[DebugInfo][SimpleLoopUnswitch] Fix missing debug location updates (#97662)

Fix #97559 .

For the change at line 1253, I propagate the debug location of the
terminator (i.e., the insertion point) to the new phi. because `MergeBB`
is generated by splitting `ExitBB` several lines above, it only has the
terminator, which could provide a reasonable debug location.

For the change at line 2348, I switch the order of moving and cloning
`TI`. Because `NewTI` cloned from `TI` is inserted into the original
place where `TI` is, `NewTI` should preserve the origianl debug
location. At the same time, doing this allows us to propagate the debug
location to the new branch instruction replacing `NewTI` (the change at
line 2446).

Author: Apochens

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

@@ -1245,9 +1245,12 @@ static BasicBlock *buildClonedLoopBlocks(
       if (SE && isa<PHINode>(I))
         SE->forgetValue(&I);
 
+      BasicBlock::iterator InsertPt = MergeBB->getFirstInsertionPt();
+
       auto *MergePN =
           PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi");
-      MergePN->insertBefore(MergeBB->getFirstInsertionPt());
+      MergePN->insertBefore(InsertPt);
+      MergePN->setDebugLoc(InsertPt->getDebugLoc());
       I.replaceAllUsesWith(MergePN);
       MergePN->addIncoming(&I, ExitBB);
       MergePN->addIncoming(&ClonedI, ClonedExitBB);
@@ -1306,8 +1309,9 @@ static BasicBlock *buildClonedLoopBlocks(
   else if (auto *SI = dyn_cast<SwitchInst>(ClonedTerminator))
     ClonedConditionToErase = SI->getCondition();
 
+  Instruction *BI = BranchInst::Create(ClonedSuccBB, ClonedParentBB);
+  BI->setDebugLoc(ClonedTerminator->getDebugLoc());
   ClonedTerminator->eraseFromParent();
-  BranchInst::Create(ClonedSuccBB, ClonedParentBB);
 
   if (ClonedConditionToErase)
     RecursivelyDeleteTriviallyDeadInstructions(ClonedConditionToErase, nullptr,
@@ -2334,22 +2338,27 @@ static void unswitchNontrivialInvariants(
   // nuke the initial terminator placed in the split block.
   SplitBB->getTerminator()->eraseFromParent();
   if (FullUnswitch) {
-    // Splice the terminator from the original loop and rewrite its
-    // successors.
-    TI.moveBefore(*SplitBB, SplitBB->end());
-
     // Keep a clone of the terminator for MSSA updates.
     Instruction *NewTI = TI.clone();
     NewTI->insertInto(ParentBB, ParentBB->end());
 
+    // Splice the terminator from the original loop and rewrite its
+    // successors.
+    TI.moveBefore(*SplitBB, SplitBB->end());
+    TI.dropLocation();
+
     // First wire up the moved terminator to the preheaders.
     if (BI) {
       BasicBlock *ClonedPH = ClonedPHs.begin()->second;
       BI->setSuccessor(ClonedSucc, ClonedPH);
       BI->setSuccessor(1 - ClonedSucc, LoopPH);
       Value *Cond = skipTrivialSelect(BI->getCondition());
-      if (InsertFreeze)
+      if (InsertFreeze) {
+        // We don't give any debug location to the new freeze, because the
+        // BI (`dyn_cast<BranchInst>(TI)`) is an in-loop instruction hoisted
+        // out of the loop.
         Cond = new FreezeInst(Cond, Cond->getName() + ".fr", BI->getIterator());
+      }
       BI->setCondition(Cond);
       DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
     } else {
@@ -2432,12 +2441,13 @@ static void unswitchNontrivialInvariants(
         DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB});
     }
 
-    // After MSSAU update, remove the cloned terminator instruction NewTI.
-    ParentBB->getTerminator()->eraseFromParent();
-
     // Create a new unconditional branch to the continuing block (as opposed to
     // the one cloned).
-    BranchInst::Create(RetainedSuccBB, ParentBB);
+    Instruction *NewBI = BranchInst::Create(RetainedSuccBB, ParentBB);
+    NewBI->setDebugLoc(NewTI->getDebugLoc());
+
+    // After MSSAU update, remove the cloned terminator instruction NewTI.
+    NewTI->eraseFromParent();
   } else {
     assert(BI && "Only branches have partial unswitching.");
     assert(UnswitchedSuccBBs.size() == 1 &&
@@ -2710,6 +2720,7 @@ static BranchInst *turnSelectIntoBranch(SelectInst *SI, DominatorTree &DT,
       PHINode::Create(SI->getType(), 2, "unswitched.select", SI->getIterator());
   Phi->addIncoming(SI->getTrueValue(), ThenBB);
   Phi->addIncoming(SI->getFalseValue(), HeadBB);
+  Phi->setDebugLoc(SI->getDebugLoc());
   SI->replaceAllUsesWith(Phi);
   SI->eraseFromParent();
 

llvm/test/Transforms/SimpleLoopUnswitch/preserving-dropping-debugloc-nontrivial.ll

@@ -0,0 +1,75 @@
+; RUN: opt -passes='simple-loop-unswitch<nontrivial>' -S < %s | FileCheck %s
+
+define i32 @basic(i32 %N, i1 %cond, i32 %select_input) !dbg !5 {
+; CHECK-LABEL: define i32 @basic(
+
+; Check that SimpleLoopUnswitch's unswitchNontrivialInvariants() drops the
+; debug location of the hoisted terminator and doesn't give any debug location
+; to the new freeze, since it's inserted in a hoist block.
+; Also check that in unswitchNontrivialInvariants(), the new br instruction
+; inherits the debug location of the old terminator in the same block.
+
+; CHECK:       entry:
+; CHECK-NEXT:    [[COND_FR:%.*]] = freeze i1 [[COND:%.*]]{{$}}
+; CHECK-NEXT:    br i1 [[COND_FR]], label %[[ENTRY_SPLIT_US:.*]], label %[[ENTRY_SPLIT:.*]]{{$}}
+; CHECK:       for.body.us:
+; CHECK-NEXT:    br label %0, !dbg [[DBG13:![0-9]+]]
+
+; Check that in turnSelectIntoBranch(), the new phi inherits the debug
+; location of the old select instruction replaced.
+
+; CHECK:       1:
+; CHECK-NEXT:    [[UNSWITCHED_SELECT_US:%.*]] = phi i32 [ [[SELECT_INPUT:%.*]], %0 ], !dbg [[DBG13]]
+
+; Check that in BuildClonedLoopBlocks(), the new phi inherits the debug
+; location of the instruction at the insertion point and the new br
+; instruction inherits the debug location of the old terminator.
+
+; CHECK:       for.body:
+; CHECK-NEXT:    br label %2, !dbg [[DBG13]]
+; CHECK:       for.cond.cleanup:
+; CHECK:         [[DOTUS_PHI:%.*]] = phi i32 [ [[RES_LCSSA:%.*]], %[[FOR_COND_CLEANUP_SPLIT:.*]] ], [ [[RES_LCSSA_US:%.*]], %[[FOR_COND_CLEANUP_SPLIT_US:.*]] ], !dbg [[DBG17:![0-9]+]]
+entry:
+  br label %for.cond, !dbg !8
+
+for.cond:                                         ; preds = %for.body, %entry
+  %res = phi i32 [ 0, %entry ], [ %add, %for.body ], !dbg !9
+  %i = phi i32 [ 0, %entry ], [ %inc, %for.body ], !dbg !10
+  %cmp = icmp slt i32 %i, %N, !dbg !11
+  br i1 %cmp, label %for.body, label %for.cond.cleanup, !dbg !12
+
+for.body:                                         ; preds = %for.cond
+  %cond1 = select i1 %cond, i32 %select_input, i32 42, !dbg !13
+  %add = add nuw nsw i32 %cond1, %res, !dbg !14
+  %inc = add nuw nsw i32 %i, 1, !dbg !15
+  br label %for.cond, !dbg !16
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  ret i32 %res, !dbg !17
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+; CHECK: [[DBG13]] = !DILocation(line: 6,
+; CHECK: [[DBG17]] = !DILocation(line: 10,
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "main.ll", directory: "/")
+!2 = !{i32 10}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "basic", linkageName: "basic", 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)