[libunwind] [libunwind][arm64e] Mach-O Support unwinding PAuth_LR frames (PR #202772)

[Daniil Kovalev via cfe-commits]

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@@ -1956,19 +1972,52 @@ bool UnwindCursor<A, R>::getInfoFromCompactEncodingSection(
         high = mid;
       }
     }
-    funcStart = pageIndex.functionOffset(low) + firstLevelFunctionOffset
-                                                              + sects.dso_base;
+    rangeStart = pageIndex.functionOffset(low) + firstLevelFunctionOffset +
+                 sects.dso_base;
     if (low < last)
       funcEnd =
           pageIndex.functionOffset(low + 1) + firstLevelFunctionOffset
                                                               + sects.dso_base;
     else
       funcEnd = firstLevelNextPageFunctionOffset + sects.dso_base;
+
+    auto encodingAtIndex = [&](uint32_t idx) -> uint32_t {
+      uint16_t encIdx = pageIndex.encodingIndex(idx);
+      if (encIdx < sectionHeader.commonEncodingsArrayCount()) {
+        return _addressSpace.get32(
+            sects.compact_unwind_section +
+            sectionHeader.commonEncodingsArraySectionOffset() +
+            encIdx * sizeof(uint32_t));
+      } else {
+        uint16_t pageEncIdx =
+            encIdx - (uint16_t)sectionHeader.commonEncodingsArrayCount();
+        return _addressSpace.get32(secondLevelAddr +
+                                   pageHeader.encodingsPageOffset() +
+                                   pageEncIdx * sizeof(uint32_t));
+      }
+    };
+
+    encoding = encodingAtIndex(low);
+
+    // If UNWIND_IS_NOT_FUNCTION_START is set, walk backwards to find the actual
+    // function start.
+    funcStart = rangeStart;
+    if (encoding & UNWIND_IS_NOT_FUNCTION_START) {
+      uint32_t backIndex = low;
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kovdan01 wrote:

Same comment as https://github.com/llvm/llvm-project/pull/202772/changes#r3481571229

https://github.com/llvm/llvm-project/pull/202772