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

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 llvm/llvm-project#53243.

Differential Revision: https://reviews.llvm.org/D117489

Author: nikic

llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp

@@ -1057,12 +1057,13 @@ EmitSchedule(MachineBasicBlock::iterator &InsertPos) {
            "first terminator cannot be a debug value");
     for (MachineInstr &MI : make_early_inc_range(
              make_range(std::next(FirstTerm), InsertBB->end()))) {
+      // Only scan up to insertion point.
+      if (&MI == InsertPos)
+        break;
+
       if (!MI.isDebugValue())
         continue;
 
-      if (&MI == InsertPos)
-        InsertPos = std::prev(InsertPos->getIterator());
-
       // 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);

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

@@ -0,0 +1,39 @@
+; 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 !10, metadata !DIExpression()), !dbg !16
+  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", file: !5, baseType: !6, size: 8, align: 8, flags: DIFlagEnumClass, elements: !7)
+!5 = !DIFile(filename: "<unknown>", directory: "")
+!6 = !DIBasicType(name: "u8", size: 8, encoding: DW_ATE_unsigned)
+!7 = !{!8, !9}
+!8 = !DIEnumerator(name: "None", value: 0)
+!9 = !DIEnumerator(name: "Some", value: 1)
+!10 = !DILocalVariable(name: "msg", arg: 2, scope: !11, file: !12, line: 689, type: !6)
+!11 = distinct !DISubprogram(name: "expect<()>", linkageName: "_ZN4core6option15Option$LT$T$GT$6expect17h9a574c18f194c213E", scope: !4, file: !12, line: 689, type: !13, scopeLine: 689, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !1, templateParams: !15, retainedNodes: !15)
+!12 = !DIFile(filename: "/rustc/ec4bcaac450279b029f3480b8b8f1b82ab36a5eb/library/core/src/option.rs", directory: "", checksumkind: CSK_MD5, checksum: "4120c8557937a0772190a676ec193800")
+!13 = !DISubroutineType(types: !14)
+!14 = !{null, !4}
+!15 = !{}
+!16 = !DILocation(line: 0, scope: !11)