[indvars] Missing variables at Og: (#69920)

https://bugs.llvm.org/show_bug.cgi?id=51735
#51077

In the given test case:
 ```
4 ...
 5 void bar() {
 6   int End = 777;
 7   int Index = 27;
 8   char Var = 1;
 9   for (; Index < End; ++Index)
10     ;
11   nop(Index);
12 }
13 ...
```
Missing local variable `Index` after loop `Induction Variable Elimination`. When adding a breakpoint at line `11`, LLDB does not have information on the variable. But it has info on `Var` and `End`.

llvm/include/llvm/Support/GenericLoopInfo.h

@@ -44,6 +44,8 @@
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetOperations.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/ValueHandle.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/GenericDomTree.h"
 
@@ -75,6 +77,13 @@ template <class BlockT, class LoopT> class LoopBase {
   const LoopBase<BlockT, LoopT> &
   operator=(const LoopBase<BlockT, LoopT> &) = delete;
 
+  // Induction variable exit value and its debug users, preserved by the
+  // 'indvars' pass, when it detects that the loop can be deleted and the
+  // there are no PHIs to be rewritten.
+  // For now, we only preserve single induction variables.
+  Value *IndVarFinalValue = nullptr;
+  SmallVector<WeakVH> IndVarDebugUsers;
+
 public:
   /// Return the nesting level of this loop.  An outer-most loop has depth 1,
   /// for consistency with loop depth values used for basic blocks, where depth
@@ -251,6 +260,21 @@ template <class BlockT, class LoopT> class LoopBase {
                           [&](BlockT *Pred) { return contains(Pred); });
   }
 
+  /// Preserve the induction variable exit value and its debug users by the
+  /// 'indvars' pass if the loop can deleted. Those debug users will be used
+  /// by the 'loop-delete' pass.
+  void preserveDebugInductionVariableInfo(
+      Value *FinalValue, SmallVector<DbgVariableIntrinsic *> DbgUsers) {
+    IndVarFinalValue = FinalValue;
+    for (DbgVariableIntrinsic *DebugUser : DbgUsers)
+      IndVarDebugUsers.push_back(DebugUser);
+  }
+
+  Value *getDebugInductionVariableFinalValue() { return IndVarFinalValue; }
+  SmallVector<WeakVH> &getDebugInductionVariableDebugUsers() {
+    return IndVarDebugUsers;
+  }
+
   //===--------------------------------------------------------------------===//
   // APIs for simple analysis of the loop.
   //

llvm/include/llvm/Transforms/Utils/LoopUtils.h

@@ -468,6 +468,12 @@ int rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
                           ReplaceExitVal ReplaceExitValue,
                           SmallVector<WeakTrackingVH, 16> &DeadInsts);
 
+/// Assign exit values to variables that use this loop variable during the loop.
+void addDebugValuesToIncomingValue(BasicBlock *Successor, Value *IndVar,
+                                   PHINode *PN);
+void addDebugValuesToLoopVariable(BasicBlock *Successor, Value *ExitValue,
+                                  PHINode *PN);
+
 /// Set weights for \p UnrolledLoop and \p RemainderLoop based on weights for
 /// \p OrigLoop and the following distribution of \p OrigLoop iteration among \p
 /// UnrolledLoop and \p RemainderLoop. \p UnrolledLoop receives weights that

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -31,6 +31,7 @@
 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/IR/DIBuilder.h"
+#include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
@@ -608,6 +609,17 @@ void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
   llvm::SmallVector<DbgVariableRecord *, 4> DeadDbgVariableRecords;
 
   if (ExitBlock) {
+    if (ExitBlock->phis().empty()) {
+      // As the loop is deleted, replace the debug users with the preserved
+      // induction variable final value recorded by the 'indvar' pass.
+      Value *FinalValue = L->getDebugInductionVariableFinalValue();
+      SmallVector<WeakVH> &DbgUsers = L->getDebugInductionVariableDebugUsers();
+      for (WeakVH &DebugUser : DbgUsers)
+        if (DebugUser)
+          cast<DbgVariableIntrinsic>(DebugUser)->replaceVariableLocationOp(
+              0u, FinalValue);
+    }
+
     // Given LCSSA form is satisfied, we should not have users of instructions
     // within the dead loop outside of the loop. However, LCSSA doesn't take
     // unreachable uses into account. We handle them here.
@@ -1401,6 +1413,36 @@ static bool checkIsIndPhi(PHINode *Phi, Loop *L, ScalarEvolution *SE,
   return InductionDescriptor::isInductionPHI(Phi, L, SE, ID);
 }
 
+void llvm::addDebugValuesToIncomingValue(BasicBlock *Successor, Value *IndVar,
+                                         PHINode *PN) {
+  SmallVector<DbgVariableIntrinsic *> DbgUsers;
+  findDbgUsers(DbgUsers, IndVar);
+  for (auto *DebugUser : DbgUsers) {
+    // Skip debug-users with variadic variable locations; they will not,
+    // get updated, which is fine as that is the existing behaviour.
+    if (DebugUser->hasArgList())
+      continue;
+    auto *Cloned = cast<DbgVariableIntrinsic>(DebugUser->clone());
+    Cloned->replaceVariableLocationOp(0u, PN);
+    Cloned->insertBefore(*Successor, Successor->getFirstNonPHIIt());
+  }
+}
+
+void llvm::addDebugValuesToLoopVariable(BasicBlock *Successor, Value *ExitValue,
+                                        PHINode *PN) {
+  SmallVector<DbgVariableIntrinsic *> DbgUsers;
+  findDbgUsers(DbgUsers, PN);
+  for (auto *DebugUser : DbgUsers) {
+    // Skip debug-users with variadic variable locations; they will not,
+    // get updated, which is fine as that is the existing behaviour.
+    if (DebugUser->hasArgList())
+      continue;
+    auto *Cloned = cast<DbgVariableIntrinsic>(DebugUser->clone());
+    Cloned->replaceVariableLocationOp(0u, ExitValue);
+    Cloned->insertBefore(*Successor, Successor->getFirstNonPHIIt());
+  }
+}
+
 int llvm::rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
                                 ScalarEvolution *SE,
                                 const TargetTransformInfo *TTI,
@@ -1542,6 +1584,10 @@ int llvm::rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
           (isa<PHINode>(Inst) || isa<LandingPadInst>(Inst)) ?
           &*Inst->getParent()->getFirstInsertionPt() : Inst;
         RewritePhiSet.emplace_back(PN, i, ExitValue, InsertPt, HighCost);
+
+        // Add debug values for the candidate PHINode incoming value.
+        if (BasicBlock *Successor = ExitBB->getSingleSuccessor())
+          addDebugValuesToIncomingValue(Successor, PN->getIncomingValue(i), PN);
       }
     }
   }
@@ -1600,11 +1646,30 @@ int llvm::rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
     // Replace PN with ExitVal if that is legal and does not break LCSSA.
     if (PN->getNumIncomingValues() == 1 &&
         LI->replacementPreservesLCSSAForm(PN, ExitVal)) {
+      addDebugValuesToLoopVariable(PN->getParent(), ExitVal, PN);
       PN->replaceAllUsesWith(ExitVal);
       PN->eraseFromParent();
     }
   }
 
+  // If the loop can be deleted and there are no PHIs to be rewritten (there
+  // are no loop live-out values), record debug variables corresponding to the
+  // induction variable with their constant exit-values. Those values will be
+  // inserted by the 'deletion loop' logic.
+  if (LoopCanBeDel && RewritePhiSet.empty()) {
+    if (auto *IndVar = L->getInductionVariable(*SE)) {
+      const SCEV *PNSCEV = SE->getSCEVAtScope(IndVar, L->getParentLoop());
+      if (auto *Const = dyn_cast<SCEVConstant>(PNSCEV)) {
+        Value *FinalIVValue = Const->getValue();
+        if (L->getUniqueExitBlock()) {
+          SmallVector<DbgVariableIntrinsic *> DbgUsers;
+          findDbgUsers(DbgUsers, IndVar);
+          L->preserveDebugInductionVariableInfo(FinalIVValue, DbgUsers);
+        }
+      }
+    }
+  }
+
   // The insertion point instruction may have been deleted; clear it out
   // so that the rewriter doesn't trip over it later.
   Rewriter.clearInsertPoint();

llvm/test/Transforms/IndVarSimplify/pr51735-1.ll

@@ -0,0 +1,129 @@
+; RUN: opt -passes="loop(indvars)" \
+; RUN:     --experimental-debuginfo-iterators=false -S -o - < %s | \
+; RUN: FileCheck --check-prefix=CHECK %s
+; RUN: opt -passes="loop(indvars,loop-deletion)" \
+; RUN:     --experimental-debuginfo-iterators=false -S -o - < %s | \
+; RUN: FileCheck --check-prefix=CHECK %s
+
+; Make sure that when we delete the loop, that the variable Index has
+; the 777 value.
+
+; As this test case does fire the 'indvars' transformation, the debug values
+; are added to the 'for.end' exit block. No debug values are preserved by the
+; pass to be used by the 'loop-deletion' pass.
+
+; CHECK: for.cond:
+; CHECK:   call void @llvm.dbg.value(metadata i32 %[[SSA_INDEX_0:.+]], metadata ![[DBG:[0-9]+]], {{.*}}
+
+; CHECK: for.extra:
+; CHECK:   %[[SSA_CALL_0:.+]] = call noundef i32 @"?nop@@YAHH@Z"(i32 noundef %[[SSA_INDEX_0]]), {{.*}}
+; CHECK:   br i1 %[[SSA_CMP_0:.+]], label %for.cond, label %if.else, {{.*}}
+
+; CHECK: if.then:
+; CHECK:   call void @llvm.dbg.value(metadata i32 777, metadata ![[DBG]], {{.*}}
+; CHECK:   call void @llvm.dbg.value(metadata i32 %[[SSA_VAR_1:.+]], metadata ![[VAR:[0-9]+]], {{.*}}
+; CHECK:   br label %for.end, {{.*}}
+
+; CHECK: if.else:
+; CHECK:   call void @llvm.dbg.value(metadata i32 %[[SSA_VAR_2:.+]], metadata ![[VAR:[0-9]+]], {{.*}}
+; CHECK:   br label %for.end, {{.*}}
+
+; CHECK: for.end:
+; CHECK:   call void @llvm.dbg.value(metadata i32 777, metadata ![[DBG]], {{.*}}
+
+; CHECK-DAG: ![[DBG]] = !DILocalVariable(name: "Index"{{.*}})
+; CHECK-DAG: ![[VAR]] = !DILocalVariable(name: "Var"{{.*}})
+
+define dso_local noundef i32 @"?nop@@YAHH@Z"(i32 noundef %Param) !dbg !11 {
+entry:
+  %Param.addr = alloca i32, align 4
+  store i32 %Param, ptr %Param.addr, align 4
+  call void @llvm.dbg.declare(metadata ptr %Param.addr, metadata !32, metadata !DIExpression()), !dbg !35
+  ret i32 0, !dbg !36
+}
+
+define dso_local void @_Z3barv() local_unnamed_addr #1 !dbg !12 {
+entry:
+  call void @llvm.dbg.value(metadata i32 777, metadata !16, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 27, metadata !18, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 1, metadata !19, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 1, metadata !30, metadata !DIExpression()), !dbg !17
+  br label %for.cond, !dbg !20
+
+for.cond:                                         ; preds = %for.cond, %entry
+  %Index.0 = phi i32 [ 27, %entry ], [ %inc, %for.extra ], !dbg !17
+  call void @llvm.dbg.value(metadata i32 %Index.0, metadata !18, metadata !DIExpression()), !dbg !17
+  %cmp = icmp ult i32 %Index.0, 777, !dbg !21
+  %inc = add nuw nsw i32 %Index.0, 1, !dbg !24
+  call void @llvm.dbg.value(metadata i32 %inc, metadata !18, metadata !DIExpression()), !dbg !17
+  br i1 %cmp, label %for.extra, label %if.then, !dbg !25, !llvm.loop !26
+
+for.extra:
+  %call.0 = call noundef i32 @"?nop@@YAHH@Z"(i32 noundef %Index.0), !dbg !21
+  %cmp.0 = icmp ult i32 %Index.0, %call.0, !dbg !21
+  br i1 %cmp.0, label %for.cond, label %if.else, !dbg !25, !llvm.loop !26
+
+if.then:                                          ; preds = %for.cond
+  %Var.1 = add nsw i32 %Index.0, 1, !dbg !20
+  call void @llvm.dbg.value(metadata i32 %Var.1, metadata !19, metadata !DIExpression()), !dbg !20
+  br label %for.end, !dbg !20
+
+if.else:
+  %Var.2 = add nsw i32 %Index.0, 2, !dbg !20
+  call void @llvm.dbg.value(metadata i32 %Var.2, metadata !19, metadata !DIExpression()), !dbg !20
+  br label %for.end, !dbg !20
+
+for.end:                                          ; preds = %if.else, %if.then
+  %Zeta.0 = phi i32 [ %Var.1, %if.then ], [ %Var.2, %if.else ], !dbg !20
+  call void @llvm.dbg.value(metadata i32 %Zeta.0, metadata !30, metadata !DIExpression()), !dbg !20
+  %Var.3 = add nsw i32 %Index.0, 1, !dbg !20
+  call void @llvm.dbg.value(metadata i32 %Var.3, metadata !19, metadata !DIExpression()), !dbg !20
+  %call = call noundef i32 @"?nop@@YAHH@Z"(i32 noundef %Index.0), !dbg !37
+  ret void, !dbg !29
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5, !6, !7, !8}
+!llvm.ident = !{!9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", 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 = !{i32 7, !"frame-pointer", i32 2}
+!9 = !{!"clang version 18.0.0"}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = distinct !DISubprogram(name: "nop", linkageName: "?nop@@YAHH@Z", scope: !1, file: !1, line: 1, type: !33, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !31)
+!12 = distinct !DISubprogram(name: "bar", linkageName: "_Z3barv", scope: !1, file: !1, line: 5, type: !13, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !15)
+!13 = !DISubroutineType(types: !14)
+!14 = !{null}
+!15 = !{}
+!16 = !DILocalVariable(name: "End", scope: !12, file: !1, line: 6, type: !10)
+!17 = !DILocation(line: 0, scope: !12)
+!18 = !DILocalVariable(name: "Index", scope: !12, file: !1, line: 7, type: !10)
+!19 = !DILocalVariable(name: "Var", scope: !12, file: !1, line: 8, type: !10)
+!20 = !DILocation(line: 9, column: 3, scope: !12)
+!21 = !DILocation(line: 9, column: 16, scope: !22)
+!22 = distinct !DILexicalBlock(scope: !23, file: !1, line: 9, column: 3)
+!23 = distinct !DILexicalBlock(scope: !12, file: !1, line: 9, column: 3)
+!24 = !DILocation(line: 9, column: 23, scope: !22)
+!25 = !DILocation(line: 9, column: 3, scope: !23)
+!26 = distinct !{!26, !25, !27, !28}
+!27 = !DILocation(line: 10, column: 5, scope: !23)
+!28 = !{!"llvm.loop.mustprogress"}
+!29 = !DILocation(line: 12, column: 1, scope: !12)
+!30 = !DILocalVariable(name: "Zeta", scope: !12, file: !1, line: 8, type: !10)
+!31 = !{!32}
+!32 = !DILocalVariable(name: "Param", arg: 1, scope: !11, file: !1, line: 1, type: !10)
+!33 = !DISubroutineType(types: !34)
+!34 = !{!10, !10}
+!35 = !DILocation(line: 1, scope: !11)
+!36 = !DILocation(line: 2, scope: !11)
+!37 = !DILocation(line: 20, scope: !12)