Make the code after throwing an error unreachable and do not even emit GC pop after throw

Author: yuyichao

src/codegen.cpp

@@ -634,10 +634,11 @@ class LocalGCFrame {
         }
     }
     void
-    restore()
+    restore(bool unreachable=false)
     {
         if (frame_swapped) {
-            emit_gcpop(ctx);
+            if (!unreachable)
+                emit_gcpop(ctx);
             finalize_gc_frame(ctx);
             std::swap(gc, ctx->gc);
             if (gc.argSpaceOffs + gc.maxDepth != 0) {
@@ -2231,7 +2232,15 @@ static Value *emit_known_call(jl_value_t *ff, jl_value_t **args, size_t nargs,
         builder.CreateCall2(prepare_call(jlthrow_line_func), arg1,
                             ConstantInt::get(T_int32, ctx->lineno));
 #endif
-        local_gc_frame.restore();
+        builder.CreateUnreachable();
+        // New (unreachable) block since LLVM is not happy about terminator
+        // in the middle of a block
+        BasicBlock *nextBB =
+            BasicBlock::Create(getGlobalContext(), "unreachable", ctx->f);
+        builder.SetInsertPoint(nextBB);
+        // Update GC frame creation. The GC pop instructions should be removed
+        // by LLVM anyway but not emitting those instructions save us some work
+        local_gc_frame.restore(true); // unreachable=true
         JL_GC_POP();
         return V_null;
     }