypes[edge] === Union{} && !isa(code[edge], PhiNode))) && continue
+                    push!(new_edges, edge)
+                    if isassigned(expr.values, j)
+                        push!(new_vals, expr.values[j])
+                    else
+                        resize!(new_vals, length(new_edges))
+                    end
+                end
+                code[i] = PhiNode(new_edges, new_vals)
             end
         end
     end

test/compiler/ssair.jl

@@ -9,16 +9,6 @@ include("irutils.jl")
 
 make_bb(preds, succs) = BasicBlock(Compiler.StmtRange(0, 0), preds, succs)
 
-function make_ci(code)
-    ci = (Meta.@lower 1 + 1).args[1]
-    ci.code = code
-    nstmts = length(ci.code)
-    ci.ssavaluetypes = nstmts
-    ci.codelocs = fill(Int32(1), nstmts)
-    ci.ssaflags = fill(Int32(0), nstmts)
-    return ci
-end
-
 # TODO: this test is broken
 #let code = Any[
 #        GotoIfNot(SlotNumber(2), 4),
@@ -763,3 +753,75 @@ end
         end
     end
 end
+
+# Test that things don't break if one branch of the frontend PhiNode becomes unreachable
+const global_error_switch_const1::Bool = false
+function gen_unreachable_phinode_edge1(world::UInt, source, args...)
+    ci = make_codeinfo(Any[
+        # block 1
+        GlobalRef(@__MODULE__, :global_error_switch_const1),
+        GotoIfNot(SSAValue(1), 4),
+        # block 2
+        Expr(:call, identity, Argument(3)),
+        # block 3
+        PhiNode(Int32[2, 3], Any[Argument(2), SSAValue(3)]),
+        ReturnNode(SSAValue(4))
+    ]; slottypes=Any[Any,Int,Int])
+    ci.slotnames = Symbol[:var"#self#", :x, :y]
+    return ci
+end
+@eval function f_unreachable_phinode_edge1(x, y)
+    $(Expr(:meta, :generated, gen_unreachable_phinode_edge1))
+    $(Expr(:meta, :generated_only))
+    #= no body =#
+end
+@test f_unreachable_phinode_edge1(1, 2) == 1
+
+const global_error_switch_const2::Bool = true
+function gen_unreachable_phinode_edge2(world::UInt, source, args...)
+    ci = make_codeinfo(Any[
+        # block 1
+        GlobalRef(@__MODULE__, :global_error_switch_const2),
+        GotoIfNot(SSAValue(1), 4),
+        # block 2
+        Expr(:call, identity, Argument(3)),
+        # block 3
+        PhiNode(Int32[2, 3], Any[Argument(2), SSAValue(3)]),
+        ReturnNode(SSAValue(4))
+    ]; slottypes=Any[Any,Int,Int])
+    ci.slotnames = Symbol[:var"#self#", :x, :y]
+    return ci
+end
+@eval function f_unreachable_phinode_edge2(x, y)
+    $(Expr(:meta, :generated, gen_unreachable_phinode_edge2))
+    $(Expr(:meta, :generated_only))
+    #= no body =#
+end
+@test f_unreachable_phinode_edge2(1, 2) == 2
+
+global global_error_switch::Bool = true
+function gen_must_throw_phinode_edge(world::UInt, source, _)
+    ci = make_codeinfo(Any[
+        # block 1
+        GlobalRef(@__MODULE__, :global_error_switch),
+        GotoIfNot(SSAValue(1), 4),
+        # block 2
+        Expr(:call, error, "This error is expected"),
+        # block 3
+        PhiNode(Int32[2, 3], Any[1, 2]),
+        ReturnNode(SSAValue(4))
+    ]; slottypes=Any[Any])
+    ci.slotnames = Symbol[:var"#self#"]
+    return ci
+end
+@eval function f_must_throw_phinode_edge()
+    $(Expr(:meta, :generated, gen_must_throw_phinode_edge))
+    $(Expr(:meta, :generated_only))
+    #= no body =#
+end
+let ir = first(only(Base.code_ircode(f_must_throw_phinode_edge)))
+    @test !any(@nospecialize(x)->isa(x,PhiNode), ir.stmts.stmt)
+end
+@test_throws ErrorException f_must_throw_phinode_edge()
+global global_error_switch = false
+@test f_must_throw_phinode_edge() == 1