[Verifier] Avoid visiting DIGlobalVariables twice

[davide]

Summary

We currently visit them twice.
Once, through visitMDNode() -> (the code generated by) ../include/llvm/IR/Metadata.def:109 -> visitDIGlobalVariable()
Then, through visitMDNode() -> visitDIGlobalVariableExpression() -> visitDIGlobalVariable()

This results in verification failures printed twice, e.g.:

$ ./opt -verify ../../test/DebugInfo/pr34186.ll
missing global variable type
!4 = distinct !DIGlobalVariable(name: "pat", scope: !0, file: !1, line: 27, isLocal: true, isDefinition: true)
missing global variable type
!4 = distinct !DIGlobalVariable(name: "pat", scope: !0, file: !1, line: 27, isLocal: true, isDefinition: true)
./opt: ../../test/DebugInfo/pr34186.ll: error: input module is broken!

The patch removes one call so we ensure each GV is visited exactly once.

[aprantl]

Does this still catch the case where that variable is a nullptr? If yes, this LGTM.

[davide]

In D36797#843446, @aprantl wrote:

Does this still catch the case where that variable is a nullptr? If yes, this LGTM.

Yes, it does from what I can see.

lib/IR/Verifier.cpp

 
 void Verifier::visitDIExpression(const DIExpression &N) {
   AssertDI(N.isValid(), "invalid expression", &N);
 }
 
 void Verifier::visitDIGlobalVariableExpression(
     const DIGlobalVariableExpression &GVE) {
   AssertDI(GVE.getVariable(), "missing variable");
-  if (auto *Var = GVE.getVariable())
-    visitDIGlobalVariable(*Var);
   if (auto *Expr = GVE.getExpression())
     visitDIExpression(*Expr);
 }
 
 void Verifier::visitDIObjCProperty(const DIObjCProperty &N) {
   AssertDI(N.getTag() == dwarf::DW_TAG_APPLE_property, "invalid tag", &N);
   if (auto *T = N.getRawType())
     AssertDI(isType(T), "invalid type ref", &N, T);

test/DebugInfo/pr34186.ll

+; Make sure we reject GVs without a type and we verify each exactly once.
 ; RUN: not llc %s 2>&1 | FileCheck %s
 ; CHECK: missing global variable type
+; CHECK-NOT: missing global variable type
 
 !llvm.dbg.cu = !{!2}
 !llvm.module.flags = !{!63, !64}
 !1 = distinct !DIGlobalVariable(name: "pat", scope: !2, file: !3, line: 27, isLocal: true, isDefinition: true)
 !2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "", emissionKind: FullDebug, globals: !5)
 !3 = !DIFile(filename: "patatino.c", directory: "/")
 !5 = !{!6}
 !6 = !DIGlobalVariableExpression(var: !1)
 !63 = !{i32 2, !"Dwarf Version", i32 4}
 !64 = !{i32 2, !"Debug Info Version", i32 3}