[DebugInfo][LoopFlatten] Fix missing debug location update for new br instruction (#97085)

Fix #97084 .

Author: Apochens

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

@@ -783,8 +783,10 @@ static bool DoFlattenLoopPair(FlattenInfo &FI, DominatorTree *DT, LoopInfo *LI,
   // Replace the inner loop backedge with an unconditional branch to the exit.
   BasicBlock *InnerExitBlock = FI.InnerLoop->getExitBlock();
   BasicBlock *InnerExitingBlock = FI.InnerLoop->getExitingBlock();
-  InnerExitingBlock->getTerminator()->eraseFromParent();
-  BranchInst::Create(InnerExitBlock, InnerExitingBlock);
+  Instruction *Term = InnerExitingBlock->getTerminator();
+  Instruction *BI = BranchInst::Create(InnerExitBlock, InnerExitingBlock);
+  BI->setDebugLoc(Term->getDebugLoc());
+  Term->eraseFromParent();
 
   // Update the DomTree and MemorySSA.
   DT->deleteEdge(InnerExitingBlock, FI.InnerLoop->getHeader());

llvm/test/Transforms/LoopFlatten/preserving_debugloc_br.ll

@@ -0,0 +1,74 @@
+; RUN: opt -S -passes="loop(loop-flatten)" < %s | FileCheck %s
+
+; Check that LoopFlatten's DoFlattenLoopPair() propagates the debug location of the
+; original terminator to the new branch instruction.
+
+define i32 @test1(i32 %val, ptr nocapture %A) !dbg !5 {
+; CHECK-LABEL: define i32 @test1(
+; CHECK-LABEL: for.body3:
+; CHECK:         br label %for.inc6, !dbg [[DBG22:![0-9]+]]
+; CHECK-LABEL: for.inc6:
+;
+entry:
+  br label %for.body, !dbg !8
+
+for.body:                                         ; preds = %for.inc6, %entry
+  %i.018 = phi i32 [ 0, %entry ], [ %inc7, %for.inc6 ], !dbg !9
+  %mul = mul nuw nsw i32 %i.018, 20, !dbg !10
+  br label %for.body3, !dbg !11
+
+for.body3:                                        ; preds = %for.body3, %for.body
+  %j.017 = phi i32 [ 0, %for.body ], [ %inc, %for.body3 ], !dbg !12
+  %add = add nuw nsw i32 %j.017, %mul, !dbg !13
+  %arrayidx = getelementptr inbounds i16, ptr %A, i32 %add, !dbg !14
+  %0 = load i16, ptr %arrayidx, align 2, !dbg !15
+  %conv16 = zext i16 %0 to i32, !dbg !16
+  %add4 = add i32 %conv16, %val, !dbg !17
+  %conv5 = trunc i32 %add4 to i16, !dbg !18
+  store i16 %conv5, ptr %arrayidx, align 2, !dbg !19
+  %inc = add nuw nsw i32 %j.017, 1, !dbg !20
+  %exitcond = icmp ne i32 %inc, 20, !dbg !21
+  br i1 %exitcond, label %for.body3, label %for.inc6, !dbg !22
+
+for.inc6:                                         ; preds = %for.body3
+  %inc7 = add nuw nsw i32 %i.018, 1, !dbg !23
+  %exitcond19 = icmp ne i32 %inc7, 10, !dbg !24
+  br i1 %exitcond19, label %for.body, label %for.end8, !dbg !25
+
+for.end8:                                         ; preds = %for.inc6
+  ret i32 10, !dbg !26
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+; CHECK: [[DBG22]] = !DILocation(line: 15,
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "temp.ll", directory: "/")
+!2 = !{i32 19}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "test1", linkageName: "test1", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!6 = !DISubroutineType(types: !7)
+!7 = !{}
+!8 = !DILocation(line: 1, column: 1, scope: !5)
+!9 = !DILocation(line: 2, column: 1, scope: !5)
+!10 = !DILocation(line: 3, column: 1, scope: !5)
+!11 = !DILocation(line: 4, column: 1, scope: !5)
+!12 = !DILocation(line: 5, column: 1, scope: !5)
+!13 = !DILocation(line: 6, column: 1, scope: !5)
+!14 = !DILocation(line: 7, column: 1, scope: !5)
+!15 = !DILocation(line: 8, column: 1, scope: !5)
+!16 = !DILocation(line: 9, column: 1, scope: !5)
+!17 = !DILocation(line: 10, column: 1, scope: !5)
+!18 = !DILocation(line: 11, column: 1, scope: !5)
+!19 = !DILocation(line: 12, column: 1, scope: !5)
+!20 = !DILocation(line: 13, column: 1, scope: !5)
+!21 = !DILocation(line: 14, column: 1, scope: !5)
+!22 = !DILocation(line: 15, column: 1, scope: !5)
+!23 = !DILocation(line: 16, column: 1, scope: !5)
+!24 = !DILocation(line: 17, column: 1, scope: !5)
+!25 = !DILocation(line: 18, column: 1, scope: !5)
+!26 = !DILocation(line: 19, column: 1, scope: !5)