DIE iteration optimizations

When iterating through all the CU DIEs, only inspect those that have the
DW_TAG_compile_unit tag. When finishing the iteration, only keep going
to reset libdwarf's internal CU iterator if we found a DIE.

backward.hpp

@@ -3032,28 +3032,37 @@ class TraceResolverLinuxImpl<trace_resolver_tag::libdwarf>:
 		// The search for aranges failed. Try to find our address by scanning
 		// all compilation units.
 		Dwarf_Unsigned next_cu_header;
-		while (dwarf_next_cu_header_d(dwarf, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+		Dwarf_Half tag = 0;
+		returnDie = 0;
+
+		while (!found && dwarf_next_cu_header_d(dwarf, 1, 0, 0, 0, 0, 0, 0, 0, 0,
 				&next_cu_header, 0, &error) == DW_DLV_OK) {
-			if (dwarf_siblingof(dwarf, 0, &returnDie, &error) == DW_DLV_OK) {
-				if (die_has_pc(fobj, returnDie, addr)) {
-					found = true;
-					break;
-				}
+
+			if (returnDie)
 				dwarf_dealloc(dwarf, returnDie, DW_DLA_DIE);
+
+			if (dwarf_siblingof(dwarf, 0, &returnDie, &error) == DW_DLV_OK) {
+				if ((dwarf_tag(returnDie, &tag, &error) == DW_DLV_OK)
+					&& tag == DW_TAG_compile_unit) {
+					if (die_has_pc(fobj, returnDie, addr)) {
+						found = true;
+					}
+                }
 			}
 		}
 
-		while (dwarf_next_cu_header_d(dwarf, 1, 0, 0, 0, 0, 0, 0, 0, 0,
-				&next_cu_header, 0, &error) == DW_DLV_OK) {
-			// Reset the cu header state. Unfortunately, libdwarf's
-			// next_cu_header API keeps its own iterator per Dwarf_Debug that
-			// can't be reset. We need to keep fetching elements until the end.
+		if (found) {
+			while (dwarf_next_cu_header_d(dwarf, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+					&next_cu_header, 0, &error) == DW_DLV_OK) {
+				// Reset the cu header state. Libdwarf's next_cu_header API
+				// keeps its own iterator per Dwarf_Debug that can't be reset.
+				// We need to keep fetching elements until the end.
+			}
 		}
 
 		if (found)
 			return returnDie;
 
-
 		// We couldn't find any compilation units with ranges or a high/low pc.
 		// Try again by looking at all DIEs in all compilation units.
 		Dwarf_Die cudie;
@@ -3071,11 +3080,13 @@ class TraceResolverLinuxImpl<trace_resolver_tag::libdwarf>:
 			}
 		}
 
-		while (dwarf_next_cu_header_d(dwarf, 1, 0, 0, 0, 0, 0, 0, 0, 0,
-				&next_cu_header, 0, &error) == DW_DLV_OK) {
-			// Reset the cu header state. Unfortunately, libdwarf's
-			// next_cu_header API keeps its own iterator per Dwarf_Debug that
-			// can't be reset. We need to keep fetching elements until the end.
+		if (found) {
+			while (dwarf_next_cu_header_d(dwarf, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+					&next_cu_header, 0, &error) == DW_DLV_OK) {
+				// Reset the cu header state. Libdwarf's next_cu_header API
+				// keeps its own iterator per Dwarf_Debug that can't be reset.
+				// We need to keep fetching elements until the end.
+			}
 		}
 
 		if (found)