[Assignment Tracking][22/*] Add loop-deletion test

[Orlando]

Summary

Check that dbg.assign intrinsics are not accidently converted into dbg.value intrinsics.

[jmorse]

LGTM with nit.

I find the reproducer utterly fascinating -- because of the leak, the variable is partially promoted, and the store in the condition block dominates the leak site so there's no loop-exit-PHI that a dbg.value gets attached to. Instead the last dbg.value is inside the loop, and gets set to undef when the loop is loop-reduced away. LowerDbgDeclare has a partial save by instrumenting the call site, but that's something dbg.assign does better.

Will this stop working if the fix to TryToSimplifyUncondBranchFromEmptyBlock in D133310 doesn't land?

llvm/test/DebugInfo/Generic/assignment-tracking/loop-deletion/dead-loop.ll

22

I'd suggest more detail in what the operands to dbg.assign are, just to confirm it isn't stripped of all information. Ensuring it still refers to the alloca would be good.

[jmorse]

(One wonders why the dbg.values get set to undef when the sunk store still retains a value -- possibly the PHI put in by lcssa gets RAUW'd but the source value doesn't? This raises the possibility that scenarios like this might benefit from forward salvaging, i.e. "we know this value is dead, but something using it isn't, so re-point the dbg intrinsics at that).

[Orlando]

I tidied up the test a little before landing

llvm/test/DebugInfo/Generic/assignment-tracking/loop-deletion/dead-loop.ll

+; RUN: opt %s -passes=loop-deletion -S -o - -experimental-assignment-tracking \
+; RUN: | FileCheck %s
+
+;; $ cat test.cpp:
+;; void esc(int*);
+;; void fun() {
+;;   int Counter = 0;
+;;   for (; Counter < 2; Counter++) {}
+;;   esc(&Counter);
+;; }
+;;
+;; IR grabbed before loop-deletion in:
+;; $ clang++ -O2 -g -Xclang -fexperimental-assignment-tracking
+;;
+;; for.cond is a dead loop - the debug intrinsic inside will be moved to the
+;; exit block (and made undef) and then the loop will be deleted. Ensure that
+;; the dbg.assign intrinsic doesn't get transformed into a dbg.value by
+;; mistake.
+
+; CHECK: for.end:
+; CHECK-NEXT: call void @llvm.dbg.assign(metadata i32 undef,{{.+}}, metadata !DIExpression({{.+}}), metadata ![[ID:[0-9]+]], metadata ptr %Counter, metadata !DIExpression())
+; CHECK-NEXT: store i32 2, ptr %Counter, align 4,{{.*}}!DIAssignID ![[ID]]
+
+define dso_local void @_Z3funv() local_unnamed_addr #0 !dbg !7 {
+entry:
+  %Counter = alloca i32, align 4, !DIAssignID !13
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !11, metadata !DIExpression(), metadata !13, metadata ptr %Counter, metadata !DIExpression()), !dbg !14
+  call void @llvm.dbg.assign(metadata i32 0, metadata !11, metadata !DIExpression(), metadata !16, metadata ptr %Counter, metadata !DIExpression()), !dbg !14
+  br label %for.cond, !dbg !17
+
+for.cond:                                         ; preds = %for.cond, %entry
+  call void @llvm.dbg.assign(metadata i32 undef, metadata !11, metadata !DIExpression(DW_OP_plus_uconst, 1, DW_OP_stack_value), metadata !16, metadata ptr %Counter, metadata !DIExpression()), !dbg !14
+  br i1 false, label %for.cond, label %for.end, !dbg !18, !llvm.loop !20
+
+for.end:                                          ; preds = %for.cond
+  store i32 2, ptr %Counter, align 4, !dbg !14, !DIAssignID !16
+  call void @_Z3escPi(ptr noundef nonnull %Counter), !dbg !27
+  ret void, !dbg !28
+}
+
+declare void @llvm.lifetime.start.p0i8(i64 immarg, ptr nocapture)
+declare !dbg !29 dso_local void @_Z3escPi(ptr noundef) local_unnamed_addr
+declare void @llvm.lifetime.end.p0i8(i64 immarg, ptr nocapture)
+declare void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 14.0.0", isOptimized: true, 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 7, !"uwtable", i32 1}
+!6 = !{!"clang version 14.0.0)"}
+!7 = distinct !DISubprogram(name: "fun", linkageName: "_Z3funv", scope: !1, file: !1, line: 2, type: !8, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !10)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null}
+!10 = !{!11}
+!11 = !DILocalVariable(name: "Counter", scope: !7, file: !1, line: 3, type: !12)
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = distinct !DIAssignID()
+!14 = !DILocation(line: 0, scope: !7)
+!15 = !DILocation(line: 3, column: 3, scope: !7)
+!16 = distinct !DIAssignID()
+!17 = !DILocation(line: 4, column: 3, scope: !7)
+!18 = !DILocation(line: 4, column: 3, scope: !19)
+!19 = distinct !DILexicalBlock(scope: !7, file: !1, line: 4, column: 3)
+!20 = distinct !{!20, !18, !21, !22}
+!21 = !DILocation(line: 4, column: 35, scope: !19)
+!22 = !{!"llvm.loop.mustprogress"}
+!27 = !DILocation(line: 5, column: 3, scope: !7)
+!28 = !DILocation(line: 6, column: 1, scope: !7)
+!29 = !DISubprogram(name: "esc", linkageName: "_Z3escPi", scope: !1, file: !1, line: 1, type: !30, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !33)
+!30 = !DISubroutineType(types: !31)
+!31 = !{null, !32}
+!32 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!33 = !{}