[Assignment Tracking][15/*] Account for assignment tracking in simplifycfg

[Orlando]

Summary

A few of the changes in this patch update the behaviour from "delete debug intrinsics" to "move dbg.assigns to a new block and set them to undef". There's an argument that we could just keep the existing behaviour in these cases and accept that occasionally we will lose some debug info accuracy, for the sake of fewer special cases in llvm. Furthermore, we can't / don't always perform the "move + undef", such as in the case of DeleteDeadBlocks. There we do indeed just delete them for the sake of practicality. I don't particularly like this inconsistency, but it might just be the kind of thing we need to decide on a case by case basis.

---

I'd be keen to hear what others think about this one as I don't have a strong gut feeling for what is best, overall.

[jmorse]

Soooo, for the TryToSimplifyUncondBranchFromEmptyBlock case I think this would expose the "LLVM can't represent things that happen on critical edges" issue [0], in that sinking the dbg.assigns out of a trivial block might put them in a block with other predecessors. IMO, it's preferable to maintain what dbg.values currently do, which is to vanish (sometimes causing wrong debug-info to be produced), because it's not a regression.

Addressing this is a much larger issue; possibly we can deal with that in the future, but I'd suggest not as part of this patch series. Same with the "hoist all..." situation -- that might be an improvement, but we should start off with no regressions, then consider the improvements we can make.

~

For the store-speculation that happens -- should there not be a call somewhere to merge the assign-IDs of these stores? (Possibly not). Plus, is there a possibility / chance that

• stores are duplicated for some reason (unrolling?) and dbg.assigns with them, using different values,
• one of them is speculated in SpeculativelyExecuteBB,
• We end up setting the Value operand of all the dbg.assigns with the assign ID, to the speculated value, where some of them referred to a non-speculated value.

Maybe there's a reason why this doesn't happen. If it does, I suppose all that needs to happen is the assignment markers are filtered by whether their Value is the same as the speculated stores value.

(Note to self, didn't look at tests)

[0] https://github.com/llvm/llvm-project/issues/55115#issuecomment-1118420725

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

1897–1898

This can (should!) be asserted; also, techncially can be peeled away from dbg.assign improvements if this also affects dbg.values?

[jmorse]

(Sending back to "revise" queue given review above)

[Orlando]

Soooo, for the TryToSimplifyUncondBranchFromEmptyBlock case I think this would expose the "LLVM can't represent things that happen on critical edges" issue [0], in that sinking the dbg.assigns out of a trivial block might put them in a block with other predecessors. IMO, it's preferable to maintain what dbg.values currently do, which is to vanish (sometimes causing wrong debug-info to be produced), because it's not a regression.

Addressing this is a much larger issue; possibly we can deal with that in the future, but I'd suggest not as part of this patch series. Same with the "hoist all..." situation -- that might be an improvement, but we should start off with no regressions, then consider the improvements we can make.

In the past (prototype's past) this was a necessary change because dropping linked dbg.assign intrinsics led to incorrect locations. I think these changes have become less important now that we've introduced the "untagged stores still count as an assignment" rule. I have removed the changes - if this turns out to have caused trouble for some reason we can always re-evaluate.

For the store-speculation that happens -- should there not be a call somewhere to merge the assign-IDs of these stores? (Possibly not)

Good spot - I didn't realise I had missed the test for this in the patch. Looking at it again, it turns out my comment (in the code) is misleading, as the other block's store isn't actually deleted (/merged) with the speculated store (it is deleted by DSE later). I've updated the code comment and added speculated-store.ll.

Plus, is there a possibility / chance that
...

Yeah that sounds right. I've now guarded that with a check.

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

1897–1898

I spent some time trying to find an interesting source reproducer without assignment tracking but failed to found one. If you're happy with an artificial (/hand-written) test then I could move this into its own patch.

[chrisjackson]

Just a couple of typos.

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

1899

2576

[Orlando]

Thanks for taking a look

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

1899

I was referring to the variable I there, but Instruction makes more sense and is not ambiguous. I'll update when I come to land this (or if major updates are required) if that's ok?

2576

I didn't add the comment above, but happy to fold in the update if you want.

[chrisjackson]

In D133310#3792255, @Orlando wrote:

Thanks for taking a look

My pleasure.

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

1899

Yep!

2576

I see. I think it should be corrected but if you'd rather leave it alone, no worries.

[jmorse]

LGTM, with one or two questions, and the middle test needs -simplifycfg adding

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

2542

s/run/executed/, for formality?

llvm/test/DebugInfo/Generic/assignment-tracking/simplifycfg/sink-erase.ll

1

This needs to run simplifycfg, right?

27–29

Is that just checking a single block, and if not should there be an implicit-check-not happening?

[Orlando]

Removed sink-erase.ll as it was testing the TryToSimplifyUncondBranchFromEmptyBlock changes that we removed from this patch earlier.

llvm/test/DebugInfo/Generic/assignment-tracking/simplifycfg/sink-erase.ll

27–29

"Check that there is only one dbg.assign (for c) in the final block." -> "Check that the final block only contains one dbg.assign"

This test isn't actually needed anymore - it was testing the TryToSimplifyUncondBranchFromEmptyBlock changes that we removed from this patch earlier.

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/ConstantRange.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
if (!I0->user_empty()) {
     // one and only one PHI node. Find that now, RAUW it to our common
     // instruction and nuke it.
     auto *PN = cast<PHINode>(*I0->user_begin());
     PN->replaceAllUsesWith(I0);
     PN->eraseFromParent();
   }
 
   // Finally nuke all instructions apart from the common instruction.
-  for (auto *I : Insts)
-    if (I != I0)
-      I->eraseFromParent();
+  for (auto *I : Insts) {
+    if (I == I0)
+      continue;
+    // The remaining uses are debug users, replace those with the common inst.
+    // In most (all?) cases this just introduces a use-before-def.
+    assert(I->user_empty() && "I unexpectedly still has non-dbg users");
+    I->replaceAllUsesWith(I0);
+  }
 
   return true;
 }
 
 namespace {
 
   // LockstepReverseIterator - Iterates through instructions
   // in a set of blocks in reverse order from the first non-terminator.
if (!Convert || Cost > Budget)
     return false;
 
   // If we get here, we can hoist the instruction and if-convert.
   LLVM_DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *ThenBB << "\n";);
 
   // Insert a select of the value of the speculated store.
   if (SpeculatedStoreValue) {
     IRBuilder<NoFolder> Builder(BI);
+    Value *OrigV = SpeculatedStore->getValueOperand();
     Value *TrueV = SpeculatedStore->getValueOperand();
     Value *FalseV = SpeculatedStoreValue;
     if (Invert)
       std::swap(TrueV, FalseV);
     Value *S = Builder.CreateSelect(
         BrCond, TrueV, FalseV, "spec.store.select", BI);
     SpeculatedStore->setOperand(0, S);
     SpeculatedStore->applyMergedLocation(BI->getDebugLoc(),
                                          SpeculatedStore->getDebugLoc());
+    // The value stored is still conditional, but the store itself is now
+    // unconditonally run, so we must be sure that any linked dbg.assign
+    // intrinsics are tracking the new stored value (the result of the
+    // select). If we don't, and the store were to be removed by another pass
+    // (e.g. DSE), then we'd eventually end up emitting a location describing
+    // the conditional value, unconditionally.
+    //
+    // === Before this transformation ===
+    // pred:
+    //   store %one, %x.dest, !DIAssignID !1
+    //   dbg.assign %one, "x", ..., !1, ...
+    //   br %cond if.then
+    //
+    // if.then:
+    //   store %two, %x.dest, !DIAssignID !2
+    //   dbg.assign %two, "x", ..., !2, ...
+    //
+    // === After this transformation ===
+    // pred:
+    //   store %one, %x.dest, !DIAssignID !1
+    //   dbg.assign %one, "x", ..., !1
+    ///  ...
+    //   %merge = select %cond, %two, %one
+    //   store %merge, %x.dest, !DIAssignID !2
+    //   dbg.assign %merge, "x", ..., !2
+    for (auto *DAI : at::getAssignmentMarkers(SpeculatedStore)) {
+      if (any_of(DAI->location_ops(), [&](Value *V) { return V == OrigV; }))
+        DAI->replaceVariableLocationOp(OrigV, S);
+    }
   }
 
   // Metadata can be dependent on the condition we are hoisting above.
   // Conservatively strip all metadata on the instruction. Drop the debug loc
   // to avoid making it appear as if the condition is a constant, which would
   // be misleading while debugging.
   // Similarly strip attributes that maybe dependent on condition we are
   // hoisting above.
   for (auto &I : *ThenBB) {
-    if (!SpeculatedStoreValue || &I != SpeculatedStore)
+    if (!SpeculatedStoreValue || &I != SpeculatedStore) {
+      // Don't update the DILocation of dbg.assign intrinsics.
+      if (!isa<DbgAssignIntrinsic>(&I))
         I.setDebugLoc(DebugLoc());
+    }
     I.dropUndefImplyingAttrsAndUnknownMetadata();
   }
 
   // Hoist the instructions.
   BB->getInstList().splice(BI->getIterator(), ThenBB->getInstList(),
                            ThenBB->begin(), std::prev(ThenBB->end()));
 
   // Insert selects and rewrite the PHI operands.
for (PHINode &PN : EndBB->phis()) {
     Value *V = Builder.CreateSelect(BrCond, TrueV, FalseV, "spec.select", BI);
     PN.setIncomingValue(OrigI, V);
     PN.setIncomingValue(ThenI, V);
   }
 
   // Remove speculated dbg intrinsics.
   // FIXME: Is it possible to do this in a more elegant way? Moving/merging the
   // dbg value for the different flows and inserting it after the select.
-  for (Instruction *I : SpeculatedDbgIntrinsics)
+  for (Instruction *I : SpeculatedDbgIntrinsics) {
+    // We still want to know that an assignment took place so don't remove
+    // dbg.assign intrinsics.
+    if (!isa<DbgAssignIntrinsic>(I))
       I->eraseFromParent();
+  }
 
   ++NumSpeculations;
   return true;
 }
 
 /// Return true if we can thread a branch across this block.
 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
   int Size = 0;

llvm/test/DebugInfo/Generic/assignment-tracking/simplifycfg/empty-block.ll

+; RUN: opt -S %s -simplifycfg -o - -experimental-assignment-tracking | FileCheck %s
+
+;; $ cat test.cpp
+;; class a {};
+;; void operator*(a, float &);
+;; class b {
+;; public:
+;;   a c;
+;; };
+;; int d;
+;; class e {
+;;   b g[3];
+;;   float f;
+;;   void i();
+;; };
+;; void e::i() {
+;;   float h;
+;;   g[d].c *h;
+;;   if (h)
+;;     h = f;
+;;   else
+;;     h = f;
+;; }
+;; Generated by grabbing IR before simplifycfg in:
+;; $ clang++ -O2 -g -c test.cpp -Xclang -fexperimental-assignment-tracking
+
+;; if.then and if.else each only have a dbg.assign and br instruction.
+;; SimplifyCFG will remove these blocks. Check that the dbg.assign intrinsics
+;; are sunk into the succ beforehand.
+
+; CHECK: entry:
+;; -- alloca dbg.assign
+; CHECK: call void @llvm.dbg.assign(metadata i1 undef
+;; -- sunk dbg.assigns
+; CHECK: call void @llvm.dbg.assign(metadata float undef, metadata ![[var:[0-9]+]], metadata !DIExpression(), metadata ![[id:[0-9]+]], metadata float* %h, metadata !DIExpression()), !dbg
+; CHECK-NEXT: call void @llvm.dbg.assign(metadata float undef, metadata ![[var]], metadata !DIExpression(), metadata ![[id]], metadata float* %h, metadata !DIExpression()), !dbg
+; CHECK-NEXT: %storemerge.in = getelementptr
+; CHECK-NEXT: %storemerge = load float
+; CHECK-NEXT: store float %storemerge, float* %h, align 4{{.+}}!DIAssignID ![[id]]
+; CHECK: ret void
+
+%class.e = type { [3 x %class.b], float }
+%class.b = type { %class.a }
+%class.a = type { i8 }
+
+@d = dso_local local_unnamed_addr global i32 0, align 4, !dbg !0
+
+; Function Attrs: uwtable
+define dso_local void @_ZN1e1iEv(%class.e* %this) local_unnamed_addr #0 align 2 !dbg !11 {
+entry:
+  %h = alloca float, align 4, !DIAssignID !32
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !31, metadata !DIExpression(), metadata !32, metadata float* %h, metadata !DIExpression()), !dbg !33
+  %0 = bitcast float* %h to i8*, !dbg !34
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0) #4, !dbg !34
+  call void @_Zml1aRf(float* nonnull align 4 dereferenceable(4) %h), !dbg !35
+  %1 = load float, float* %h, align 4, !dbg !36, !tbaa !38
+  %tobool = fcmp une float %1, 0.000000e+00, !dbg !36
+  br i1 %tobool, label %if.then, label %if.else, !dbg !42
+
+if.then:                                          ; preds = %entry
+  call void @llvm.dbg.assign(metadata float undef, metadata !31, metadata !DIExpression(), metadata !43, metadata float* %h, metadata !DIExpression()), !dbg !33
+  br label %if.end, !dbg !44
+
+if.else:                                          ; preds = %entry
+  call void @llvm.dbg.assign(metadata float undef, metadata !31, metadata !DIExpression(), metadata !43, metadata float* %h, metadata !DIExpression()), !dbg !33
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  %storemerge.in = getelementptr inbounds %class.e, %class.e* %this, i64 0, i32 1, !dbg !45
+  %storemerge = load float, float* %storemerge.in, align 4, !dbg !45, !tbaa !46
+  store float %storemerge, float* %h, align 4, !dbg !45, !tbaa !38, !DIAssignID !43
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0) #4, !dbg !48
+  ret void, !dbg !48
+}
+
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #1
+
+declare !dbg !49 dso_local void @_Zml1aRf(float* nonnull align 4 dereferenceable(4)) local_unnamed_addr #2
+
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #1
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata) #3
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "d", scope: !2, file: !3, line: 7, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, producer: "clang version 12.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "test.cpp", directory: "/")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 12.0.0"}
+!11 = distinct !DISubprogram(name: "i", linkageName: "_ZN1e1iEv", scope: !12, file: !3, line: 13, type: !25, scopeLine: 13, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, declaration: !24, retainedNodes: !28)
+!12 = distinct !DICompositeType(tag: DW_TAG_class_type, name: "e", file: !3, line: 8, size: 64, flags: DIFlagTypePassByValue, elements: !13, identifier: "_ZTS1e")
+!13 = !{!14, !22, !24}
+!14 = !DIDerivedType(tag: DW_TAG_member, name: "g", scope: !12, file: !3, line: 9, baseType: !15, size: 24)
+!15 = !DICompositeType(tag: DW_TAG_array_type, baseType: !16, size: 24, elements: !20)
+!16 = distinct !DICompositeType(tag: DW_TAG_class_type, name: "b", file: !3, line: 3, size: 8, flags: DIFlagTypePassByValue, elements: !17, identifier: "_ZTS1b")
+!17 = !{!18}
+!18 = !DIDerivedType(tag: DW_TAG_member, name: "c", scope: !16, file: !3, line: 5, baseType: !19, size: 8, flags: DIFlagPublic)
+!19 = distinct !DICompositeType(tag: DW_TAG_class_type, name: "a", file: !3, line: 1, size: 8, flags: DIFlagTypePassByValue, elements: !4, identifier: "_ZTS1a")
+!20 = !{!21}
+!21 = !DISubrange(count: 3)
+!22 = !DIDerivedType(tag: DW_TAG_member, name: "f", scope: !12, file: !3, line: 10, baseType: !23, size: 32, offset: 32)
+!23 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+!24 = !DISubprogram(name: "i", linkageName: "_ZN1e1iEv", scope: !12, file: !3, line: 11, type: !25, scopeLine: 11, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!25 = !DISubroutineType(types: !26)
+!26 = !{null, !27}
+!27 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64, flags: DIFlagArtificial | DIFlagObjectPointer)
+!28 = !{!29, !31}
+!29 = !DILocalVariable(name: "this", arg: 1, scope: !11, type: !30, flags: DIFlagArtificial | DIFlagObjectPointer)
+!30 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!31 = !DILocalVariable(name: "h", scope: !11, file: !3, line: 14, type: !23)
+!32 = distinct !DIAssignID()
+!33 = !DILocation(line: 0, scope: !11)
+!34 = !DILocation(line: 14, column: 3, scope: !11)
+!35 = !DILocation(line: 15, column: 10, scope: !11)
+!36 = !DILocation(line: 16, column: 7, scope: !37)
+!37 = distinct !DILexicalBlock(scope: !11, file: !3, line: 16, column: 7)
+!38 = !{!39, !39, i64 0}
+!39 = !{!"float", !40, i64 0}
+!40 = !{!"omnipotent char", !41, i64 0}
+!41 = !{!"Simple C++ TBAA"}
+!42 = !DILocation(line: 16, column: 7, scope: !11)
+!43 = distinct !DIAssignID()
+!44 = !DILocation(line: 17, column: 5, scope: !37)
+!45 = !DILocation(line: 0, scope: !37)
+!46 = !{!47, !39, i64 4}
+!47 = !{!"_ZTS1e", !40, i64 0, !39, i64 4}
+!48 = !DILocation(line: 20, column: 1, scope: !11)
+!49 = !DISubprogram(name: "operator*", linkageName: "_Zml1aRf", scope: !3, file: !3, line: 2, type: !50, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !4)
+!50 = !DISubroutineType(types: !51)
+!51 = !{null, !19, !52}
+!52 = !DIDerivedType(tag: DW_TAG_reference_type, baseType: !23, size: 64)

llvm/test/DebugInfo/Generic/assignment-tracking/simplifycfg/sink-erase.ll

+; RUN: opt %s -S -o - -experimental-assignment-tracking | FileCheck %s
+
+;; Reduced from function cpmx_calc_new_bk in
+;; MultiSource/Benchmarks/mafft/tddis.c in the llvm test suite.
+;;
+;; $ cat reduce.cpp
+;; int a;
+;; void b() {
+;;   int c = 0;
+;;   for (; c < 6; c++)
+;;     for (; a;)
+;;       ;
+;; }
+;;
+;; IR grabbed before simplifycfg in:
+;; clang++ -O2 -g -Xclang -fexperimental-assignment-tracking
+;;
+;; simplifycfg will transform this cfg into:
+;;
+;; [entry]
+;; |    |       +---+
+;; |    v       v   |
+;; |    [for.cond1]-+
+;; v
+;; [for.inc.split.5]
+;;
+;; Check that there is only one dbg.assign (for c) in the final block.
+; CHECK: for.inc.split.5:
+; CHECK-COUNT-1: call void @llvm.dbg.assign
+
+@a = dso_local local_unnamed_addr global i32 0, align 4, !dbg !0
+
+; Function Attrs: norecurse nounwind readonly uwtable mustprogress
+define dso_local void @_Z1bv() local_unnamed_addr #0 !dbg !11 {
+entry:
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !15, metadata !DIExpression(), metadata !16, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.assign(metadata i32 0, metadata !15, metadata !DIExpression(), metadata !18, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  %.pr = load i32, i32* @a, align 4, !tbaa !19
+  %tobool.not = icmp eq i32 %.pr, 0
+  call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression()), !dbg !17
+  br i1 %tobool.not, label %for.inc.split, label %for.cond1.preheader, !dbg !23
+
+for.cond1.preheader:                              ; preds = %for.inc.split.4.for.cond1.preheader_crit_edge, %for.inc.split.3.for.cond1.preheader_crit_edge, %for.inc.split.2.for.cond1.preheader_crit_edge, %for.inc.split.1.for.cond1.preheader_crit_edge, %for.inc.split.for.cond1.preheader_crit_edge, %entry
+  br label %for.cond1, !dbg !28
+
+for.cond1:                                        ; preds = %for.cond1.preheader, %for.cond1
+  br label %for.cond1, !dbg !28
+
+for.inc.split:                                    ; preds = %entry
+  call void @llvm.dbg.assign(metadata i32 1, metadata !15, metadata !DIExpression(), metadata !29, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 1, metadata !15, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 1, metadata !15, metadata !DIExpression()), !dbg !17
+  br i1 true, label %for.inc.split.1, label %for.inc.split.for.cond1.preheader_crit_edge, !dbg !23
+
+for.inc.split.for.cond1.preheader_crit_edge:      ; preds = %for.inc.split
+  br label %for.cond1.preheader, !dbg !23
+
+for.inc.split.1:                                  ; preds = %for.inc.split
+  call void @llvm.dbg.assign(metadata i32 2, metadata !15, metadata !DIExpression(), metadata !29, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 2, metadata !15, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 2, metadata !15, metadata !DIExpression()), !dbg !17
+  br i1 true, label %for.inc.split.2, label %for.inc.split.1.for.cond1.preheader_crit_edge, !dbg !23
+
+for.inc.split.1.for.cond1.preheader_crit_edge:    ; preds = %for.inc.split.1
+  br label %for.cond1.preheader, !dbg !23
+
+for.inc.split.2:                                  ; preds = %for.inc.split.1
+  call void @llvm.dbg.assign(metadata i32 3, metadata !15, metadata !DIExpression(), metadata !29, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 3, metadata !15, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 3, metadata !15, metadata !DIExpression()), !dbg !17
+  br i1 true, label %for.inc.split.3, label %for.inc.split.2.for.cond1.preheader_crit_edge, !dbg !23
+
+for.inc.split.2.for.cond1.preheader_crit_edge:    ; preds = %for.inc.split.2
+  br label %for.cond1.preheader, !dbg !23
+
+for.inc.split.3:                                  ; preds = %for.inc.split.2
+  call void @llvm.dbg.assign(metadata i32 4, metadata !15, metadata !DIExpression(), metadata !29, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 4, metadata !15, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 4, metadata !15, metadata !DIExpression()), !dbg !17
+  br i1 true, label %for.inc.split.4, label %for.inc.split.3.for.cond1.preheader_crit_edge, !dbg !23
+
+for.inc.split.3.for.cond1.preheader_crit_edge:    ; preds = %for.inc.split.3
+  br label %for.cond1.preheader, !dbg !23
+
+for.inc.split.4:                                  ; preds = %for.inc.split.3
+  call void @llvm.dbg.assign(metadata i32 5, metadata !15, metadata !DIExpression(), metadata !29, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 5, metadata !15, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 5, metadata !15, metadata !DIExpression()), !dbg !17
+  br i1 true, label %for.inc.split.5, label %for.inc.split.4.for.cond1.preheader_crit_edge, !dbg !23
+
+for.inc.split.4.for.cond1.preheader_crit_edge:    ; preds = %for.inc.split.4
+  br label %for.cond1.preheader, !dbg !23
+
+for.inc.split.5:                                  ; preds = %for.inc.split.4
+  call void @llvm.dbg.assign(metadata i32 6, metadata !15, metadata !DIExpression(), metadata !29, metadata i32* undef, metadata !DIExpression()), !dbg !17
+  call void @llvm.dbg.value(metadata i32 6, metadata !15, metadata !DIExpression()), !dbg !17
+  ret void, !dbg !30
+}
+
+declare void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata) #1
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !3, producer: "clang version 12.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "reduce.cpp", directory: "/")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 12.0.0"}
+!11 = distinct !DISubprogram(name: "b", linkageName: "_Z1bv", scope: !3, file: !3, line: 2, type: !12, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !14)
+!12 = !DISubroutineType(types: !13)
+!13 = !{null}
+!14 = !{!15}
+!15 = !DILocalVariable(name: "c", scope: !11, file: !3, line: 3, type: !6)
+!16 = distinct !DIAssignID()
+!17 = !DILocation(line: 0, scope: !11)
+!18 = distinct !DIAssignID()
+!19 = !{!20, !20, i64 0}
+!20 = !{!"int", !21, i64 0}
+!21 = !{!"omnipotent char", !22, i64 0}
+!22 = !{!"Simple C++ TBAA"}
+!23 = !DILocation(line: 5, column: 12, scope: !24)
+!24 = distinct !DILexicalBlock(scope: !25, file: !3, line: 5, column: 5)
+!25 = distinct !DILexicalBlock(scope: !26, file: !3, line: 5, column: 5)
+!26 = distinct !DILexicalBlock(scope: !27, file: !3, line: 4, column: 3)
+!27 = distinct !DILexicalBlock(scope: !11, file: !3, line: 4, column: 3)
+!28 = !DILocation(line: 5, column: 5, scope: !25)
+!29 = distinct !DIAssignID()
+!30 = !DILocation(line: 7, column: 1, scope: !11)

llvm/test/DebugInfo/Generic/assignment-tracking/simplifycfg/speculated-store.ll

+; RUN: opt -simplifycfg %s -S -experimental-assignment-tracking  \
+; RUN: | FileCheck %s
+
+;; Ensure that we correctly update the value component of dbg.assign intrinsics
+;; after merging a conditional block with a store its the predecessor. The
+;; value stored is still conditional, but the store itself is now
+;; unconditionally run, so we must be sure that any linked dbg.assign intrinsics
+;; are tracking the new stored value (the result of the select). If we don't,
+;; and the store were to be removed by another pass (e.g. DSE), then we'd
+;; eventually end up emitting a location describing the conditional value,
+;; unconditionally.
+
+;; Created from the following source and command, with dbg.assign and DIAssignID
+;; metadata added and some other metadata removed by hand:
+;; $ cat test.c
+;; int a;
+;; void b() {
+;;   int c = 0;
+;;   if (a)
+;;      c = 1;
+;; }
+;; $ clang -O2 -g -emit-llvm -S test.c -Xclang -fexperimental-assignment-tracking
+
+; CHECK: %[[SELECT:.*]] = select i1 %tobool
+; CHECK-NEXT: store i32 %[[SELECT]], i32* %c{{.*}}, !DIAssignID ![[ID:[0-9]+]]
+; CHECK-NEXT: call void @llvm.dbg.assign(metadata i32 %[[SELECT]], metadata ![[VAR_C:[0-9]+]], metadata !DIExpression(), metadata ![[ID]], metadata i32* %c, metadata !DIExpression()), !dbg
+; CHECK: ![[VAR_C]] = !DILocalVariable(name: "c",
+
+@a = dso_local global i32 0, align 4, !dbg !0
+
+define dso_local void @b() !dbg !11 {
+entry:
+  %c = alloca i32, align 4
+  %0 = bitcast i32* %c to i8*, !dbg !16
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* %0), !dbg !16
+  store i32 0, i32* %c, align 4, !dbg !17, !DIAssignID !36
+  call void @llvm.dbg.assign(metadata i32 0, metadata !15, metadata !DIExpression(), metadata !36, metadata i32* %c, metadata !DIExpression()), !dbg !17
+  %1 = load i32, i32* @a, align 4, !dbg !22
+  %tobool = icmp ne i32 %1, 0, !dbg !22
+  br i1 %tobool, label %if.then, label %if.end, !dbg !24
+
+if.then:                                          ; preds = %entry
+  store i32 1, i32* %c, align 4, !dbg !25, !DIAssignID !37
+  call void @llvm.dbg.assign(metadata i32 1, metadata !15, metadata !DIExpression(), metadata !37, metadata i32* %c, metadata !DIExpression()), !dbg !17
+  br label %if.end, !dbg !26
+
+if.end:                                           ; preds = %if.then, %entry
+  %2 = bitcast i32* %c to i8*, !dbg !27
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* %2), !dbg !27
+  ret void, !dbg !27
+}
+
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture)
+declare void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata)
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture)
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 14.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "test.c", directory: "/")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 12.0.0"}
+!11 = distinct !DISubprogram(name: "b", scope: !3, file: !3, line: 2, type: !12, scopeLine: 2, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !14)
+!12 = !DISubroutineType(types: !13)
+!13 = !{null}
+!14 = !{!15}
+!15 = !DILocalVariable(name: "c", scope: !11, file: !3, line: 3, type: !6)
+!16 = !DILocation(line: 3, column: 3, scope: !11)
+!17 = !DILocation(line: 3, column: 7, scope: !11)
+!22 = !DILocation(line: 4, column: 7, scope: !23)
+!23 = distinct !DILexicalBlock(scope: !11, file: !3, line: 4, column: 7)
+!24 = !DILocation(line: 4, column: 7, scope: !11)
+!25 = !DILocation(line: 5, column: 7, scope: !23)
+!26 = !DILocation(line: 5, column: 5, scope: !23)
+!27 = !DILocation(line: 6, column: 1, scope: !11)
+!36 = distinct !DIAssignID()
+!37 = distinct !DIAssignID()