Emit traps for unreachable nodes (JuliaLang#41465)

Raw `unreachable` in LLVM is somewhat dangerous since LLVM will literally
stop emitting code there, so if the unreachable is wrong, execution will
crash into whatever subsequent code exists, causing strange and mysterious
errors and incorrect backtraces that are hard to debug without rr. As a
result, we basically always emit a safety trap call before an `unreachable`
terminator, which will abort execution at the point where unreachable was
executed and at least provide proper backtraces. However, we neglected
to do this for literal unreachables that came from Julia IR (i.e. ReturnNodes
with undef val field). Fix that to make debugging easier if unreachable
ever gets accidentally executed.

src/codegen.cpp

@@ -1157,7 +1157,7 @@ static void allocate_gc_frame(jl_codectx_t &ctx, BasicBlock *b0);
 static Value *get_current_task(jl_codectx_t &ctx);
 static Value *get_current_ptls(jl_codectx_t &ctx);
 static Value *get_current_signal_page(jl_codectx_t &ctx);
-static void CreateTrap(IRBuilder<> &irbuilder);
+static void CreateTrap(IRBuilder<> &irbuilder, bool create_new_block = true);
 static CallInst *emit_jlcall(jl_codectx_t &ctx, Function *theFptr, Value *theF,
                              jl_cgval_t *args, size_t nargs, CallingConv::ID cc);
 static CallInst *emit_jlcall(jl_codectx_t &ctx, JuliaFunction *theFptr, Value *theF,
@@ -1456,16 +1456,21 @@ static Constant *undef_value_for_type(Type *T) {
     return undef;
 }
 
-static void CreateTrap(IRBuilder<> &irbuilder)
+static void CreateTrap(IRBuilder<> &irbuilder, bool create_new_block)
 {
     Function *f = irbuilder.GetInsertBlock()->getParent();
     Function *trap_func = Intrinsic::getDeclaration(
             f->getParent(),
             Intrinsic::trap);
     irbuilder.CreateCall(trap_func);
     irbuilder.CreateUnreachable();
-    BasicBlock *newBB = BasicBlock::Create(irbuilder.getContext(), "after_noret", f);
-    irbuilder.SetInsertPoint(newBB);
+    if (create_new_block) {
+        BasicBlock *newBB = BasicBlock::Create(irbuilder.getContext(), "after_noret", f);
+        irbuilder.SetInsertPoint(newBB);
+    }
+    else {
+        irbuilder.ClearInsertionPoint();
+    }
 }
 
 #if 0 // this code is likely useful, but currently unused
@@ -6901,8 +6906,8 @@ static std::pair<std::unique_ptr<Module>, jl_llvm_functions_t>
         if (seq_next >= 0 && (unsigned)seq_next < stmtslen) {
             workstack.push_back(seq_next);
         }
-        else if (!ctx.builder.GetInsertBlock()->getTerminator()) {
-            ctx.builder.CreateUnreachable();
+        else if (ctx.builder.GetInsertBlock() && !ctx.builder.GetInsertBlock()->getTerminator()) {
+            CreateTrap(ctx.builder, false);
         }
         while (!workstack.empty()) {
             int item = workstack.back();
@@ -6912,7 +6917,7 @@ static std::pair<std::unique_ptr<Module>, jl_llvm_functions_t>
                 cursor = item;
                 return;
             }
-            if (seq_next != -1 && !ctx.builder.GetInsertBlock()->getTerminator()) {
+            if (seq_next != -1 && ctx.builder.GetInsertBlock() && !ctx.builder.GetInsertBlock()->getTerminator()) {
                 come_from_bb[cursor + 1] = ctx.builder.GetInsertBlock();
                 ctx.builder.CreateBr(nextbb->second);
             }
@@ -7050,7 +7055,7 @@ static std::pair<std::unique_ptr<Module>, jl_llvm_functions_t>
         if (jl_is_returnnode(stmt)) {
             jl_value_t *retexpr = jl_returnnode_value(stmt);
             if (retexpr == NULL) {
-                ctx.builder.CreateUnreachable();
+                CreateTrap(ctx.builder, false);
                 find_next_stmt(-1);
                 continue;
             }
@@ -7059,7 +7064,7 @@ static std::pair<std::unique_ptr<Module>, jl_llvm_functions_t>
             jl_cgval_t retvalinfo = emit_expr(ctx, retexpr);
             retvalinfo = convert_julia_type(ctx, retvalinfo, jlrettype);
             if (retvalinfo.typ == jl_bottom_type) {
-                ctx.builder.CreateUnreachable();
+                CreateTrap(ctx.builder, false);
                 find_next_stmt(-1);
                 continue;
             }