[SDAG] Don't move DBG_VALUE instructions after insertion point during scheduling (PR53243)

[nikic]

Summary

EmitSchedule() shouldn't be touching instructions after the provided insertion point. The change introduced in D83561 performs a scan to the end of the block, and thus may move unrelated instructions. In particular, this ends up moving instructions that have been produced by FastISel and will later be deleted. Moving them means that more instructions than intended are removed.

Fix this by stopping the iteration when the insertion point is reached.

Fixes https://github.com/llvm/llvm-project/issues/53243.

[nikic]

Swap order of checks. We should stop at the insertion point even if it doesn't happen to be DBG_VALUE.

[fhahn]

LGTM, thanks

llvm/test/CodeGen/X86/pr53243-tail-call-fastisel.ll

30

nit: most of those namespaces/enumerates should not be needed I think.

44

nit: most of the types shouldn't be needed.

llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp

EmitSchedule(MachineBasicBlock::iterator &InsertPos) {
   if (FirstTerm != InsertBB->end()) {
     assert(!FirstTerm->isDebugValue() &&
            "first terminator cannot be a debug value");
     for (MachineInstr &MI : make_early_inc_range(
              make_range(std::next(FirstTerm), InsertBB->end()))) {
       if (!MI.isDebugValue())
         continue;
 
+      // Only scan up to insertion point.
       if (&MI == InsertPos)
-        InsertPos = std::prev(InsertPos->getIterator());
+        break;
 
       // The DBG_VALUE was referencing a value produced by a terminator. By
       // moving the DBG_VALUE, the referenced value also needs invalidating.
       MI.getOperand(0).ChangeToRegister(0, false);
       MI.moveBefore(&*FirstTerm);
     }
   }
   return InsertBB;
 }
 
 /// Return the basic block label.
 std::string ScheduleDAGSDNodes::getDAGName() const {
   return "sunit-dag." + BB->getFullName();
 }

llvm/test/CodeGen/X86/pr53243-tail-call-fastisel.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O0 -fast-isel -mtriple=x86_64-- < %s | FileCheck %s
+
+define void @test() {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    jmp set_state@PLT # TAILCALL
+  tail call void @set_state()
+  call void @llvm.dbg.value(metadata i64 0, metadata !12, metadata !DIExpression()), !dbg !27
+  ret void
+}
+
+declare void @set_state()
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+attributes #0 = { nofree nosync nounwind readnone speculatable willreturn }
+
+!llvm.module.flags = !{!0}
+!llvm.dbg.cu = !{!1}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = distinct !DICompileUnit(language: DW_LANG_Rust, file: !2, producer: "clang LLVM (rustc version 1.60.0-nightly (ec4bcaac4 2022-01-15))", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !3)
+!2 = !DIFile(filename: "src/lib.rs/@/bug.63e521cd-cgu.0", directory: "/tmp/rust-bug")
+!3 = !{!4}
+!4 = !DICompositeType(tag: DW_TAG_enumeration_type, name: "Option", scope: !6, file: !5, baseType: !8, size: 8, align: 8, flags: DIFlagEnumClass, elements: !9)
+!5 = !DIFile(filename: "<unknown>", directory: "")
+!6 = !DINamespace(name: "option", scope: !7)
+!7 = !DINamespace(name: "core", scope: null)
+!8 = !DIBasicType(name: "u8", size: 8, encoding: DW_ATE_unsigned)
+!9 = !{!10, !11}
+!10 = !DIEnumerator(name: "None", value: 0)
+!11 = !DIEnumerator(name: "Some", value: 1)
+!12 = !DILocalVariable(name: "msg", arg: 2, scope: !13, file: !14, line: 689, type: !17)
+!13 = distinct !DISubprogram(name: "expect<()>", linkageName: "_ZN4core6option15Option$LT$T$GT$6expect17h9a574c18f194c213E", scope: !4, file: !14, line: 689, type: !15, scopeLine: 689, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !1, templateParams: !24, retainedNodes: !23)
+!14 = !DIFile(filename: "/rustc/ec4bcaac450279b029f3480b8b8f1b82ab36a5eb/library/core/src/option.rs", directory: "", checksumkind: CSK_MD5, checksum: "4120c8557937a0772190a676ec193800")
+!15 = !DISubroutineType(types: !16)
+!16 = !{null, !4, !17}
+!17 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "&str", file: !5, size: 128, align: 64, elements: !18, templateParams: !23, identifier: "84eec819988617519061e0b609a72fe3")
+!18 = !{!19, !21}
+!19 = !DIDerivedType(tag: DW_TAG_member, name: "data_ptr", scope: !17, file: !5, baseType: !20, size: 64, align: 64)
+!20 = !DIDerivedType(tag: DW_TAG_pointer_type, name: "*const u8", baseType: !8, size: 64, align: 64, dwarfAddressSpace: 0)
+!21 = !DIDerivedType(tag: DW_TAG_member, name: "length", scope: !17, file: !5, baseType: !22, size: 64, align: 64, offset: 64)
+!22 = !DIBasicType(name: "usize", size: 64, encoding: DW_ATE_unsigned)
+!23 = !{}
+!24 = !{!25}
+!25 = !DITemplateTypeParameter(name: "T", type: !26)
+!26 = !DIBasicType(name: "()", encoding: DW_ATE_unsigned)
+!27 = !DILocation(line: 0, scope: !13)