boots: Fix inlining of functions with unreachable return

pkgs/boots/graph.dora

@@ -556,6 +556,18 @@ impl Block {
         inst
     }
 
+    pub fn prependInst(inst: Inst): Inst {
+        assert(inst.block.isNone() && !inst.hasId());
+
+        inst.block = Some(self);
+        self.graph.getOrPanic().assignNextInstId(inst);
+
+        self.instructions.prependInst(inst);
+        inst.registerUses();
+
+        inst
+    }
+
     pub fn splitAfter(inst: Inst): Block {
         assert(inst.getBlock() === self);
         assert(!inst.isTerminator());
@@ -3167,6 +3179,16 @@ impl InstLinkedList {
         self.last.getOrPanic()
     }
 
+    fn prependInst(inst: Inst) {
+        let first = self.first.getOrPanic();
+        first.previous = Some(inst);
+
+        inst.previous = None[Inst];
+        inst.next = Some[Inst](first);
+
+        self.first = Some(inst);
+    }
+
     fn appendInst(inst: Inst) {
         if self.first.isNone() {
             inst.previous = None;

pkgs/boots/graph_builder.dora

@@ -1206,9 +1206,7 @@ impl SsaGen {
                 let value = self.readVariable(opnd, self.current());
                 let ty = self.graphTy(ty);
 
-                if self.returnValue.isSome() {
-                    let returnValue = self.returnValue.getOrPanic();
-
+                if self.returnValue is Some(returnValue) {
                     let phi = if returnValue.getBlock() == exitBlock {
                         assert(returnValue.isPhi());
                         returnValue
@@ -1219,8 +1217,8 @@ impl SsaGen {
                         phi
                     };
 
-                    // Here we add input to phi after the phi was  already appended to
-                    // the graph. Therefore we need to register the input manually.
+                    // Here we add another input to the phi after the phi was already appended
+                    // to the graph. Therefore we need to register the input manually.
                     let input = phi.addInput(value);
                     value.addUse(input);
                     self.returnValue = Some(phi);

pkgs/boots/inlining.dora

@@ -1,6 +1,6 @@
 use package::bytecode::{BytecodeType, FunctionId};
 use package::compilation::CompilationInfo;
-use package::graph::{Graph, Inst, Op};
+use package::graph::{createUndefInst, Graph, Inst, Op};
 use package::graph_builder;
 use package::interface as iface;
 
@@ -43,11 +43,18 @@ fn inline(ci: CompilationInfo, graph: Graph, inst: Inst, fctId: FunctionId, type
     if returnValue.isSome() {
         let returnValue = returnValue.getOrPanic();
         inst.replaceAllUsesWith(returnValue);
-    }
-    if inst.hasUses() {
-        println("ouch: ${ci.getDisplayName()}");
-        // println(dumpGraph(Some[CompilationInfo](ci), graph, config.architecture));
-        // println("ouch");
+    } else if inst.hasUses() {
+        // This call has users but the inlined function has no return value
+        // because it e.g. crashes unconditionally. In this case newBlock is
+        // unreachable and will be removed by cfg_simplification.
+        // There is no value to replace all uses of the call return value, so
+        // replace it with Undef. Undef also needs to be in newBlock to not
+        // break SSA.
+        let undef = createUndefInst();
+        newBlock.prependInst(undef);
+        inst.replaceAllUsesWith(undef);
+    } else {
+        assert(inst.getValueType().isUnit());
     }
     inst.remove();
 }

tests/boots/opt/inline-unreachable.dora

@@ -0,0 +1,18 @@
+//= boots
+//= error code 1
+
+fn main() {
+    testme();
+}
+
+@Optimize
+fn testme(): Int {
+    let x = myFatalError[Int]();
+    // Everything here is unreachable after inlining.
+    println("x = ${x}");
+    x + 1
+}
+
+fn myFatalError[T](): T {
+  std::fatalError[T]("not yet implemented")
+}