DWARF: Fix abbreviation lookups, they are relative to 1 (#3158)

Apparently I misunderstood the DWARF spec on this. Abbreviation offsets are all
relative to 1 (as 0 is not a valid number). For some reason I thought the first DIE's
index was the "base", as in practice LLVM always emits the lowest index there,
and that's what the LLVM YAML code suggested to me.

1 files changed, 6 insertions, 6 deletions

diff --git a/third_party/llvm-project/DWARFVisitor.cpp b/third_party/llvm-project/DWARFVisitor.cpp
index 4b6ed5ada..22df78639 100644
--- a/third_party/llvm-project/DWARFVisitor.cpp
+++ b/third_party/llvm-project/DWARFVisitor.cpp
@@ -101,19 +101,19 @@ template <typename T> void DWARFYAML::VisitorImpl<T>::traverseDebugInfo() {
     if (Unit.Entries.empty()) { // XXX BINARYEN
       continue;
     }
-    auto FirstAbbrevCode = Unit.Entries[0].AbbrCode;
     for (auto &Entry : Unit.Entries) {
       onStartDIE(Unit, Entry);
       if (Entry.AbbrCode == 0u)
         continue;
-      // XXX BINARYEN
-      if (Entry.AbbrCode - FirstAbbrevCode + AbbrevStart >= AbbrevEnd) {
+      // XXX BINARYEN valid abbreviation codes start from 1, so subtract that,
+      //              and are relative to the start of the abbrev table
+      auto RelativeAbbrIndex = Entry.AbbrCode - 1 + AbbrevStart;
+      if (RelativeAbbrIndex >= AbbrevEnd) {
         errs() << "warning: invalid abbreviation code " << Entry.AbbrCode
-               << " (range: " << FirstAbbrevCode << " : " << AbbrevStart
-               << ".." << AbbrevEnd << ")\n";
+               << " (range: " << AbbrevStart << ".." << AbbrevEnd << ")\n";
         continue;
       }
-      auto &Abbrev = DebugInfo.AbbrevDecls[Entry.AbbrCode - FirstAbbrevCode + AbbrevStart];
+      auto &Abbrev = DebugInfo.AbbrevDecls[RelativeAbbrIndex];
       auto FormVal = Entry.Values.begin();
       auto AbbrForm = Abbrev.Attributes.begin();
       for (;