[libunwind] [libunwind] Be more careful about enabling GCS (PR #101973)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libunwind

Author: John Brawn (john-brawn-arm)

<details>
<summary>Changes</summary>

We need both GCS to be enabled by the compiler (which we do by checking if __ARM_FEATURE_GCS_DEFAULT is defined) and for arm_acle.h to define the GCS intrinsics. Check the latter by checking if _CHKFEAT_GCS is defined.

---
Full diff: https://github.com/llvm/llvm-project/pull/101973.diff


1 Files Affected:

- (modified) libunwind/src/cet_unwind.h (+5-1) 


``````````diff
diff --git a/libunwind/src/cet_unwind.h b/libunwind/src/cet_unwind.h
index 45c11973cb7fa..47d7616a7322c 100644
--- a/libunwind/src/cet_unwind.h
+++ b/libunwind/src/cet_unwind.h
@@ -39,9 +39,13 @@
 // need to guard any use of GCS instructions with __chkfeat though, as GCS may
 // not be enabled.
 #if defined(_LIBUNWIND_TARGET_AARCH64) && defined(__ARM_FEATURE_GCS_DEFAULT)
-#define _LIBUNWIND_USE_GCS 1
 #include <arm_acle.h>
 
+// We can only use GCS if arm_acle.h defines the GCS intrinsics.
+#ifdef _CHKFEAT_GCS
+#define _LIBUNWIND_USE_GCS 1
+#endif
+
 #define _LIBUNWIND_POP_CET_SSP(x)                                              \
   do {                                                                         \
     if (__chkfeat(_CHKFEAT_GCS)) {                                             \

``````````

</details>


https://github.com/llvm/llvm-project/pull/101973