[DebugInfo] Recover debug intrinsics when killing duplicated/empty basic blocks

When basic blocks are killed, either due to being empty or to being an if.then
or if.else block whose complement contains identical instructions, some of the
debug intrinsics in that block are lost. This patch sinks those intrinsics
into the single successor block, setting them Undef if necessary to
prevent debug info from falling out-of-date.

Differential Revision: https://reviews.llvm.org/D70318

5 files changed, 204 insertions, 22 deletions

diff --git a/llvm/include/llvm/Transforms/Utils/Local.h b/llvm/include/llvm/Transforms/Utils/Local.h
index d1dc0b3..0b50ebd 100644
--- a/llvm/include/llvm/Transforms/Utils/Local.h
+++ b/llvm/include/llvm/Transforms/Utils/Local.h
@@ -178,6 +178,8 @@ bool SimplifyInstructionsInBlock(BasicBlock *BB,
 /// Returns true if the dbg values have been changed.
 bool replaceDbgUsesWithUndef(Instruction *I);
 
+void setDbgVariableUndef(DbgVariableIntrinsic *DVI);
+
 //===----------------------------------------------------------------------===//
 //  Control Flow Graph Restructuring.
 //
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index eaccaa1..94edf8c 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -485,15 +485,19 @@ void llvm::RecursivelyDeleteTriviallyDeadInstructions(
     I.eraseFromParent();
   }
 }
+void llvm::setDbgVariableUndef(DbgVariableIntrinsic *DVI) {
+  Value *DbgValue = DVI->getVariableLocation(false);
+  Value *Undef = UndefValue::get(DbgValue ? DbgValue->getType()
+                                          : Type::getInt1Ty(DVI->getContext()));
+  DVI->setOperand(
+      0, MetadataAsValue::get(DVI->getContext(), ValueAsMetadata::get(Undef)));
+}
 
 bool llvm::replaceDbgUsesWithUndef(Instruction *I) {
   SmallVector<DbgVariableIntrinsic *, 1> DbgUsers;
   findDbgUsers(DbgUsers, I);
-  for (auto *DII : DbgUsers) {
-    Value *Undef = UndefValue::get(I->getType());
-    DII->setOperand(0, MetadataAsValue::get(DII->getContext(),
-                                            ValueAsMetadata::get(Undef)));
-  }
+  for (auto *DII : DbgUsers)
+    setDbgVariableUndef(DII);
   return !DbgUsers.empty();
 }
 
@@ -1040,6 +1044,19 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
       assert(PN->use_empty() && "There shouldn't be any uses here!");
       PN->eraseFromParent();
     }
+    // If Succ has multiple predecessors, each debug intrinsic in BB may or may
+    // not be valid when we reach Succ, so the debug variable should be set
+    // undef since its value is unknown.
+    Instruction *DbgInsertPoint = Succ->getFirstNonPHI();
+    while (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(&BB->front())) {
+      if (auto DVI = cast<DbgVariableIntrinsic>(DI)) {
+        if (!isa<DbgDeclareInst>(DVI))
+          setDbgVariableUndef(DVI);
+        DVI->moveBefore(DbgInsertPoint);
+      } else {
+        break;
+      }
+    }
   }
 
   // If the unconditional branch we replaced contains llvm.loop metadata, we
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 79d4857..0e74a26 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -13,6 +13,7 @@
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetOperations.h"
@@ -38,6 +39,7 @@
 #include "llvm/IR/ConstantRange.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalValue.h"
@@ -1250,14 +1252,38 @@ static bool HoistThenElseCodeToIf(BranchInst *BI,
 
   Instruction *I1 = &*BB1_Itr++, *I2 = &*BB2_Itr++;
   // Skip debug info if it is not identical.
-  DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
-  DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
-  if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
-    while (isa<DbgInfoIntrinsic>(I1))
-      I1 = &*BB1_Itr++;
-    while (isa<DbgInfoIntrinsic>(I2))
-      I2 = &*BB2_Itr++;
-  }
+
+  // If the terminator instruction is hoisted, and any variable locations have
+  // non-identical debug intrinsics, then those variable locations must be set
+  // as undef.
+  // FIXME: If each block contains identical debug variable intrinsics in a
+  // different order, they will be considered non-identical and be dropped.
+  MapVector<DebugVariable, DbgVariableIntrinsic *> UndefDVIs;
+
+  auto SkipDbgInfo = [&UndefDVIs](Instruction *&I,
+                                  BasicBlock::iterator &BB_Itr) {
+    while (isa<DbgInfoIntrinsic>(I)) {
+      if (DbgVariableIntrinsic *DVI = dyn_cast<DbgVariableIntrinsic>(I))
+        UndefDVIs.insert(
+            {DebugVariable(DVI->getVariable(), DVI->getExpression(),
+                           DVI->getDebugLoc()->getInlinedAt()),
+             DVI});
+      I = &*BB_Itr++;
+    }
+  };
+
+  auto SkipNonIdenticalDbgInfo =
+      [&BB1_Itr, &BB2_Itr, &SkipDbgInfo](Instruction *&I1, Instruction *&I2) {
+        DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
+        DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
+        if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
+          SkipDbgInfo(I1, BB1_Itr);
+          SkipDbgInfo(I2, BB2_Itr);
+        }
+      };
+
+  SkipNonIdenticalDbgInfo(I1, I2);
+
   // FIXME: Can we define a safety predicate for CallBr?
   if (isa<PHINode>(I1) || !I1->isIdenticalToWhenDefined(I2) ||
       (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)) ||
@@ -1330,15 +1356,7 @@ static bool HoistThenElseCodeToIf(BranchInst *BI,
 
     I1 = &*BB1_Itr++;
     I2 = &*BB2_Itr++;
-    // Skip debug info if it is not identical.
-    DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
-    DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
-    if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
-      while (isa<DbgInfoIntrinsic>(I1))
-        I1 = &*BB1_Itr++;
-      while (isa<DbgInfoIntrinsic>(I2))
-        I2 = &*BB2_Itr++;
-    }
+    SkipNonIdenticalDbgInfo(I1, I2);
   } while (I1->isIdenticalToWhenDefined(I2));
 
   return true;
@@ -1374,6 +1392,13 @@ HoistTerminator:
   }
 
   // Okay, it is safe to hoist the terminator.
+  for (auto DIVariableInst : UndefDVIs) {
+    DbgVariableIntrinsic *DVI = DIVariableInst.second;
+    DVI->moveBefore(BI);
+    if (Value *V = DVI->getVariableLocation())
+      setDbgVariableUndef(DVI);
+  }
+
   Instruction *NT = I1->clone();
   BIParent->getInstList().insert(BI->getIterator(), NT);
   if (!NT->getType()->isVoidTy()) {
diff --git a/llvm/test/Transforms/SimplifyCFG/hoist-dbgvalue-else.ll b/llvm/test/Transforms/SimplifyCFG/hoist-dbgvalue-else.ll
new file mode 100644
index 0000000..3296288
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/hoist-dbgvalue-else.ll
@@ -0,0 +1,67 @@
+; RUN: opt -simplifycfg -S < %s | FileCheck %s
+; Checks that when the if.then and if.else blocks are both eliminated by
+; SimplifyCFG, as the common code is hoisted out, the variables with a
+; dbg.value in either of those blocks are set undef just before the
+; conditional branch instruction.
+
+define i32 @"?fn@@YAHH@Z"(i32 %foo) !dbg !8 {
+; CHECK-LABEL: entry:
+entry:
+  call void @llvm.dbg.value(metadata i32 %foo, metadata !13, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i32 0, metadata !14, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression()), !dbg !16
+  %cmp = icmp eq i32 %foo, 4, !dbg !17
+; CHECK: call void @llvm.dbg.value(metadata i32 undef, metadata [[BEARDS:![0-9]+]]
+; CHECK: call void @llvm.dbg.value(metadata i32 undef, metadata [[BIRDS:![0-9]+]]
+  br i1 %cmp, label %if.then, label %if.else, !dbg !17
+
+if.then:                                          ; preds = %entry
+  call void @llvm.dbg.value(metadata i32 8, metadata !14, metadata !DIExpression()), !dbg !16
+  br label %if.end, !dbg !18
+
+if.else:                                          ; preds = %entry
+  call void @llvm.dbg.value(metadata i32 4, metadata !14, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i32 8, metadata !15, metadata !DIExpression()), !dbg !16
+  br label %if.end, !dbg !21
+
+if.end:                                           ; preds = %if.else, %if.then
+  %beards.0 = phi i32 [ 8, %if.then ], [ 4, %if.else ], !dbg !23
+  call void @llvm.dbg.value(metadata i32 %beards.0, metadata !14, metadata !DIExpression()), !dbg !16
+  ret i32 %beards.0, !dbg !24
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+; CHECK-LABEL: }
+; CHECK: [[BEARDS]] = !DILocalVariable(name: "beards"
+; CHECK: [[BIRDS]] = !DILocalVariable(name: "birds"
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 10.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "C:\5Cdev\5Cllvm-project", checksumkind: CSK_MD5, checksum: "64604a72fdf5b6db8aa2328236bedd6b")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 10.0.0 "}
+!8 = distinct !DISubprogram(name: "fn", linkageName: "?fn@@YAHH@Z", scope: !1, file: !1, line: 1, type: !9, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11, !11}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !{!13, !14, !15}
+!13 = !DILocalVariable(name: "foo", arg: 1, scope: !8, file: !1, line: 1, type: !11)
+!14 = !DILocalVariable(name: "beards", scope: !8, file: !1, line: 2, type: !11)
+!15 = !DILocalVariable(name: "birds", scope: !8, file: !1, line: 3, type: !11)
+!16 = !DILocation(line: 0, scope: !8)
+!17 = !DILocation(line: 5, scope: !8)
+!18 = !DILocation(line: 8, scope: !19)
+!19 = distinct !DILexicalBlock(scope: !20, file: !1, line: 5)
+!20 = distinct !DILexicalBlock(scope: !8, file: !1, line: 5)
+!21 = !DILocation(line: 11, scope: !22)
+!22 = distinct !DILexicalBlock(scope: !20, file: !1, line: 8)
+!23 = !DILocation(line: 0, scope: !20)
+!24 = !DILocation(line: 13, scope: !8)
diff --git a/llvm/test/Transforms/SimplifyCFG/hoist-dbgvalue-empty.ll b/llvm/test/Transforms/SimplifyCFG/hoist-dbgvalue-empty.ll
new file mode 100644
index 0000000..7d5e32a2
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/hoist-dbgvalue-empty.ll
@@ -0,0 +1,71 @@
+; RUN: opt -simplifycfg -S < %s | FileCheck %s
+; Checks that when the if.then block is eliminated due to containing no
+; instructions, the debug intrinsics are hoisted out of the block before its
+; deletion. The hoisted intrinsics should have undef values as the branch
+; behaviour is unknown to the intrinsics after hoisting.
+
+define dso_local i32 @"?fn@@YAHH@Z"(i32 %foo) local_unnamed_addr !dbg !8 {
+entry:
+  call void @llvm.dbg.value(metadata i32 %foo, metadata !13, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i32 0, metadata !14, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression()), !dbg !16
+  %cmp = icmp eq i32 %foo, 4, !dbg !17
+  br i1 %cmp, label %if.then, label %if.else, !dbg !17
+
+if.then:                                          ; preds = %entry
+  call void @llvm.dbg.value(metadata i32 8, metadata !14, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i32 3, metadata !15, metadata !DIExpression()), !dbg !16
+  br label %if.end, !dbg !18
+
+if.else:                                          ; preds = %entry
+  call void @"?side@@YAXXZ"(), !dbg !21
+  call void @llvm.dbg.value(metadata i32 4, metadata !14, metadata !DIExpression()), !dbg !16
+  call void @llvm.dbg.value(metadata i32 6, metadata !15, metadata !DIExpression()), !dbg !16
+  br label %if.end, !dbg !23
+
+; CHECK-LABEL: if.end:
+if.end:                                           ; preds = %if.else, %if.then
+; CHECK: call void @llvm.dbg.value(metadata i32 undef, metadata [[BEARDS:![0-9]+]]
+; CHECK: call void @llvm.dbg.value(metadata i32 undef, metadata [[BIRDS:![0-9]+]]
+  %beards.0 = phi i32 [ 8, %if.then ], [ 4, %if.else ], !dbg !24
+  call void @llvm.dbg.value(metadata i32 %beards.0, metadata !14, metadata !DIExpression()), !dbg !16
+  ret i32 %beards.0, !dbg !25
+}
+
+declare dso_local void @"?side@@YAXXZ"() local_unnamed_addr
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+; CHECK: [[BEARDS]] = !DILocalVariable(name: "beards"
+; CHECK: [[BIRDS]] = !DILocalVariable(name: "birds"
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 10.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test2.cpp", directory: "C:\5Cdev\5Cllvm-project", checksumkind: CSK_MD5, checksum: "8ac5d40fcc9914d6479c1a770dfdc176")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 10.0.0 "}
+!8 = distinct !DISubprogram(name: "fn", linkageName: "?fn@@YAHH@Z", scope: !1, file: !1, line: 3, type: !9, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11, !11}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !{!13, !14, !15}
+!13 = !DILocalVariable(name: "foo", arg: 1, scope: !8, file: !1, line: 3, type: !11)
+!14 = !DILocalVariable(name: "beards", scope: !8, file: !1, line: 4, type: !11)
+!15 = !DILocalVariable(name: "birds", scope: !8, file: !1, line: 5, type: !11)
+!16 = !DILocation(line: 0, scope: !8)
+!17 = !DILocation(line: 7, scope: !8)
+!18 = !DILocation(line: 10, scope: !19)
+!19 = distinct !DILexicalBlock(scope: !20, file: !1, line: 7)
+!20 = distinct !DILexicalBlock(scope: !8, file: !1, line: 7)
+!21 = !DILocation(line: 11, scope: !22)
+!22 = distinct !DILexicalBlock(scope: !20, file: !1, line: 10)
+!23 = !DILocation(line: 14, scope: !22)
+!24 = !DILocation(line: 0, scope: !20)
+!25 = !DILocation(line: 16, scope: !8)