symbolizer: cache each DWARF DIE's nextOffset and nextSiblingOffset

folly/experimental/symbolizer/DwarfImpl.cpp

@@ -375,9 +375,13 @@ size_t DwarfImpl::forEachChild(
     const CompilationUnit& cu,
     const Die& die,
     folly::FunctionRef<bool(const Die& die)> f) const {
-  size_t nextDieOffset =
-      forEachAttribute(cu, die, [&](const Attribute&) { return true; });
+  size_t nextDieOffset = die.nextOffset;
+  if (!nextDieOffset) {
+    nextDieOffset =
+        forEachAttribute(cu, die, [&](const Attribute&) { return true; });
+  }
   if (!die.abbr.hasChildren) {
+    die.nextSiblingOffset = nextDieOffset;
     return nextDieOffset;
   }
 
@@ -387,15 +391,20 @@ size_t DwarfImpl::forEachChild(
       return childDie.offset;
     }
 
-    // NOTE: Don't run `f` over grandchildren, just skip over them.
-    size_t siblingOffset =
-        forEachChild(cu, childDie, [](const Die&) { return true; });
+    // Use childDie.nextSiblingOffset to skip grandchildren if known
+    size_t siblingOffset = childDie.nextSiblingOffset;
+    if (!siblingOffset) {
+      // NOTE: Don't run `f` over grandchildren, just skip over them.
+      siblingOffset =
+          forEachChild(cu, childDie, [](const Die&) { return true; });
+    }
     childDie = getDieAtOffset(cu, siblingOffset);
   }
 
   // childDie is now a dummy die whose offset is to the code 0 marking the
   // end of the children. Need to add one to get the offset of the next die.
-  return childDie.offset + 1;
+  die.nextSiblingOffset = childDie.offset + 1;
+  return die.nextSiblingOffset;;
 }
 
 bool DwarfImpl::isAddrInRangeList(

folly/experimental/symbolizer/DwarfUtil.cpp

@@ -782,8 +782,13 @@ size_t forEachAttribute(
     if (!f(attr)) {
       return static_cast<size_t>(-1);
     }
+    if (spec.name == DW_AT_sibling && !die.nextSiblingOffset &&
+        spec.form != DW_FORM_ref_addr) {
+      die.nextSiblingOffset = cu.offset + boost::get<uint64_t>(attr.attrValue);
+    }
   }
-  return values.data() - cu.debugSections.debugInfo.data();
+  die.nextOffset = values.data() - cu.debugSections.debugInfo.data();
+  return die.nextOffset;
 }
 
 folly::StringPiece getFunctionNameFromDie(

folly/experimental/symbolizer/DwarfUtil.h

@@ -133,6 +133,23 @@ struct Die {
   uint8_t attrOffset = 0;
   // Offset within debug info.
   uint32_t offset = 0;
+
+  // If nonzero, nextOffset is the offset within .debug_info
+  // corresponding to the beginning of this DIE's first child or next
+  // sibling (depending on whether abbr.hasChildren). This is what
+  // forEachAttribute() would return. So nextOffset - offset is the
+  // length of this DIE. If this points to a zero byte then there
+  // are no children or no further siblings. If the offset itself
+  // is zero, we don't know yet what it should be.
+  mutable uint32_t nextOffset = 0;
+
+  // If nonzero, nextSiblingOffset is the offset within .debug_info
+  // corresponding to the beginning of this DIE's next sibling.
+  // This is what forEachChild() would return. If this points to
+  // a zero byte then there are no further siblings. If the offset
+  // itself is zero, we don't know yet what it should be.
+  mutable uint32_t nextSiblingOffset = 0;
+
   uint64_t code = 0;
   DIEAbbreviation abbr;
 };