Panic on prompt duplication (#810)

Detect an invalid state dynamically and panic (#244).

Todo: print proper error message

examples/llvm/prompt-duplication.check

@@ -0,0 +1,2 @@
+[error] Process exited with non-zero exit code 1.
+[error] Valgrind log:

examples/llvm/prompt-duplication.effekt

@@ -0,0 +1,16 @@
+
+effect E() : (() => Unit at {io})
+
+
+def main() : Unit = {
+    try {
+      val f = do E()
+      f()
+    } with E { () =>
+        def f() = { () }
+        def g() = {
+          resume(f)
+        }
+        resume(g)
+    }
+}

libraries/llvm/forward-declare-c.ll

@@ -9,6 +9,7 @@ declare void @c_io_println_Double(%Double)
 declare void @c_io_println_String(%Pos)
 
 declare void @hole()
+declare void @duplicated_prompt()
 
 declare %Pos @c_ref_fresh(%Pos)
 declare %Pos @c_ref_get(%Pos)

libraries/llvm/main.c

@@ -12,7 +12,7 @@
 #include "types.c"
 #include "bytearray.c"
 #include "io.c"
-#include "hole.c"
+#include "panic.c"
 #include "ref.c"
 #include "array.c"
 

libraries/llvm/panic.c

@@ -0,0 +1,16 @@
+#ifndef EFFEKT_PANIC_C
+#define EFFEKT_PANIC_C
+
+#include <stdio.h>
+
+void hole() {
+    fprintf(stderr, "PANIC: Reached a hole in the program\n");
+    exit(1);
+}
+
+void duplicated_prompt() {
+    fprintf(stderr, "PANIC: Continuation invoked itself\n");
+    exit(1);
+}
+
+#endif

libraries/llvm/rts.ll

@@ -386,7 +386,7 @@ define private %Stack @reset(%Stack %oldStack) {
     ret %Stack %stack
 }
 
-define private void @updatePrompts(%Stack %stack) {
+define private void @revalidate(%Stack %stack) {
     %prompt_pointer = getelementptr %StackValue, %Stack %stack, i64 0, i32 3
     %prompt = load %Prompt, ptr %prompt_pointer
     %stack_pointer = getelementptr %PromptValue, %Prompt %prompt, i64 0, i32 1
@@ -402,42 +402,52 @@ continue:
     br i1 %isOccupied, label %displace, label %update
 
 displace:
-    call void @displace(%Stack %promptStack, %Stack %promptStack)
+    call void @invalidate(%Stack %promptStack, %Stack %promptStack)
     br label %update
 
 update:
     store %Stack %stack, ptr %stack_pointer
 
     %next_pointer = getelementptr %StackValue, %Stack %stack, i64 0, i32 4
     %next = load %Stack, ptr %next_pointer
-    tail call void @updatePrompts(%Stack %next)
+    tail call void @revalidate(%Stack %next)
     ret void
 }
 
-define private void @displace(%Stack %stack, %Stack %end) {
+; This panics if we invalidate the meta stack
+define private void @invalidate(%Stack %stack, %Stack %end) {
     %prompt_pointer = getelementptr %StackValue, %Stack %stack, i64 0, i32 3
     %next_pointer = getelementptr %StackValue, %Stack %stack, i64 0, i32 4
     %prompt = load %Prompt, ptr %prompt_pointer
     %stack_pointer = getelementptr %PromptValue, %Prompt %prompt, i64 0, i32 1
     store %Stack null, ptr %stack_pointer
 
     %next = load %Stack, ptr %next_pointer
+
+    %isNull = icmp eq %Stack %next, null
+    br i1 %isNull, label %error, label %check
+
+error:
+    call void @duplicated_prompt()
+    ret void
+
+check:
     %isEnd = icmp eq %Stack %next, %end
     br i1 %isEnd, label %done, label %continue
 
 done:
     ret void
 
 continue:
-    tail call void @displace(%Stack %next, %Stack %end)
+    tail call void @invalidate(%Stack %next, %Stack %end)
     ret void
 }
 
 define private %Stack @resume(%Resumption %resumption, %Stack %oldStack) {
     %uniqueResumption = call %Resumption @uniqueStack(%Resumption %resumption)
     %rest_pointer = getelementptr %StackValue, %Resumption %uniqueResumption, i64 0, i32 4
     %start = load %Stack, ptr %rest_pointer
-    call void @updatePrompts(%Stack %start)
+    call void @revalidate(%Stack %start)
 
     store %Stack %oldStack, ptr %rest_pointer