Fix panic in p_block when all control-point live values are globals

tests/c/guard_body_no_args.c

@@ -0,0 +1,43 @@
+// Run-time:
+//   env-var: YKD_LOG=4
+//   env-var: YKD_SERIALISE_COMPILATION=1
+//   stderr:
+//     yk-tracing: start-tracing
+//     ...
+//     yk-tracing: stop-tracing
+//     ...
+//     yk-execution: enter-jit-code
+//     ...
+
+// Testing that when all interpreter state is in globals the JIT produces
+// a trace block with no Arg instructions.
+// Previously this caused an unconditional panic in p_block.
+
+#include <stdlib.h>
+#include <yk.h>
+#include <yk_testing.h>
+
+long g_pc = 0;
+YkMT *g_mt = NULL;
+YkLocation i;
+
+__attribute__((yk_outline)) static void debug_fn(void) {}
+
+int main(void) {
+  g_mt = yk_mt_new(NULL);
+  yk_mt_hot_threshold_set(g_mt, 0);
+  yk_mt_sidetrace_threshold_set(g_mt, 4);
+  i = yk_location_new();
+
+  for (; g_pc < 20; g_pc++) {
+    yk_mt_control_point(g_mt, &i);
+    debug_fn();
+    if (g_pc >  15) {
+      break;
+    }
+  }
+
+  yk_location_drop(i);
+  yk_mt_shutdown(g_mt);
+  return EXIT_SUCCESS;
+}

ykrt/src/compile/j2/hir_to_asm.rs

@@ -860,11 +860,9 @@ impl<'a, AB: HirToAsmBackend> HirToAsm<'a, AB> {
                 break;
             }
             let Some((iidx, hinst)) = insts_iter.next() else {
-                // By definition there must be no `Arg` instructions in this trace, which is only
-                // plausible in testing mode.
-                #[cfg(not(test))]
-                panic!();
-                #[cfg(test)]
+                // No Arg instructions: Can happen when all live values at this control point are globals.
+                // Their addresses are baked into the IR, so nothing needs to be passed in
+                // via registers or stack at trace entry.
                 break;
             };
             match hinst {