[DebugInfo] Remove redundant initialization of DbgVariable's MInsn, NFC

[dstenb]

Summary

When building a location list, the first debug value for a variable was
passed to DebugLocStream::ListBuilder. Later on, in ListBuilder()'s
destructor, initializeDbgValue() was called with the passed debug value,
setting the DbgVariable's MInsn pointer. A non-null MInsn says that the
variable is described by a single debug value through the variable's
scope, which is not the case when we have begun building a location
list.

In DwarfCompileUnit::constructVariableDIEImpl(), the presence of a
location list index is checked before emitting a single location, so
setting MInsn for variables expressed by location lists did not have any
effect in practice. However, as it is unnecessary with the current code
base, remove that initialization.

I have verified that `llvm-dwarfdump -v' produces the same location
output with respectively without this patch when building a
RelWithDebInfo-profiled clang-tblgen binary.

[dstenb]

Ping.

[StephenTozer]

Looks like a sensible straightforward change. Someone closer to the code in particular should approve as well before this lands, but this should serve as a ping.

lib/CodeGen/AsmPrinter/DebugLocStream.h

private:
   }
 };
 
 /// Builder for DebugLocStream lists.
 class DebugLocStream::ListBuilder {
   DebugLocStream &Locs;
   AsmPrinter &Asm;
   DbgVariable &V;
-  const MachineInstr &MI;
   size_t ListIndex;
 
 public:
   ListBuilder(DebugLocStream &Locs, DwarfCompileUnit &CU, AsmPrinter &Asm,
-              DbgVariable &V, const MachineInstr &MI)
-      : Locs(Locs), Asm(Asm), V(V), MI(MI), ListIndex(Locs.startList(&CU)) {}
+              DbgVariable &V)
+      : Locs(Locs), Asm(Asm), V(V), ListIndex(Locs.startList(&CU)) {}
 
   /// Finalize the list.
   ///
   /// If the list is empty, delete it.  Otherwise, finalize it by creating a
   /// temp symbol in \a Asm and setting up the \a DbgVariable.
   ~ListBuilder();
 
   DebugLocStream &getLocs() { return Locs; }

lib/CodeGen/AsmPrinter/DebugLocStream.cpp

void DebugLocStream::finalizeEntry() {
 
   assert(Lists.back().EntryOffset <= Entries.size() &&
          "Popped off more entries than are in the list");
 }
 
 DebugLocStream::ListBuilder::~ListBuilder() {
   if (!Locs.finalizeList(Asm))
     return;
-  V.initializeDbgValue(&MI);
+  assert(!V.getMInsn() && "Already initialized with a single DBG_VALUE");
   V.setDebugLocListIndex(ListIndex);
 }

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

if (Ranges.size() == 1 &&
       RegVar->initializeDbgValue(MInsn);
       continue;
     }
     // Do not emit location lists if .debug_loc secton is disabled.
     if (!useLocSection())
       continue;
 
     // Handle multiple DBG_VALUE instructions describing one variable.
-    DebugLocStream::ListBuilder List(DebugLocs, TheCU, *Asm, *RegVar, *MInsn);
+    DebugLocStream::ListBuilder List(DebugLocs, TheCU, *Asm, *RegVar);
 
     // Build the location list for this variable.
     SmallVector<DebugLocEntry, 8> Entries;
     buildLocationList(Entries, Ranges);
 
     // If the variable has a DIBasicType, extract it.  Basic types cannot have
     // unique identifiers, so don't bother resolving the type with the
     // identifier map.