[libunwind] Fix unwind_leaffunction.pass.cpp

[leonardchan]

Summary

It's possible for this test not to pass if the libc used does not provide unwind info for raise. We can replace it with __builtin_cast, which can lead to a SIGTRAP on x86_64 and a SIGILL on aarch64.

Using this alternative, a nop is needed before the __builtin_cast. This is because libunwind incorrectly decrements pc, which can cause pc to jump into the previous function and use the incorrect FDE.

[leonardchan]

I meant to commandeer https://reviews.llvm.org/D114818 rather than make a new patch.

libunwind/test/unwind_leaffunction.pass.cpp

 
 void signal_handler(int signum) {
   (void)signum;
   _Unwind_Backtrace(frame_handler, NULL);
   _Exit(-1);
 }
 
 __attribute__((noinline)) void crashing_leaf_func(void) {
-  raise(SIGSEGV);
+  // 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();
 }
 
 int main(int, char**) {
-  signal(SIGSEGV, signal_handler);
+  signal(SIGTRAP, signal_handler);
+  signal(SIGILL, signal_handler);
   crashing_leaf_func();
   return -2;
 }