[StripDeadDebugInfo] Drop dead CUs entirely

[loladiro]

Summary

Prior to this while it would delete the dead DIGlobalVariables, it would
leave dead DICompileUnits and everything referenced therefrom. For a bit
bitcode file with thousands of compile units those dead nodes easily
outnumbered the real ones. Clean that up.

[aprantl]

Can you add a CU that only has a DIGlobalVariableExpression with a DIExpression(DW_OP_constu 0, DW_OP_stack_value) to your testcase and verify that it survives?
Otherwise this LGTM.

lib/Transforms/IPO/StripSymbols.cpp

327

. at the end :-)

353

remove extra parens?

373

It might be faster to just call clearOperands() instead of deleting the entire node?

lib/Transforms/IPO/StripSymbols.cpp

bool StripDeadDebugInfo::runOnModule(Module &M) {
   std::set<DIGlobalVariableExpression *> LiveGVs;
   for (GlobalVariable &GV : M.globals()) {
     SmallVector<DIGlobalVariableExpression *, 1> GVEs;
     GV.getDebugInfo(GVEs);
     for (auto *GVE : GVEs)
       LiveGVs.insert(GVE);
   }
 
+  std::set<DICompileUnit *> LiveCUs;
+  // Any CU referenced from a function is live
+  for (Function &F : M.functions()) {
+    DISubprogram *SP = F.getSubprogram();
+    if (SP && SP->getUnit())
+      LiveCUs.insert(SP->getUnit());
+  }
+
+  bool HasDeadCUs = false;
   for (DICompileUnit *DIC : F.compile_units()) {
     // Create our live global variable list.
     bool GlobalVariableChange = false;
     for (auto *DIG : DIC->getGlobalVariables()) {
       if (DIG->getExpression() && DIG->getExpression()->isConstant())
         LiveGVs.insert(DIG);
 
       // Make sure we only visit each global variable only once.
       if (!VisitedSet.insert(DIG).second)
         continue;
 
       // If a global variable references DIG, the global variable is live.
       if (LiveGVs.count(DIG))
         LiveGlobalVariables.push_back(DIG);
       else
         GlobalVariableChange = true;
     }
 
+    if (!LiveGlobalVariables.empty()) {
+      LiveCUs.insert(DIC);
+    } else if (!LiveCUs.count(DIC)) {
+      HasDeadCUs = true;
+    }
+
     // If we found dead global variables, replace the current global
     // variable list with our new live global variable list.
     if (GlobalVariableChange) {
       DIC->replaceGlobalVariables(MDTuple::get(C, LiveGlobalVariables));
       Changed = true;
     }
 
     // Reset lists for the next iteration.
     LiveGlobalVariables.clear();
   }
 
+  if (HasDeadCUs) {
+    // Delete the old node and replace it with a new one
+    NamedMDNode *NMD = M.getOrInsertNamedMetadata("llvm.dbg.cu");
+    NMD->eraseFromParent();
+    if (!LiveCUs.empty()) {
+      NMD = M.getOrInsertNamedMetadata("llvm.dbg.cu");
+      for (DICompileUnit *CU : LiveCUs)
+        NMD->addOperand(CU);
+    }
+    Changed = true;
+  }
+
   return Changed;
 }

test/Transforms/StripSymbols/strip-dead-debug-info.ll

 ; RUN: opt -strip-dead-debug-info -verify %s -S | FileCheck %s
 
 ; CHECK: ModuleID = '{{.*}}'
 ; CHECK-NOT: "bar"
 ; CHECK-NOT: "abcd"
+; CHECK-NOT: "GCC"
 
 source_filename = "test/Transforms/StripSymbols/strip-dead-debug-info.ll"
 
 @xyz = global i32 2, !dbg !0
 
 ; Function Attrs: nounwind readnone
 declare void @llvm.dbg.value(metadata, i64, metadata, metadata) #0
 
entry:
   %.0 = load i32, i32* @xyz, align 4
   ret i32 %.0, !dbg !21
 }
 
 attributes #0 = { nounwind readnone }
 attributes #1 = { nounwind readnone ssp }
 attributes #2 = { nounwind readonly ssp }
 
-!llvm.dbg.cu = !{!4}
+!llvm.dbg.cu = !{!4, !23}
 !llvm.module.flags = !{!9}
 
 !0 = !DIGlobalVariableExpression(var: !1)
 !1 = !DIGlobalVariable(name: "xyz", scope: !2, file: !2, line: 3, type: !3, isLocal: false, isDefinition: true)
 !2 = !DIFile(filename: "g.c", directory: "/tmp/")
 !3 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
 !4 = distinct !DICompileUnit(language: DW_LANG_C89, file: !2, producer: "4.2.1 (Based on Apple Inc. build 5658) (LLVM build)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, retainedTypes: !5, globals: !6)
 !5 = !{}

 !15 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: null, file: !2, line: 7, type: !16, isLocal: false, isDefinition: true, virtualIndex: 6, isOptimized: true, unit: !4)
 !16 = !DISubroutineType(types: !17)
 !17 = !{!3, !3}
 !18 = !DILocalVariable(name: "i", arg: 1, scope: !15, file: !2, line: 7, type: !3)
 !19 = !DIExpression()
 !20 = !DILocation(line: 7, scope: !15)
 !21 = !DILocation(line: 10, scope: !22)
 !22 = distinct !DILexicalBlock(scope: !15, file: !2, line: 7)
-
+!23 = distinct !DICompileUnit(language: DW_LANG_C89, file: !2, producer: "GCC", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, retainedTypes: !5, globals: !5)