[llvm] [NFC] Leave a comment in `Local.cpp` about debug info & sample profiling (PR #155296)

[Mircea Trofin via llvm-commits]

https://github.com/mtrofin updated https://github.com/llvm/llvm-project/pull/155296

>From f04017077b9d8f398b1721e8e794bfdcfbd81794 Mon Sep 17 00:00:00 2001
From: Mircea Trofin <mtrofin at google.com>
Date: Wed, 20 Aug 2025 15:04:49 -0700
Subject: [PATCH 1/2] [Local] preserve `MD_prof` in `hoistAllInstructionsInto`

---
 llvm/lib/IR/Instruction.cpp                   |  7 ++++---
 llvm/lib/Transforms/Scalar/LICM.cpp           |  5 +----
 .../Transforms/SimplifyCFG/PhiBlockMerge.ll   | 21 ++++++++++++-------
 3 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index 5e87b5ff941ad..c1fafd759b5ab 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -553,16 +553,17 @@ void Instruction::dropUBImplyingAttrsAndUnknownMetadata(
 }
 
 void Instruction::dropUBImplyingAttrsAndMetadata(ArrayRef<unsigned> Keep) {
-  // !annotation metadata does not impact semantics.
+  // !annotation and !prof metadata does not impact semantics.
   // !range, !nonnull and !align produce poison, so they are safe to speculate.
   // !noundef and various AA metadata must be dropped, as it generally produces
   // immediate undefined behavior.
   static const unsigned KnownIDs[] = {
       LLVMContext::MD_annotation, LLVMContext::MD_range,
-      LLVMContext::MD_nonnull, LLVMContext::MD_align};
+      LLVMContext::MD_nonnull, LLVMContext::MD_align, LLVMContext::MD_prof};
   SmallVector<unsigned> KeepIDs;
   KeepIDs.reserve(Keep.size() + std::size(KnownIDs));
-  append_range(KeepIDs, KnownIDs);
+  append_range(KeepIDs, (!ProfcheckDisableMetadataFixes ? KnownIDs
+                                                        : drop_end(KnownIDs)));
   append_range(KeepIDs, Keep);
   dropUBImplyingAttrsAndUnknownMetadata(KeepIDs);
 }
diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 40104e8fb4249..092a0fb264c28 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -1705,10 +1705,7 @@ static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
       // time in isGuaranteedToExecute if we don't actually have anything to
       // drop.  It is a compile time optimization, not required for correctness.
       !SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop)) {
-    if (ProfcheckDisableMetadataFixes)
-      I.dropUBImplyingAttrsAndMetadata();
-    else
-      I.dropUBImplyingAttrsAndMetadata({LLVMContext::MD_prof});
+    I.dropUBImplyingAttrsAndMetadata();
   }
 
   if (isa<PHINode>(I))
diff --git a/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll b/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll
index 2c5889a981db2..08397b5755a3f 100644
--- a/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll
+++ b/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll
@@ -1,20 +1,21 @@
-; NOTE: Assertions have been autogenerated by update_test_checks.py
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals all --version 5
 ; Test merging of blocks that only have PHI nodes in them
 ;
 ; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
 ;
 
 define i32 @test(i1 %a, i1 %b) {
-; CHECK-LABEL: @test(
-; CHECK:       M:
-; CHECK-NEXT:    [[DOT:%.*]] = select i1 %b, i32 0, i32 1
-; CHECK-NEXT:    [[W:%.*]] = select i1 %a, i32 2, i32 [[DOT]]
+; CHECK-LABEL: define i32 @test(
+; CHECK-SAME: i1 [[A:%.*]], i1 [[B:%.*]]) {
+; CHECK-NEXT:  [[M:.*:]]
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[B]], i32 0, i32 1, !prof [[PROF0:![0-9]+]]
+; CHECK-NEXT:    [[W:%.*]] = select i1 [[A]], i32 2, i32 [[SPEC_SELECT]], !prof [[PROF1:![0-9]+]]
 ; CHECK-NEXT:    [[R:%.*]] = add i32 [[W]], 1
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
-  br i1 %a, label %M, label %O
+  br i1 %a, label %M, label %O, !prof !0
 O:              ; preds = %0
-  br i1 %b, label %N, label %Q
+  br i1 %b, label %N, label %Q, !prof !1
 Q:              ; preds = %O
   br label %N
 N:              ; preds = %Q, %O
@@ -27,3 +28,9 @@ M:              ; preds = %N, %0
   ret i32 %R
 }
 
+!0 = !{!"branch_weights", i32 11, i32 7}
+!1 = !{!"branch_weights", i32 3, i32 5}
+;.
+; CHECK: [[PROF0]] = !{!"branch_weights", i32 3, i32 5}
+; CHECK: [[PROF1]] = !{!"branch_weights", i32 11, i32 7}
+;.

>From 2362af9d23a45db4bb85381539630be98703a2c3 Mon Sep 17 00:00:00 2001
From: Mircea Trofin <mtrofin at google.com>
Date: Mon, 25 Aug 2025 21:04:05 +0000
Subject: [PATCH 2/2] [NFC] Leave a comment in `Local.cpp` about debug info &
 sample profiling

---
 llvm/lib/Transforms/Utils/Local.cpp | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 2cfd70a1746c8..57dc1b38b8ec3 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -3342,8 +3342,11 @@ void llvm::hoistAllInstructionsInto(BasicBlock *DomBlock, Instruction *InsertPt,
   // retain their original debug locations (DILocations) and debug intrinsic
   // instructions.
   //
-  // Doing so would degrade the debugging experience and adversely affect the
-  // accuracy of profiling information.
+  // Doing so would degrade the debugging experience.
+  //
+  // FIXME: Issue #152767: debug info should also be the same as the
+  // original branch, **if** the user explicitly indicated that (for sampling
+  // PGO)
   //
   // Currently, when hoisting the instructions, we take the following actions:
   // - Remove their debug intrinsic instructions.