[libunwind] Use `_dl_find_object` if available

As shown in P2544R0 [1] and the accompanying benchmark [2], the
current unwinding logic does not scale for multi-threaded programs.
This is because `dl_iterate_phdr` takes a global lock.

glibc 2.35 added `_dl_find_object` which directly returns the unwind
info for a given target address. `_dl_find_object` is fully lock-free
and hence allows parallel exception unwinding on multiple threads.

With this commit, libunwind now takes advantage of `_dl_find_object`.
Thereby, this commit improves libunwind's performance on benchmark [2]
for unwinding exception on 20 threads from 1103ms to 78ms.
(measured on Intel Xeon Silver 4114 with 20 physical cores)

[1] https://isocpp.org/files/papers/P2544R0.html
[2] https://github.com/neumannt/exceptionperformance

Detailed performance numbers from the benchmark:

Before:
> Testing unwinding performance: sqrt computation with occasional errors
>
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 34 35 34 35 35 36
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 16 32 33 34 35 36
> failure rate 0.1%: 16 32 34 36 35 36
> failure rate 1%: 20 40 40 43 90 113
> failure rate 10%: 59 92 140 304 880 1103
> [...]
>
> Testing invocation overhead: recursive fib with occasional errors
>
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 19 32 37 38 39 36
> failure rate 0.1%: 22 32 40 40 39 34
> failure rate 1%: 20 28 38 39 48 40
> failure rate 10%: 25 39 44 50 92 113

After:
> Testing unwinding performance: sqrt computation with occasional errors
>
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 19 30 35 38 39 35
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 32 35 33 34 34 36
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 16 35 33 37 35 35
> failure rate 0.1%: 16 32 36 33 34 37
> failure rate 1%: 21 37 39 40 40 41
> failure rate 10%: 72 75 76 80 80 78
> [...]
>
> Testing invocation overhead: recursive fib with occasional errors
>
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 18 35 37 34 38 37
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 19 33 40 40 41 39
> failure rate 0.1%: 21 33 39 38 39 38
> failure rate 1%: 20 36 39 40 41 40
> failure rate 10%: 25 45 41 42 44 43

Differential Revision: https://reviews.llvm.org/D130668

1 files changed, 55 insertions, 0 deletions

diff --git a/libunwind/src/AddressSpace.hpp b/libunwind/src/AddressSpace.hpp
index 36c9f5a9..f1ba94e 100644
--- a/libunwind/src/AddressSpace.hpp
+++ b/libunwind/src/AddressSpace.hpp
@@ -601,6 +601,61 @@ inline bool LocalAddressSpace::findUnwindSections(pint_t targetAddr,
   if (info.arm_section && info.arm_section_length)
     return true;
 #elif defined(_LIBUNWIND_USE_DL_ITERATE_PHDR)
+  // Use DLFO_STRUCT_HAS_EH_DBASE to determine the existence of
+  // `_dl_find_object`. Use _LIBUNWIND_SUPPORT_DWARF_INDEX, because libunwind
+  // support for _dl_find_object on other unwind formats is not implemented,
+  // yet.
+#if defined(DLFO_STRUCT_HAS_EH_DBASE) & defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
+  // We expect to run on a platform which does not use a base address for
+  // exception information.
+#if DLFO_STRUCT_HAS_EH_DBASE
+#error dlfo_eh_dbase is not supported for DWARF-based unwinding
+#endif
+  // We expect `_dl_find_object` to return PT_GNU_EH_FRAME.
+#if DLFO_EH_SEGMENT_TYPE != PT_GNU_EH_FRAME
+#error _dl_find_object retrieves an unexpected section type
+#endif
+  // We look-up `dl_find_object` dynamically at runtime to ensure backwards
+  // compatibility with earlier version of glibc not yet providing it. On older
+  // systems, we gracefully fallback to `dl_iterate_phdr`. Cache the pointer
+  // so we only look it up once. Do manual lock to avoid _cxa_guard_acquire.
+  static decltype(_dl_find_object) *dlFindObject;
+  static bool dlFindObjectChecked = false;
+  if (!dlFindObjectChecked) {
+    dlFindObject = reinterpret_cast<decltype(_dl_find_object) *>(
+        dlsym(RTLD_DEFAULT, "_dl_find_object"));
+    dlFindObjectChecked = true;
+  }
+  // Try to find the unwind info using `dl_find_object`
+  dl_find_object findResult;
+  if (dlFindObject && dlFindObject((void *)targetAddr, &findResult) == 0) {
+    if (findResult.dlfo_eh_frame == nullptr) {
+      // Found an entry for `targetAddr`, but there is no unwind info.
+      return false;
+    }
+    info.dso_base = reinterpret_cast<uintptr_t>(findResult.dlfo_map_start);
+    info.text_segment_length = static_cast<size_t>(
+        (char *)findResult.dlfo_map_end - (char *)findResult.dlfo_map_start);
+
+    // Record the start of PT_GNU_EH_FRAME.
+    info.dwarf_index_section =
+        reinterpret_cast<uintptr_t>(findResult.dlfo_eh_frame);
+    // `_dl_find_object` does not give us the size of PT_GNU_EH_FRAME.
+    // Setting length to `SIZE_MAX` effectively disables all range checks.
+    info.dwarf_index_section_length = SIZE_MAX;
+    EHHeaderParser<LocalAddressSpace>::EHHeaderInfo hdrInfo;
+    if (!EHHeaderParser<LocalAddressSpace>::decodeEHHdr(
+            *this, info.dwarf_index_section, info.dwarf_index_section_length,
+            hdrInfo)) {
+      return false;
+    }
+    // Record the start of the FDE and use SIZE_MAX to indicate that we do
+    // not know the end address.
+    info.dwarf_section = hdrInfo.eh_frame_ptr;
+    info.dwarf_section_length = SIZE_MAX;
+    return true;
+  }
+#endif
   dl_iterate_cb_data cb_data = {this, &info, targetAddr};
   int found = dl_iterate_phdr(findUnwindSectionsByPhdr, &cb_data);
   return static_cast<bool>(found);