[GWP-ASan] Change unreachable -> trap to work around DCE bug.

[hctim]

Summary

trapOnAddress is designed to SEGV on a specific address. Unfortunately,
with an IR change, __builtin_unreachable() ends up doing DCE on things
that have side effects, like the load that causes the trap.

Change to __builtin_trap() to avoid the optimisation.

Root cause is still an LLVM bug, and tracked in
https://bugs.llvm.org/show_bug.cgi?id=47480.

[lebedev.ri]

compiler-rt/lib/gwp_asan/guarded_pool_allocator.cpp

258–265

Not commenting on the rest of the comment, but volatile *store*.

[eugenis]

LGTM

[hctim]

compiler-rt/lib/gwp_asan/guarded_pool_allocator.cpp

258–265

fixed, thanks

[hctim]

/s/load/store

compiler-rt/lib/gwp_asan/guarded_pool_allocator.cpp

void *GuardedPoolAllocator::allocate(size_t Size, size_t Alignment) {
 
   return reinterpret_cast<void *>(UserPtr);
 }
 
 void GuardedPoolAllocator::trapOnAddress(uintptr_t Address, Error E) {
   State.FailureType = E;
   State.FailureAddress = Address;
 
   // Raise a SEGV by touching first guard page.
   volatile char *p = reinterpret_cast<char *>(State.GuardedPagePool);
   *p = 0;
-  __builtin_unreachable();
+  // Normally, would be __builtin_unreachable(), but because of
+  // https://bugs.llvm.org/show_bug.cgi?id=47480, unreachable will DCE the
+  // volatile store above, even though it has side effects.
+  __builtin_trap();
 }
 
 void GuardedPoolAllocator::stop() {
   getThreadLocals()->RecursiveGuard = true;
   PoolMutex.tryLock();
 }
 
 void GuardedPoolAllocator::deallocate(void *Ptr) {
   assert(pointerIsMine(Ptr) && "Pointer is not mine!");