Propagate DebugLocs on phis in BreakCriticalEdges (#133492)

The pull request discusses whether this change is needed or not. We leant
towards "it can't hurt" on the basis that it's at worst slightly unecessary
(but not incorret).

The motivation for the patch came from reviewing code duplication sites to
update for Key Instructions, finding this, trying to generate a test case and
seeing the DebugLocs aren't propagated.

Author: OCHyams

llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp

@@ -454,13 +454,16 @@ bool llvm::SplitIndirectBrCriticalEdges(Function &F,
       PHINode *NewIndPHI = PHINode::Create(IndPHI->getType(), 1, "ind", InsertPt);
       NewIndPHI->addIncoming(IndPHI->getIncomingValueForBlock(IBRPred),
                              IBRPred);
+      NewIndPHI->setDebugLoc(IndPHI->getDebugLoc());
 
       // Create a PHI in the body block, to merge the direct and indirect
       // predecessors.
       PHINode *MergePHI = PHINode::Create(IndPHI->getType(), 2, "merge");
       MergePHI->insertBefore(MergeInsert);
       MergePHI->addIncoming(NewIndPHI, Target);
       MergePHI->addIncoming(DirPHI, DirectSucc);
+      MergePHI->applyMergedLocation(DirPHI->getDebugLoc(),
+                                    IndPHI->getDebugLoc());
 
       IndPHI->replaceAllUsesWith(MergePHI);
       IndPHI->eraseFromParent();

llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll

@@ -0,0 +1,51 @@
+; RUN: opt -passes='require<profile-summary>,function(codegenprepare)' -S -mtriple=x86_64 < %s \
+; RUN: | FileCheck %s
+
+;; Check debug locations are propagated onto new PHIs.
+
+; CHECK: .split:
+; CHECK-NEXT: %merge = phi i32 [ poison, %while.body ], [ %dest.sroa.clone, %while.body.clone ], !dbg [[DBG:!.*]]
+
+; CHECK: while.body.clone:
+; CHECK-NEXT: %dest.sroa.clone = phi i32 [ %1, %.split ], [ poison, %if.else ], !dbg [[DBG]]
+
+; CHECK: [[DBG]] = !DILocation(line: 1, column: 1, scope: ![[#]])
+
+define void @test(i1 %c) !dbg !5 {
+entry:
+  br label %if.else
+
+if.else:                                          ; preds = %if.else1, %entry
+  br i1 %c, label %while.body, label %preheader
+
+preheader:                                        ; preds = %if.else
+  br label %if.else1
+
+if.then:                                          ; preds = %if.else1
+  unreachable
+
+while.body:                                       ; preds = %if.else1, %while.body, %if.else
+  %dest.sroa = phi i32 [ %1, %while.body ], [ poison, %if.else1 ], [ poison, %if.else ], !dbg !8
+  %0 = inttoptr i32 %dest.sroa to ptr
+  %incdec.ptr = getelementptr inbounds i8, ptr %0, i32 -1
+  %1 = ptrtoint ptr %incdec.ptr to i32
+  store i8 0, ptr %incdec.ptr, align 1
+  br label %while.body
+
+if.else1:                                         ; preds = %if.else1, %preheader
+  indirectbr ptr poison, [label %if.then, label %while.body, label %if.else, label %if.else1]
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "test.ll", directory: "/")
+!2 = !{i32 11}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "test", linkageName: "test", 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)