[lldb/DWARF] Fix DW_AT_addr_base & DW_AT_low_pc interaction

In DWARF5 DW_AT_low_pc (and DW_AT_entry_pc, and possibly others) can use
DW_FORM_addrx to refer to the address indirectly. This means we need to
have processed the DW_AT_addr_base attribute before we can do anything
with these.

Since we were processing the unit attributes serially, this created a
problem in cases where the DW_AT_addr_base comes after DW_AT_low_pc --
we would end up computing the wrong unit base address, which also
corrupted any values which later depended on that (for instance range
lists). Clang currently always emits DW_AT_addr_base last.

The fix is simple -- process DW_AT_addr_base first, regardless of its
position in the attribute list.

1 files changed, 13 insertions, 4 deletions

diff --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp b/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp
index 35f5c30e177..072cbc2abc3 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp
@@ -287,16 +287,25 @@ void DWARFUnit::AddUnitDIE(const DWARFDebugInfoEntry &cu_die) {
 
   DWARFAttributes attributes;
   size_t num_attributes = cu_die.GetAttributes(this, attributes);
+
+  // Extract DW_AT_addr_base first, as other attributes may need it.
+  for (size_t i = 0; i < num_attributes; ++i) {
+    if (attributes.AttributeAtIndex(i) != DW_AT_addr_base)
+      continue;
+    DWARFFormValue form_value;
+    if (attributes.ExtractFormValueAtIndex(i, form_value)) {
+      addr_base = form_value.Unsigned();
+      SetAddrBase(*addr_base);
+      break;
+    }
+  }
+
   for (size_t i = 0; i < num_attributes; ++i) {
     dw_attr_t attr = attributes.AttributeAtIndex(i);
     DWARFFormValue form_value;
     if (!attributes.ExtractFormValueAtIndex(i, form_value))
       continue;
     switch (attr) {
-    case DW_AT_addr_base:
-      addr_base = form_value.Unsigned();
-      SetAddrBase(*addr_base);
-      break;
     case DW_AT_rnglists_base:
       ranges_base = form_value.Unsigned();
       SetRangesBase(*ranges_base);