[Assignment Tracking] Only set module flag if pass modifies a function

[Orlando]

Summary

The module flag "debug-info-assignment-tracking" is set by the declare-to-assign pass to indicate that assignment tracking is enabled. This patch changes declare-to-assign to only set the flag if it makes a modification. This lets the compiler avoid doing extra work for no benefit, such as is currently the case if assignment tracking is requested for a build with line tables only (-gmlt) or a build without debug info.

llvm/include/llvm/IR/DebugInfo.h

 /// intrinsics by treating stores to the dbg.declare'd address as assignments
 /// to the variable. Not all kinds of variables are supported yet; those will
 /// be left with their dbg.declare intrinsics.
 /// The pass sets the debug-info-assignment-tracking module flag to true to
 /// indicate assignment tracking has been enabled.
 class AssignmentTrackingPass : public PassInfoMixin<AssignmentTrackingPass> {
   /// Note: this method does not set the debug-info-assignment-tracking module
   /// flag.
-  void runOnFunction(Function &F);
+  bool runOnFunction(Function &F);
 
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
 };
 
 /// Return true if assignment tracking is enabled for module \p M.
 bool isAssignmentTrackingEnabled(const Module &M);
 } // end namespace llvm
 
 #endif // LLVM_IR_DEBUGINFO_H

llvm/lib/IR/DebugInfo.cpp

for (Instruction &I : *BBI) {
             emitDbgAssign(*Info, ValueComponent, DestComponent, I, R, DIB);
         (void)Assign;
         LLVM_DEBUG(errs() << " > INSERT: " << *Assign << "\n");
       }
     }
   }
 }
 
-void AssignmentTrackingPass::runOnFunction(Function &F) {
+bool AssignmentTrackingPass::runOnFunction(Function &F) {
+  bool Changed = false;
   // Collect a map of {backing storage : dbg.declares} (currently "backing
   // storage" is limited to Allocas). We'll use this to find dbg.declares to
   // delete after running `trackAssignments`.
   DenseMap<const AllocaInst *, SmallPtrSet<DbgDeclareInst *, 2>> DbgDeclares;
   // Create another similar map of {storage : variables} that we'll pass to
   // trackAssignments.
   StorageToVarsMap Vars;
   for (auto &BB : F) {
for (DbgDeclareInst *DDI : P.second) {
       // describing the same variable (i.e. check that this dbg.declare
       // has been replaced by a dbg.assign).
       assert(llvm::any_of(Markers, [DDI](DbgAssignIntrinsic *DAI) {
         return DebugVariable(DAI) == DebugVariable(DDI);
       }));
       // Delete DDI because the variable location is now tracked using
       // assignment tracking.
       DDI->eraseFromParent();
+      Changed = true;
     }
   }
+  return Changed;
 }
 
 static const char *AssignmentTrackingModuleFlag =
     "debug-info-assignment-tracking";
 
 static void setAssignmentTrackingModuleFlag(Module &M) {
   M.setModuleFlag(Module::ModFlagBehavior::Max, AssignmentTrackingModuleFlag,
                   ConstantAsMetadata::get(
                       ConstantInt::get(Type::getInt1Ty(M.getContext()), 1)));
 }
 
 static bool getAssignmentTrackingModuleFlag(const Module &M) {
   Metadata *Value = M.getModuleFlag(AssignmentTrackingModuleFlag);
   return Value && !cast<ConstantAsMetadata>(Value)->getValue()->isZeroValue();
 }
 
 bool llvm::isAssignmentTrackingEnabled(const Module &M) {
   return getAssignmentTrackingModuleFlag(M);
 }
 
 PreservedAnalyses AssignmentTrackingPass::run(Function &F,
                                               FunctionAnalysisManager &AM) {
-  runOnFunction(F);
+  if (!runOnFunction(F))
+    return PreservedAnalyses::all();
 
   // Record that this module uses assignment tracking. It doesn't matter that
   // some functons in the module may not use it - the debug info in those
   // functions will still be handled properly.
   setAssignmentTrackingModuleFlag(*F.getParent());
 
   // Q: Can we return a less conservative set than just CFGAnalyses? Can we
   // return PreservedAnalyses::all()?
   PreservedAnalyses PA;
   PA.preserveSet<CFGAnalyses>();
   return PA;
 }
 
 PreservedAnalyses AssignmentTrackingPass::run(Module &M,
                                               ModuleAnalysisManager &AM) {
+  bool Changed = false;
   for (auto &F : M)
-    runOnFunction(F);
+    Changed |= runOnFunction(F);
+
+  if (!Changed)
+    return PreservedAnalyses::all();
 
   // Record that this module uses assignment tracking.
   setAssignmentTrackingModuleFlag(M);
 
   // Q: Can we return a less conservative set than just CFGAnalyses? Can we
   // return PreservedAnalyses::all()?
   PreservedAnalyses PA;
   PA.preserveSet<CFGAnalyses>();
   return PA;
 }
 
 #undef DEBUG_TYPE

llvm/test/DebugInfo/Generic/assignment-tracking/declare-to-assign/set-flag-only-if-modified.ll

+; RUN: opt -passes=declare-to-assign -S %s -o - \
+; RUN: | FileCheck %s --check-prefix=WITHOUT-INTRINSIC
+
+; RUN: sed 's/;Uncomment-with-sed//g' < %s \
+; RUN: | opt -passes=declare-to-assign -S - -o - \
+; RUN: | FileCheck %s --check-prefix=WITH-INTRINSIC
+
+; WITHOUT-INTRINSIC-NOT: "debug-info-assignment-tracking"
+; WITH-INTRINSIC: "debug-info-assignment-tracking"
+
+;; Check that the module flag "debug-info-assignment-tracking" is only added if
+;; declare-to-assign actually modifies the function. In the first run the
+;; dbg.declare is commented out and the pass leaves the function unmodified.
+;; In the second run sed is used to uncomment the dbg.declare and
+;; declare-to-assign replaces it with a dbg.assign.
+
+define dso_local void @f() !dbg !9 {
+entry:
+  %a = alloca i32, align 4
+;Uncomment-with-sed  call void @llvm.dbg.declare(metadata ptr %a, metadata !13, metadata !DIExpression()), !dbg !16
+  ret void, !dbg !17
+}
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5, !6, !7}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 17.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 8, !"PIC Level", i32 2}
+!6 = !{i32 7, !"PIE Level", i32 2}
+!7 = !{i32 7, !"uwtable", i32 2}
+!8 = !{!"clang version 17.0.0"}
+!9 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 1, type: !10, scopeLine: 1, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!10 = !DISubroutineType(types: !11)
+!11 = !{null}
+!12 = !{!13}
+!13 = !DILocalVariable(name: "a", scope: !9, file: !1, line: 1, type: !14)
+!14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!15 = !DILocation(line: 1, column: 12, scope: !9)
+!16 = !DILocation(line: 1, column: 16, scope: !9)
+!17 = !DILocation(line: 1, column: 19, scope: !9)