Emit udf for trap vasm instead of brk

Summary:
Previously, vasm trap is implemented with a `brk` on Arm64. However, `brk` triggers a SIGTRAP signal, which is not handled by HHVM's crash reporter code, which is the root cause for `hphp/test/slow/static-analysis-error.php` test failure.

This diffs fixes the issue by implementing vasm trap on Arm64 with a `udf #1` instruction, which is Arm64's counterpart to x86's `ud2` instruction. `udf` triggers a SIGILL just like what `ud2` does. This fixes `hphp/test/slow/static-analysis-error.php` and achieves platform parity in terms of trap and crash handling.

Reviewed By: binliu19

Differential Revision: D94627667

fbshipit-source-id: 16654bd424f1147e5fb1e7e336086dbf8cda612d

Author: mcfi

hphp/runtime/base/crash-reporter.cpp

@@ -45,9 +45,7 @@
 
 #include <fcntl.h>
 #include <signal.h>
-#ifdef __x86_64__
 #include <ucontext.h>
-#endif
 
 #include <folly/portability/Fcntl.h>
 #include <folly/portability/Stdio.h>
@@ -147,8 +145,12 @@ void bt_handler(int sigin, siginfo_t* info, void* args) {
 #ifdef __x86_64__
   static uintptr_t sig_rbp = ((ucontext_t*) args)->uc_mcontext.gregs[REG_RBP];
   static uintptr_t sig_rip = ((ucontext_t*) args)->uc_mcontext.gregs[REG_RIP];
+#elif defined(__aarch64__)
+  static uintptr_t sig_rbp = ((ucontext_t*) args)->uc_mcontext.regs[29];
+  static uintptr_t sig_rip = ((ucontext_t*) args)->uc_mcontext.pc;
 #else
   static uintptr_t sig_rbp = 0;
+  static uintptr_t sig_rip = 0;
 #endif
 
   switch (s_crash_report_stage) {
@@ -322,11 +324,7 @@ void bt_handler(int sigin, siginfo_t* info, void* args) {
           auto const frame = BTFrame::regular(ar, kInvalidOffset);
           auto const addr = [&] () -> jit::CTCA {
             if (sig != SIGILL && sig != SIGSEGV) return (jit::CTCA) sig_addr;
-#if defined(__x86_64__)
             return (jit::CTCA) sig_rip;
-#else
-            return (jit::CTCA) 0;
-#endif
           }();
           auto const trace = createCrashBacktrace(frame, (jit::CTCA) addr);
 

hphp/runtime/vm/jit/vasm-arm.cpp

@@ -1252,7 +1252,9 @@ void Vgen::emit(const trap& i) {
     env.meta.trapFixups.emplace_back(a->frontier(), i.fix);
     env.record_inline_stack(a->frontier());
   }
-  a->Brk(1);
+  // UDF #1 — permanently undefined instruction that raises SIGILL, matching
+  // x86_64's ud2 behavior. TODO: switch to a->udf(1) once vixl is updated.
+  a->dc32(1);
 }
 
 void Vgen::emit(const unpcklpd& i) {