[llvm] Propagate DebugLocs on phis in BreakCriticalEdges (PR #133492)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams updated https://github.com/llvm/llvm-project/pull/133492

>From 38b230aee385bd6ea9ae59b2e9790ba9c6e42ff2 Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Wed, 26 Mar 2025 17:27:27 +0000
Subject: [PATCH 1/2] Propagate DebugLocs on phis in BreakCriticalEdges

---
 .../Transforms/Utils/BreakCriticalEdges.cpp   |  3 ++
 .../CodeGenPrepare/X86/split-dbg.ll           | 49 +++++++++++++++++++
 2 files changed, 52 insertions(+)
 create mode 100644 llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll

diff --git a/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp b/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp
index 0721358eb03bb..aea47aec7bb80 100644
--- a/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp
+++ b/llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp
@@ -454,6 +454,7 @@ 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.
@@ -461,6 +462,8 @@ bool llvm::SplitIndirectBrCriticalEdges(Function &F,
       MergePHI->insertBefore(MergeInsert);
       MergePHI->addIncoming(NewIndPHI, Target);
       MergePHI->addIncoming(DirPHI, DirectSucc);
+      MergePHI->applyMergedLocation(DirPHI->getDebugLoc(),
+                                    IndPHI->getDebugLoc());
 
       IndPHI->replaceAllUsesWith(MergePHI);
       IndPHI->eraseFromParent();
diff --git a/llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll b/llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll
new file mode 100644
index 0000000000000..773ae3ff9a3e5
--- /dev/null
+++ b/llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll
@@ -0,0 +1,49 @@
+; 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]]
+
+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)

>From fdfecd2eecfd83fa67d29fc3be42c9dc2157b190 Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Thu, 8 May 2025 12:55:28 +0100
Subject: [PATCH 2/2] add check to test

---
 llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll b/llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll
index 773ae3ff9a3e5..50ff35da175ad 100644
--- a/llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll
+++ b/llvm/test/Transforms/CodeGenPrepare/X86/split-dbg.ll
@@ -9,6 +9,8 @@
 ; 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