[libunwind] Ensure test/libunwind_01.pass is not completely inlined

[arichardson]

Summary

By adding noinline and calling fprintf before returning we ensure that
every function will have a distinct call frame and that the return address
will always be saved instead of saving the target in main as the result.

Before this change all backtraces were always backtrace -> main -> _start,
i.e. always exactly three entries. This happenend because all calls were
inlined in main() and the test just happenend to pass because there is at
least _start before main.

I found this while fixing some bugs in libunwind for CHERI and noticed that
the test was passing even though the code was completely broken.

Obtained from: https://github.com/CTSRD-CHERI/llvm-project

[arichardson]

libunwind/test/libunwind_01.pass.cpp

11

@ldionne does the noinline fix the failure? I was originally testing on macOS and when I made this change (back in 2018) I believe it fixed the tests for me.

[MaskRay]

I think it improves the robustness of the test. It'd be good to make tests at -O1 and above, not just -O0.

The test subject can mention [test] to make it clear this is a non-code change.

libunwind/test/libunwind_01.pass.cpp

167

The exit status is sufficient, no need for this log.

[arichardson]

libunwind/test/libunwind_01.pass.cpp

167

Indeed that was a debugging change to make it more obvious when the test passes. Will remove before committing.

[ldionne]

libunwind/test/libunwind_01.pass.cpp

11

It doesn't look like it does, otherwise we'd have seen some XPASS in the CI.

libunwind/test/libunwind_01.pass.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // TODO: Investigate this failure on x86_64 macOS back deployment
 // XFAIL: use_system_cxx_lib && target=x86_64-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
 
 // TODO: Figure out why this fails with Memory Sanitizer.
 // XFAIL: msan
 
 #include <libunwind.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 
 void backtrace(int lower_bound) {
   unw_context_t context;
   unw_getcontext(&context);
 
   unw_cursor_t cursor;
   unw_init_local(&cursor, &context);
 
+  char buffer[1024];
+  unw_word_t offset = 0;
+
   int n = 0;
   do {
-    ++n;
+    n++;
+    if (unw_get_proc_name(&cursor, buffer, sizeof(buffer), &offset) == 0) {
+      fprintf(stderr, "Frame %d: %s+%p\n", n, buffer, (void*)offset);
+    } else {
+      fprintf(stderr, "Frame %d: Could not get name for cursor\n", n);
+    }
     if (n > 100) {
       abort();
     }
   } while (unw_step(&cursor) > 0);
 
   if (n < lower_bound) {
     abort();
   }
 }
 
-void test1(int i) {
+__attribute__((noinline)) void test1(int i) {
+  fprintf(stderr, "starting %s\n", __func__);
   backtrace(i);
+  fprintf(stderr, "finished %s\n", __func__); // ensure return address is saved
 }
 
-void test2(int i, int j) {
+__attribute__((noinline)) void test2(int i, int j) {
+  fprintf(stderr, "starting %s\n", __func__);
   backtrace(i);
   test1(j);
+  fprintf(stderr, "finished %s\n", __func__); // ensure return address is saved
 }
 
-void test3(int i, int j, int k) {
+__attribute__((noinline)) void test3(int i, int j, int k) {
+  fprintf(stderr, "starting %s\n", __func__);
   backtrace(i);
   test2(j, k);
+  fprintf(stderr, "finished %s\n", __func__); // ensure return address is saved
 }
 
 void test_no_info() {
   unw_context_t context;
   unw_getcontext(&context);
 
   unw_cursor_t cursor;
   unw_init_local(&cursor, &context);
void test_fpreg_get_set() {
   }
 }
 #else
 void test_reg_get_set() {}
 void test_fpreg_get_set() {}
 #endif
 
 int main(int, char**) {
-  test1(1);
-  test2(1, 2);
-  test3(1, 2, 3);
+  test1(3);
+  test2(3, 4);
+  test3(3, 4, 5);
   test_no_info();
   test_reg_names();
   test_reg_get_set();
   test_fpreg_get_set();
   return 0;
 }