[DebugInfo] Ensure dead store elimination can mark an operand value as undefined

[chrisjackson]

Summary

See Bugzilla https://bugs.llvm.org/show_bug.cgi?id=45080. Dead Store Elimination was failing to mark a value as undefined, instead producing an empty dbg.value.

This patch replaces the salvageDebugInfo() call with a salvageDebugInfoOrMarkUndef() call. If these two variants are replaced by a single function, we can eliminate the cause of this bug and prevent the same mistake in future.

[aprantl]

LGTM!

[vsk]

Thanks, lgtm, +1 for the proposal to hide/delete salvageDebugInfo.

[chrisjackson]

Closed by commit 135709aa9013565b9d02000bb0ca744f7585e6a5.

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

do {
     Instruction *DeadInst = NowDeadInsts.pop_back_val();
     // Mark the DeadInst as dead in the list of throwable instructions.
     auto It = ThrowableInst.find(DeadInst);
     if (It != ThrowableInst.end())
       ThrowableInst[It->first] = false;
     ++NumFastOther;
 
     // Try to preserve debug information attached to the dead instruction.
-    salvageDebugInfo(*DeadInst);
+    salvageDebugInfoOrMarkUndef(*DeadInst);
 
     // This instruction is dead, zap it, in stages.  Start by removing it from
     // MemDep, which needs to know the operands and needs it to be in the
     // function.
     MD.removeInstruction(DeadInst);
 
     for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
       Value *Op = DeadInst->getOperand(op);

llvm/test/DebugInfo/X86/dead-store-elimination-marks-undef.ll

+; RUN: opt -mtriple=x86_64-- -S --dse %s  -o - | FileCheck %s
+; Ensure that we can mark a value as undefined when performing dead 
+; store elimination.
+; Bugzilla #45080
+
+@b = common dso_local local_unnamed_addr global i32 0, align 1
+
+define dso_local i32 @main() local_unnamed_addr !dbg !7 {
+  %1 = alloca i32, align 4
+  %2 = load i32, i32* @b, align 1, !dbg !13
+  ; CHECK: call void @llvm.dbg.value(metadata i32 undef
+  call void @llvm.dbg.value(metadata i32 %2, metadata !12, metadata !DIExpression()), !dbg !13
+  store i32 %2, i32* %1, align 4, !dbg !13
+  ret i32 0, !dbg !13
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !2, nameTableKind: None)
+!1 = !DIFile(filename: "dead-store-elimination-marks-undef.ll", directory: "/temp/bz45080")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = !{!"clang version 10.0.0"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 3, type: !8, scopeLine: 3, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!12}
+!12 = !DILocalVariable(name: "l_2864", scope: !7, file: !1, line: 4, type: !10)
+!13 = !DILocation(line: 5, column: 12, scope: !7)