[BOLT][PAC] Warn about synchronous unwind tables

BOLT currently ignores functions with synchronous PAuth DWARF info.
When more than 10% of functions get ignored for inconsistencies, we
should emit a warning to only use asynchronous unwind tables.

See also: #165215

Author: bgergely0

bolt/lib/Passes/PointerAuthCFIAnalyzer.cpp

@@ -133,11 +133,18 @@ Error PointerAuthCFIAnalyzer::runOnFunctions(BinaryContext &BC) {
   ParallelUtilities::runOnEachFunction(
       BC, ParallelUtilities::SchedulingPolicy::SP_INST_LINEAR, WorkFun,
       SkipPredicate, "PointerAuthCFIAnalyzer");
+
+  float IgnoredPercent = (100.0 * FunctionsIgnored) / Total;
   BC.outs() << "BOLT-INFO: PointerAuthCFIAnalyzer ran on " << Total
             << " functions. Ignored " << FunctionsIgnored << " functions "
-            << format("(%.2lf%%)", (100.0 * FunctionsIgnored) / Total)
+            << format("(%.2lf%%)", IgnoredPercent)
             << " because of CFI inconsistencies\n";
 
+  if (IgnoredPercent >= 10.0)
+    BC.outs() << "BOLT-WARNING: PointerAuthCFIAnalyzer only supports "
+                 "asynchronous unwind tables. For C compilers, see "
+                 "-fasynchronous-unwind-tables.\n";
+
   return Error::success();
 }
 

bolt/test/runtime/AArch64/pacret-synchronous-unwind.cpp

@@ -0,0 +1,33 @@
+// Test to demonstrate that functions compiled with synchronous unwind tables
+// are ignored by the PointerAuthCFIAnalyzer.
+// Exception handling is needed to have _any_ unwind tables, otherwise the
+// PointerAuthCFIAnalyzer does not run on these functions, so it does not ignore
+// any function.
+//
+// REQUIRES: system-linux,bolt-runtime
+//
+// RUN: %clangxx --target=aarch64-unknown-linux-gnu \
+// RUN: -mbranch-protection=pac-ret \
+// RUN: -fno-asynchronous-unwind-tables \
+// RUN: %s -o %t.exe -Wl,-q
+// RUN: llvm-bolt %t.exe -o %t.bolt | FileCheck %s --check-prefix=CHECK
+//
+// CHECK: PointerAuthCFIAnalyzer ran on 3 functions. Ignored
+// CHECK-NOT: 0 functions (0.00%) because of CFI inconsistencies
+// CHECK-SAME: 1 functions (33.33%) because of CFI inconsistencies
+// CHECK-NEXT: BOLT-WARNING: PointerAuthCFIAnalyzer only supports asynchronous
+// CHECK-SAME: unwind tables. For C compilers, see -fasynchronous-unwind-tables.
+
+#include <cstdio>
+#include <stdexcept>
+
+void foo() { throw std::runtime_error("Exception from foo()."); }
+
+int main() {
+  try {
+    foo();
+  } catch (const std::exception &e) {
+    printf("Exception caught: %s\n", e.what());
+  }
+  return 0;
+}