[StripDeadDebugInfo] Drop dead CUs entirely

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.

Reviewed By: aprantl
Differential Revision: https://reviews.llvm.org/D31720

llvm-svn: 299692

2 files changed, 34 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/IPO/StripSymbols.cpp b/llvm/lib/Transforms/IPO/StripSymbols.cpp
index 8f6f161428e..4d52f5f3110 100644
--- a/llvm/lib/Transforms/IPO/StripSymbols.cpp
+++ b/llvm/lib/Transforms/IPO/StripSymbols.cpp
@@ -323,6 +323,15 @@ bool StripDeadDebugInfo::runOnModule(Module &M) {
       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;
@@ -341,6 +350,11 @@ bool StripDeadDebugInfo::runOnModule(Module &M) {
         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) {
@@ -352,5 +366,16 @@ bool StripDeadDebugInfo::runOnModule(Module &M) {
     LiveGlobalVariables.clear();
   }
 
+  if (HasDeadCUs) {
+    // Delete the old node and replace it with a new one
+    NamedMDNode *NMD = M.getOrInsertNamedMetadata("llvm.dbg.cu");
+    NMD->clearOperands();
+    if (!LiveCUs.empty()) {
+      for (DICompileUnit *CU : LiveCUs)
+        NMD->addOperand(CU);
+    }
+    Changed = true;
+  }
+
   return Changed;
 }
diff --git a/llvm/test/Transforms/StripSymbols/strip-dead-debug-info.ll b/llvm/test/Transforms/StripSymbols/strip-dead-debug-info.ll
index 0e252d70465..eb8ff50137c 100644
--- a/llvm/test/Transforms/StripSymbols/strip-dead-debug-info.ll
+++ b/llvm/test/Transforms/StripSymbols/strip-dead-debug-info.ll
@@ -3,6 +3,9 @@
 ; CHECK: ModuleID = '{{.*}}'
 ; CHECK-NOT: "bar"
 ; CHECK-NOT: "abcd"
+; CHECK-NOT: "GCC"
+; CHECK: "Globals"
+; CHECK: "abcd2"
 
 source_filename = "test/Transforms/StripSymbols/strip-dead-debug-info.ll"
 
@@ -29,7 +32,7 @@ attributes #0 = { nounwind readnone }
 attributes #1 = { nounwind readnone ssp }
 attributes #2 = { nounwind readonly ssp }
 
-!llvm.dbg.cu = !{!4}
+!llvm.dbg.cu = !{!4, !23, !24}
 !llvm.module.flags = !{!9}
 
 !0 = !DIGlobalVariableExpression(var: !1)
@@ -55,4 +58,8 @@ attributes #2 = { nounwind readonly ssp }
 !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)
+!24 = distinct !DICompileUnit(language: DW_LANG_C89, file: !2, producer: "Globals", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, retainedTypes: !5, globals: !25)
+!25 = !{!26}
+!26 = !DIGlobalVariableExpression(var: !27, expr: !DIExpression(DW_OP_constu, 0, DW_OP_stack_value))
+!27 = !DIGlobalVariable(name: "abcd2", scope: !2, file: !2, line: 2, type: !3, isLocal: true, isDefinition: true)