[libc++] Allow running the test suite with optimizations (#68753)

This patch adds a configuration of the libc++ test suite that enables
optimizations when building the tests. It also adds a new CI
configuration to exercise this on a regular basis. This is added in the
context of [1], which requires building with optimizations in order to
hit the bug.

[1]: https://github.com/llvm/llvm-project/issues/68552

3 files changed, 36 insertions, 14 deletions

diff --git a/libunwind/test/libunwind_02.pass.cpp b/libunwind/test/libunwind_02.pass.cpp
index ea34cd5..9fd8e5d 100644
--- a/libunwind/test/libunwind_02.pass.cpp
+++ b/libunwind/test/libunwind_02.pass.cpp
@@ -21,7 +21,8 @@
 #define EXPECTED_NUM_FRAMES 50
 #define NUM_FRAMES_UPPER_BOUND 100
 
-_Unwind_Reason_Code callback(_Unwind_Context *context, void *cnt) {
+__attribute__((noinline)) _Unwind_Reason_Code callback(_Unwind_Context *context,
+                                                       void *cnt) {
   (void)context;
   int *i = (int *)cnt;
   ++*i;
@@ -31,7 +32,7 @@ _Unwind_Reason_Code callback(_Unwind_Context *context, void *cnt) {
   return _URC_NO_REASON;
 }
 
-void test_backtrace() {
+__attribute__((noinline)) void test_backtrace() {
   int n = 0;
   _Unwind_Backtrace(&callback, &n);
   if (n < EXPECTED_NUM_FRAMES) {
@@ -39,17 +40,34 @@ void test_backtrace() {
   }
 }
 
-int test(int i) {
+// These functions are effectively the same, but we have to be careful to avoid
+// unwanted optimizations that would mess with the number of frames we expect.
+// Surprisingly, slapping `noinline` is not sufficient -- we also have to avoid
+// writing the function in a way that the compiler can easily spot tail
+// recursion.
+__attribute__((noinline)) int test1(int i);
+__attribute__((noinline)) int test2(int i);
+
+__attribute__((noinline)) int test1(int i) {
+  if (i == 0) {
+    test_backtrace();
+    return 0;
+  } else {
+    return i + test2(i - 1);
+  }
+}
+
+__attribute__((noinline)) int test2(int i) {
   if (i == 0) {
     test_backtrace();
     return 0;
   } else {
-    return i + test(i - 1);
+    return i + test1(i - 1);
   }
 }
 
 int main(int, char**) {
-  int total = test(50);
+  int total = test1(50);
   assert(total == 1275);
   return 0;
 }
diff --git a/libunwind/test/unw_resume.pass.cpp b/libunwind/test/unw_resume.pass.cpp
index 08e8d4e..2b7470b 100644
--- a/libunwind/test/unw_resume.pass.cpp
+++ b/libunwind/test/unw_resume.pass.cpp
@@ -15,7 +15,7 @@
 
 #include <libunwind.h>
 
-void test_unw_resume() {
+__attribute__((noinline)) void test_unw_resume() {
   unw_context_t context;
   unw_cursor_t cursor;
 
diff --git a/libunwind/test/unwind_leaffunction.pass.cpp b/libunwind/test/unwind_leaffunction.pass.cpp
index 8c9912e..112a596 100644
--- a/libunwind/test/unwind_leaffunction.pass.cpp
+++ b/libunwind/test/unwind_leaffunction.pass.cpp
@@ -28,7 +28,7 @@ _Unwind_Reason_Code frame_handler(struct _Unwind_Context* ctx, void* arg) {
   (void)arg;
   Dl_info info = { 0, 0, 0, 0 };
 
-  // Unwind until the main is reached, above frames deeped on the platform and
+  // Unwind until the main is reached, above frames depend on the platform and
   // architecture.
   if (dladdr(reinterpret_cast<void *>(_Unwind_GetIP(ctx)), &info) &&
       info.dli_sname && !strcmp("main", info.dli_sname)) {
@@ -43,18 +43,22 @@ void signal_handler(int signum) {
   _Exit(-1);
 }
 
-__attribute__((noinline)) void crashing_leaf_func(void) {
+__attribute__((noinline)) void crashing_leaf_func(int do_trap) {
   // libunwind searches for the address before the return address which points
-  // to the trap instruction. NOP guarantees the trap instruction is not the
-  // first instruction of the function.
-  // We should keep this here for other unwinders that also decrement pc.
-  __asm__ __volatile__("nop");
-  __builtin_trap();
+  // to the trap instruction. We make the trap conditional and prevent inlining
+  // of the function to ensure that the compiler doesn't remove the `ret`
+  // instruction altogether.
+  //
+  // It's also important that the trap instruction isn't the first instruction
+  // in the function (which it isn't because of the branch) for other unwinders
+  // that also decrement pc.
+  if (do_trap)
+    __builtin_trap();
 }
 
 int main(int, char**) {
   signal(SIGTRAP, signal_handler);
   signal(SIGILL, signal_handler);
-  crashing_leaf_func();
+  crashing_leaf_func(1);
   return -2;
 }