cfg_simplify: Correctly handle must-throw blocks (JuliaLang#54216)

We have a special exception in our IR that allows `::Union{}` marked
instructions to act as an `unreachable` terminator.

Without this fix, we were advancing into the subsequent block despite it
not being a successor.

base/compiler/ssair/passes.jl

@@ -2274,6 +2274,15 @@ function cfg_simplify!(ir::IRCode)
             elseif merge_into[idx] == 0 && is_bb_empty(ir, bb) && is_legal_bb_drop(ir, idx, bb)
                 # If this BB is empty, we can still merge it as long as none of our successor's phi nodes
                 # reference our predecessors.
+                #
+                # This is for situations like:
+                #   #1 - ...
+                #        goto #3 if not ...
+                #   #2 - (empty)
+                #   #3 - ϕ(#2 => true, #1 => false)
+                #
+                # where we rely on the empty basic block to disambiguate the ϕ-node's value
+
                 found_interference = false
                 preds = Int[ascend_eliminated_preds(bbs, pred) for pred in bb.preds]
                 for idx in bbs[succ].stmts
@@ -2331,8 +2340,11 @@ function cfg_simplify!(ir::IRCode)
                     end
                     curr = merged_succ[curr]
                 end
-                terminator = ir[SSAValue(ir.cfg.blocks[curr].stmts[end])][:stmt]
-                if isa(terminator, GotoNode) || isa(terminator, ReturnNode)
+                terminator = ir[SSAValue(bbs[curr].stmts[end])][:stmt]
+                is_throw = ir[SSAValue(bbs[curr].stmts[end])][:type] === Union{} && !isa(terminator, PhiNode)
+                if isa(terminator, GotoNode) || isa(terminator, ReturnNode) || is_throw
+                    # Only advance to next block if it's a successor
+                    # (avoid GotoNode, ReturnNode, throw()/Union{})
                     break
                 elseif isa(terminator, GotoIfNot)
                     if bb_rename_succ[terminator.dest] == 0

test/compiler/irpasses.jl

@@ -1881,3 +1881,33 @@ end
 @test !fully_eliminated() do
     all(iszero, Iterators.repeated(0))
 end
+
+## Test that cfg_simplify respects implicit `unreachable` terminators
+let code = Any[
+        # block 1
+        GotoIfNot(Core.Argument(2), 4),
+        # block 2
+        Expr(:call, Base.throw, "error"), # an implicit `unreachable` terminator
+        # block 3
+        Expr(:call, :opaque),
+        # block 4
+        ReturnNode(nothing),
+    ]
+    ir = make_ircode(code; ssavaluetypes=Any[Union{}, Union{}, Any, Union{}])
+
+    # Unfortunately `compute_basic_blocks` does not notice the `throw()` so it gives us
+    # a slightly imprecise CFG. Instead manually construct the CFG we need for this test:
+    empty!(ir.cfg.blocks)
+    push!(ir.cfg.blocks, BasicBlock(StmtRange(1,1), [], [2,4]))
+    push!(ir.cfg.blocks, BasicBlock(StmtRange(2,2), [1], []))
+    push!(ir.cfg.blocks, BasicBlock(StmtRange(3,3), [], []))
+    push!(ir.cfg.blocks, BasicBlock(StmtRange(4,4), [1], []))
+    empty!(ir.cfg.index)
+    append!(ir.cfg.index, Int[2,3,4])
+    ir.stmts.stmt[1] = GotoIfNot(Core.Argument(2), 4)
+
+    Core.Compiler.verify_ir(ir)
+    ir = Core.Compiler.cfg_simplify!(ir)
+    Core.Compiler.verify_ir(ir)
+    @test length(ir.cfg.blocks) == 3 # should have removed block 3
+end