[llvm-branch-commits] [llvm] [BOLT][PAC] Warn about synchronous unwind tables (PR #165227)

[Gergely Bálint via llvm-branch-commits]

https://github.com/bgergely0 created https://github.com/llvm/llvm-project/pull/165227

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

>From 43b2e4cee68b4edb707c019e6db139f189651916 Mon Sep 17 00:00:00 2001
From: Gergely Balint <gergely.balint at arm.com>
Date: Mon, 27 Oct 2025 09:29:54 +0000
Subject: [PATCH] [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
---
 bolt/lib/Passes/PointerAuthCFIAnalyzer.cpp    |  8 ++++-
 .../AArch64/pacret-synchronous-unwind.cpp     | 32 +++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 bolt/test/runtime/AArch64/pacret-synchronous-unwind.cpp

diff --git a/bolt/lib/Passes/PointerAuthCFIAnalyzer.cpp b/bolt/lib/Passes/PointerAuthCFIAnalyzer.cpp
index 2fc5a2fda086a..6bcb5a6bd1801 100644
--- a/bolt/lib/Passes/PointerAuthCFIAnalyzer.cpp
+++ b/bolt/lib/Passes/PointerAuthCFIAnalyzer.cpp
@@ -130,11 +130,17 @@ 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.\n";
+
   return Error::success();
 }
 
diff --git a/bolt/test/runtime/AArch64/pacret-synchronous-unwind.cpp b/bolt/test/runtime/AArch64/pacret-synchronous-unwind.cpp
new file mode 100644
index 0000000000000..e90882833323d
--- /dev/null
+++ b/bolt/test/runtime/AArch64/pacret-synchronous-unwind.cpp
@@ -0,0 +1,32 @@
+// 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: PointerAuthCFIAnalyzer only supports asynchronous 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;
+}