[FastISel] Lower `llvm.dbg.value(undef, ...` correctly.

[davide]

Summary

Instead of just dropping them.

rdar://problem/58657146

[aprantl]

Thanks!

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

1417

This comment is no longer correct. It may fits for the !V case, but dbg.value(undef)s are a feature and not a bug.

llvm/test/CodeGen/AArch64/fastisel-debugvalue-undef.ll

5

Can you drop the !34? It's not relevant anyway.

53

You could drop all these by just pointing every type to the DIFile directly.

[davide]

llvm/test/CodeGen/AArch64/fastisel-debugvalue-undef.ll

5

Sure!

53

I really wish bugpoint was smart enough to do this transformation :)

[davide]

Adrian's comments.

[davide]

• [FastIsel] Update comments to reflect reality.

[davide]

@aprantl should be ready for another pass.

[vsk]

llvm/test/CodeGen/AArch64/fastisel-debugvalue-undef.ll

5

Shorter repro:

target triple = "arm64-apple-ios13.4.0"
define void @foo() !dbg !6 {
  ; CHECK: DBG_VALUE $noreg, $noreg, !"1", !DIExpression()
  call void @llvm.dbg.value(metadata i32* undef, metadata !9, metadata !DIExpression()), !dbg !11
  ret void, !dbg !12
}

declare void @llvm.dbg.value(metadata, metadata, metadata)

!llvm.dbg.cu = !{!0}
!llvm.debugify = !{!3, !4}
!llvm.module.flags = !{!5}

!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
!1 = !DIFile(filename: "t.ll", directory: "/")
!2 = !{}
!3 = !{i32 2}
!4 = !{i32 1}
!5 = !{i32 2, !"Debug Info Version", i32 3}
!6 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
!7 = !DISubroutineType(types: !2)
!8 = !{!9}
!9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
!10 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
!11 = !DILocation(line: 1, column: 1, scope: !6)
!12 = !DILocation(line: 2, column: 1, scope: !6)

[aprantl]

LGTM with Vedant's testcase!

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

bool FastISel::selectIntrinsicCall(const IntrinsicInst *II) {
   }
   case Intrinsic::dbg_value: {
     // This form of DBG_VALUE is target-independent.
     const DbgValueInst *DI = cast<DbgValueInst>(II);
     const MCInstrDesc &II = TII.get(TargetOpcode::DBG_VALUE);
     const Value *V = DI->getValue();
     assert(DI->getVariable()->isValidLocationForIntrinsic(DbgLoc) &&
            "Expected inlined-at fields to agree");
-    if (!V) {
+    if (!V || isa<UndefValue>(V)) {
       // Currently the optimizer can produce this; insert an undef to
-      // help debugging.  Probably the optimizer should not do this.
+      // help debugging.
       BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, II, false, 0U,
               DI->getVariable(), DI->getExpression());
     } else if (const auto *CI = dyn_cast<ConstantInt>(V)) {
       if (CI->getBitWidth() > 64)
         BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, II)
             .addCImm(CI)
             .addReg(0U)
             .addMetadata(DI->getVariable())
if (!V || isa<UndefValue>(V)) {
           .addMetadata(DI->getVariable())
           .addMetadata(DI->getExpression());
     } else if (unsigned Reg = lookUpRegForValue(V)) {
       // FIXME: This does not handle register-indirect values at offset 0.
       bool IsIndirect = false;
       BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, II, IsIndirect, Reg,
               DI->getVariable(), DI->getExpression());
     } else {
-      // We can't yet handle anything else here because it would require
-      // generating code, thus altering codegen because of debug info.
+      // We don't know how to handle other cases, so we drop.
       LLVM_DEBUG(dbgs() << "Dropping debug info for " << *DI << "\n");
     }
     return true;
   }
   case Intrinsic::dbg_label: {
     const DbgLabelInst *DI = cast<DbgLabelInst>(II);
     assert(DI->getLabel() && "Missing label");
     if (!FuncInfo.MF->getMMI().hasDebugInfo()) {

llvm/test/CodeGen/AArch64/fastisel-debugvalue-undef.ll

+; RUN: llc -O0 -fast-isel=1 -o - -print-after="finalize-isel" %s 2>&1 | FileCheck %s
+
+; Check that we emit a DBG_VALUE for the `@llvm.dbg.value` which has `undef` has first arg.
+
+target triple = "arm64-apple-ios13.4.0"
+define void @foo() !dbg !6 {
+  ; CHECK: DBG_VALUE $noreg, $noreg, !"1", !DIExpression()
+  call void @llvm.dbg.value(metadata i32* undef, metadata !9, metadata !DIExpression()), !dbg !11
+  ret void, !dbg !12
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "t.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 2}
+!4 = !{i32 1}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
+!7 = !DISubroutineType(types: !2)
+!8 = !{!9}
+!9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
+!10 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
+!11 = !DILocation(line: 1, column: 1, scope: !6)
+!12 = !DILocation(line: 2, column: 1, scope: !6)