[PATCH] D18477: Drop debug info for DISubprograms that are not referenced by anything

[Adrian Prantl via llvm-commits]

> On Apr 6, 2016, at 11:07 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
>> On 2016-Apr-06, at 17:09, Adrian Prantl <aprantl at apple.com> wrote:
>> 
>> aprantl added a comment.
>> 
>> Ping.
>> 
>> 
>> http://reviews.llvm.org/D18477
> 
> There's something I don't quite understand in the DwarfDebug.cpp which
> I point out below.  Heading out on vacation, so I'm happy to defer to
> Eric/David/you/etc. if it makes sense (LGTM once others are happy).
> 
>> Index: lib/CodeGen/AsmPrinter/DwarfDebug.cpp
>> ===================================================================
>> --- lib/CodeGen/AsmPrinter/DwarfDebug.cpp
>> +++ lib/CodeGen/AsmPrinter/DwarfDebug.cpp
>> @@ -527,29 +527,10 @@
>> 
>> void DwarfDebug::finishSubprogramDefinitions() {
>>   for (const auto &P : SPMap)
>> -    forBothCUs(*P.second, [&](DwarfCompileUnit &CU) {
>> -      CU.finishSubprogramDefinition(cast<DISubprogram>(P.first));
>> -    });
>> -}
>> -
>> -// Collect info for variables that were optimized out.
>> -void DwarfDebug::collectDeadVariables() {
>> -  const Module *M = MMI->getModule();
>> -
>> -  if (NamedMDNode *CU_Nodes = M->getNamedMetadata("llvm.dbg.cu")) {
>> -    for (MDNode *N : CU_Nodes->operands()) {
>> -      auto *TheCU = cast<DICompileUnit>(N);
>> -      // Construct subprogram DIE and add variables DIEs.
>> -      DwarfCompileUnit *SPCU =
>> -          static_cast<DwarfCompileUnit *>(CUMap.lookup(TheCU));
>> -      assert(SPCU && "Unable to find Compile Unit!");
>> -      for (auto *SP : TheCU->getSubprograms()) {
>> -        if (ProcessedSPNodes.count(SP) != 0)
>> -          continue;
>> -        SPCU->collectDeadVariables(SP);
>> -      }
>> -    }
>> -  }
>> +    if (ProcessedSPNodes.count(P.first))
>> +      forBothCUs(*P.second, [&](DwarfCompileUnit &CU) {
>> +          CU.finishSubprogramDefinition(cast<DISubprogram>(P.first));
>> +        });
> 
> ^ remember this...
> 
>> }
>> 
>> void DwarfDebug::finalizeModuleInfo() {
>> @@ -559,9 +540,6 @@
>> 
>>   finishVariableDefinitions();
>> 
>> -  // Collect info for variables that were optimized out.
>> -  collectDeadVariables();
>> -
>>   // Handle anything that needs to be done on a per-unit basis after
>>   // all other generation.
>>   for (const auto &P : CUMap) {
>> @@ -1121,6 +1099,10 @@
>>     PrevCU = nullptr;
>>     CurFn = nullptr;
>>     DebugHandlerBase::endFunction(MF);
>> +    // Mark functions with no debug info on any instructions, but a
>> +    // valid DISubprogram as processed.
>> +    if (auto *SP = MF->getFunction()->getSubprogram())
>> +      ProcessedSPNodes.insert(SP);
> 
> Why do you want to run the above code for these subprograms?
> Don't you want to skip them entirely?

The comment is describing the situation in DebugInfo/X86/dbg-file-name.ll
; Verify that the file name is relative to the directory.
; rdar://problem/8884898
; CHECK: file	1 "simple.c"

define i32 @main() nounwind !dbg !6 {
  ret i32 0
}

!6 = distinct !DISubprogram(name: "main", linkageName: "main", line: 9, isLocal: false, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: false, file: !10, scope: !1, type: !7)

where a function has a DISubprogram attached to it, but none of its instructions have a valid location. We still want to emit an SP DIE for these functions. (Or at least we have done so in the past).

-- adrian
> 
>>     return;
>>   }
>> 
>> 
>