Wrong debug info generated at -O2 (-O0 is correct)

Instcombiner pass was erasing trivially dead instruction without updating dependent llvm.dbg.value.
which was not showing programmer current state of variables while debugging.
As a part of this fix I did following,
Iterate throught all the users (llvm.dbg) of a instruction which is trivially dead and set each if them undef, Before deleting the instruction.
Now user will see optimized out, when try to print those variables.
This fixes
https://bugs.llvm.org/show_bug.cgi?id=43893

This is my first fix to llvm.

Patch by kamlesh kumar!

Differential Revision: https://reviews.llvm.org/D69809

include/llvm/Transforms/Utils/Local.h

@@ -354,6 +354,10 @@ AllocaInst *findAllocaForValue(Value *V,
 /// Returns true if any debug users were updated.
 bool salvageDebugInfo(Instruction &I);
 
+/// Salvage all debug users of the instruction \p I or mark it as undef if it
+/// cannot be salvaged.
+void salvageDebugInfoOrMarkUndef(Instruction &I);
+
 /// Implementation of salvageDebugInfo, applying only to instructions in
 /// \p Insns, rather than all debug users of \p I.
 bool salvageDebugInfoForDbgValues(Instruction &I,

lib/Transforms/InstCombine/InstCombineInternal.h

@@ -705,7 +705,7 @@ public:
   Instruction *eraseInstFromFunction(Instruction &I) {
     LLVM_DEBUG(dbgs() << "IC: ERASE " << I << '\n');
     assert(I.use_empty() && "Cannot erase instruction that is used!");
-    salvageDebugInfo(I);
+    salvageDebugInfoOrMarkUndef(I);
 
     // Make sure that we reprocess all operands now that we reduced their
     // use counts.

lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -3414,8 +3414,7 @@ static bool AddReachableCodeToWorklist(BasicBlock *BB, const DataLayout &DL,
       if (isInstructionTriviallyDead(Inst, TLI)) {
         ++NumDeadInst;
         LLVM_DEBUG(dbgs() << "IC: DCE: " << *Inst << '\n');
-        if (!salvageDebugInfo(*Inst))
-          replaceDbgUsesWithUndef(Inst);
+        salvageDebugInfoOrMarkUndef(*Inst);
         Inst->eraseFromParent();
         MadeIRChange = true;
         continue;

lib/Transforms/Utils/Local.cpp

@@ -1611,6 +1611,11 @@ bool llvm::salvageDebugInfo(Instruction &I) {
   return salvageDebugInfoForDbgValues(I, DbgUsers);
 }
 
+void llvm::salvageDebugInfoOrMarkUndef(Instruction &I) {
+  if (!salvageDebugInfo(I))
+    replaceDbgUsesWithUndef(&I);
+}
+
 bool llvm::salvageDebugInfoForDbgValues(
     Instruction &I, ArrayRef<DbgVariableIntrinsic *> DbgUsers) {
   auto &Ctx = I.getContext();

test/Transforms/InstCombine/cast-mul-select.ll

@@ -13,6 +13,8 @@ define i32 @mul(i32 %x, i32 %y) {
 ; we preserve the debug information in the resulting
 ; instruction.
 ; DBGINFO-LABEL: @mul(
+; DBGINFO-NEXT:    call void @llvm.dbg.value(metadata i8 undef
+; DBGINFO-NEXT:    call void @llvm.dbg.value(metadata i8 undef
 ; DBGINFO-NEXT:    [[C:%.*]] = mul i32 {{.*}}
 ; DBGINFO-NEXT:    [[D:%.*]] = and i32 {{.*}}
 ; DBGINFO-NEXT:    call void @llvm.dbg.value(metadata i32 [[C]]

test/Transforms/InstCombine/debuginfo-dce.ll

@@ -34,7 +34,7 @@ entry:
   call void @llvm.dbg.value(metadata %struct.entry* %1, metadata !18, metadata !20), !dbg !19
 ; CHECK: define void @salvage_load
 ; CHECK-NEXT: entry:
-; CHECK-NOT: dbg.value
+; CHECK-NEXT: call void @llvm.dbg.value(metadata %struct.entry* undef
   store %struct.entry* %1, %struct.entry** %im_not_dead, align 8
   ret void, !dbg !21
 }

test/Transforms/InstCombine/pr43893.ll

@@ -0,0 +1,54 @@
+; Check for setting dbg.value as undef which depends on trivially dead instructions.
+; RUN: opt -instcombine -S -o - %s | FileCheck %s
+
+@a = common dso_local global i8 0, align 1, !dbg !0
+@b = common dso_local global i8 0, align 1, !dbg !6
+
+define dso_local i32 @main() !dbg !13 {
+entry:
+  %0 = load i8, i8* @a, align 1, !dbg !17
+  %dec = add i8 %0, -1, !dbg !17
+  store i8 %dec, i8* @a, align 1, !dbg !17
+;CHECK: call void @llvm.dbg.value(metadata i32 undef
+;CHECK: call void @llvm.dbg.value(metadata i32 -8
+;CHECK: call void @llvm.dbg.value(metadata i32 undef
+  %conv = sext i8 %dec to i32, !dbg !17
+  call void @llvm.dbg.value(metadata i32 %conv, metadata !18, metadata !DIExpression()), !dbg !19
+  call void @llvm.dbg.value(metadata i32 -8, metadata !20, metadata !DIExpression()), !dbg !19
+  call void @llvm.dbg.value(metadata i32 %conv, metadata !20, metadata !DIExpression()), !dbg !19
+  store i8 0, i8* @b, align 1, !dbg !21
+  %cmp = icmp sgt i32 %conv, 0, !dbg !22
+  %conv1 = zext i1 %cmp to i32, !dbg !22
+  ret i32 0, !dbg !23
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata) 
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!9, !10, !11}
+!llvm.ident = !{!12}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 10.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "test", directory: "n")
+!4 = !{}
+!5 = !{!0, !6}
+!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+!7 = distinct !DIGlobalVariable(name: "b", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
+!8 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!9 = !{i32 2, !"Dwarf Version", i32 4}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !{i32 1, !"wchar_size", i32 4}
+!12 = !{!"clang version 10.0.0"}
+!13 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 2, type: !14, scopeLine: 2, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !4)
+!14 = !DISubroutineType(types: !15)
+!15 = !{!16}
+!16 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!17 = !DILocation(line: 4, column: 11, scope: !13)
+!18 = !DILocalVariable(name: "c", scope: !13, file: !3, line: 4, type: !16)
+!19 = !DILocation(line: 0, scope: !13)
+!20 = !DILocalVariable(name: "l_1240", scope: !13, file: !3, line: 6, type: !16)
+!21 = !DILocation(line: 10, column: 10, scope: !13)
+!22 = !DILocation(line: 10, column: 5, scope: !13)
+!23 = !DILocation(line: 12, column: 1, scope: !13)

test/Transforms/InstCombine/stacksave-debuginfo.ll

@@ -8,10 +8,11 @@ declare void @llvm.stackrestore(i8*) #0
 
 define i32* @test1(i32 %P) !dbg !6 {
 ; CHECK-LABEL: @test1(
-; CHECK-NEXT:    [[TMP1:%.*]] = zext i32 [[P:%.*]] to i64, !dbg !12
-; CHECK-NEXT:    [[A:%.*]] = alloca i32, i64 [[TMP1]], align 4, !dbg !12
-; CHECK-NEXT:    call void @llvm.dbg.value(metadata i32* [[A]], metadata !11, metadata !DIExpression()), !dbg !12
-; CHECK-NEXT:    ret i32* [[A]], !dbg !13
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8* undef
+; CHECK-NEXT:    [[TMP1:%.*]] = zext i32 [[P:%.*]] to i64, !dbg !13
+; CHECK-NEXT:    [[A:%.*]] = alloca i32, i64 [[TMP1]], align 4, !dbg !13
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i32* [[A]]
+; CHECK-NEXT:    ret i32* [[A]], !dbg !14
 ;
   %tmp = call i8* @llvm.stacksave(), !dbg !12
   call void @llvm.dbg.value(metadata i8* %tmp, metadata !9, metadata !DIExpression()), !dbg !12