[TailDuplicator] Maintain DebugLoc for branch instructions

[twoh]

Summary

Existing implementation of duplicateSimpleBB function drops DebugLoc metadata of branch instructions during the transformation. This patch addresses this issue by making newly created branch instructions to keep the metadata of replaced branch instructions.

[twoh]

nit: auto

[twoh]

Friendly Ping. Thanks!

[dblaikie]

Seems plausible, though I'm not especially familiar with this optimization, etc.

lib/CodeGen/TailDuplicator.cpp

for (MachineBasicBlock *PredBB : Preds) {
     }
 
     // Avoid adding fall through branches.
     if (PredFBB == NextBB)
       PredFBB = nullptr;
     if (PredTBB == NextBB && PredFBB == nullptr)
       PredTBB = nullptr;
 
+    auto DL = PredBB->findBranchDebugLoc();
     TII->removeBranch(*PredBB);
 
     if (!PredBB->isSuccessor(NewTarget))
       PredBB->replaceSuccessor(TailBB, NewTarget);
     else {
       PredBB->removeSuccessor(TailBB, true);
       assert(PredBB->succ_size() <= 1);
     }
 
     if (PredTBB)
-      TII->insertBranch(*PredBB, PredTBB, PredFBB, PredCond, DebugLoc());
+      TII->insertBranch(*PredBB, PredTBB, PredFBB, PredCond, DL);
 
     TDBBs.push_back(PredBB);
   }
   return Changed;
 }
 
 bool TailDuplicator::canTailDuplicate(MachineBasicBlock *TailBB,
                                       MachineBasicBlock *PredBB) {

test/CodeGen/X86/tail-dup-debugloc.ll

+; RUN: llc -stop-after=tailduplication -march=x86-64 < %s | FileCheck %s
+;
+; Check that DebugLoc attached to the branch instruction of 
+; 'while.cond1.preheader.lr.ph' survives after tailduplication pass.
+;
+; CHECK: [[DLOC:![0-9]+]] = !DILocation(line: 9, column: 5, scope: !{{[0-9]+}})
+; CHECK: [[VREG:%[^ ]+]] = COPY %rdi
+; CHECK: TEST64rr [[VREG]], [[VREG]]
+; CHECK-NEXT: JE_1 {{.+}}, debug-location [[DLOC]]
+; CHECK-NEXT: JMP_1 {{.+}}, debug-location [[DLOC]]
+
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.Node = type { %struct.Node* }
+
+define i32 @foo(%struct.Node* readonly %node, %struct.Node* readnone %root) !dbg !6 {
+entry:
+  %cmp = icmp eq %struct.Node* %node, %root, !dbg !8
+  br i1 %cmp, label %while.end4, label %while.cond1.preheader.lr.ph, !dbg !10
+
+while.cond1.preheader.lr.ph:                      ; preds = %entry
+  %tobool = icmp eq %struct.Node* %node, null
+  br i1 %tobool, label %while.cond1.preheader.us.preheader, label %while.body2.preheader, !dbg !11
+
+while.body2.preheader:                            ; preds = %while.cond1.preheader.lr.ph
+  br label %while.body2, !dbg !11
+
+while.cond1.preheader.us.preheader:               ; preds = %while.cond1.preheader.lr.ph
+  br label %while.cond1.preheader.us, !dbg !10
+
+while.cond1.preheader.us:                         ; preds = %while.cond1.preheader.us.preheader, %while.cond1.preheader.us
+  br label %while.cond1.preheader.us, !dbg !10
+
+while.body2:                                      ; preds = %while.body2.preheader, %while.body2
+  br label %while.body2, !dbg !11
+
+while.end4:                                       ; preds = %entry
+  ret i32 0, !dbg !12
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, isOptimized: true, emissionKind: LineTablesOnly)
+!1 = !DIFile(filename: "foo.c", directory: "b/")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{}
+!6 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 5, type: !7, isLocal: false, isDefinition: true, scopeLine: 5, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !2)
+!7 = !DISubroutineType(types: !2)
+!8 = !DILocation(line: 7, column: 15, scope: !9)
+!9 = !DILexicalBlockFile(scope: !6, file: !1, discriminator: 2)
+!10 = !DILocation(line: 7, column: 3, scope: !9)
+!11 = !DILocation(line: 9, column: 5, scope: !9)
+!12 = !DILocation(line: 14, column: 3, scope: !6)