[llvm] [RemoveDIs] Simplify spliceDebugInfo, fixing splice-to-end edge case (PR #105670)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams created https://github.com/llvm/llvm-project/pull/105670

Not quite NFC, fixes splitBasicBlockBefore case when we split before an instruction with debug records (but without the headBit set, i.e., we are splitting before the instruction but after the debug records that come before it). splitBasicBlockBefore splices the instructions before the split point into a new block. Prior to this patch, the debug records would get shifted up to the front of the spliced instructions (as seen in the modified unittest - I believe the unittest was checking erroneous behaviour). We instead want to leave those debug records at the end of the spliced instructions.

The functionality of the deleted `else if` branch is covered by the remaining `if` now that `DestMarker` is set to the trailing marker if `Dest` is `end()`. Previously the "===" markers were sometimes detached, now we always detach them and always reattach them.

Note: `deleteTrailingDbgRecords` only "unlinks" the tailing marker from the block, it doesn't delete anything. The trailing marker is still cleaned up properly inside the final `if` body with `DestMarker->eraseFromParent();`.

Part 1 of 2 needed for #105571 

>From 716952debaeff657af33a72c43db87a31d843a45 Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Thu, 22 Aug 2024 11:30:45 +0100
Subject: [PATCH] [RemoveDIs] Simplify spliceDebugInfo, fixing splice-to-end
 edge case

Not quite NFC, fixes splitBasicBlockBefore case when we split before an
instruction with debug records (but without the headBit set, i.e., we are
splitting before the instruction but after the debug records that come before
it). splitBasicBlockBefore splices the instructions before the split point into
a new block. Prior to this patch, the debug records would get shifted up to the
front of the spliced instructions (as seen in the modified unittest - I believe
the unittest was checking erroneous behaviour). We instead want to leave those
debug records at the end of the spliced instructions.
---
 llvm/lib/IR/BasicBlock.cpp                  | 24 +++++++++------------
 llvm/unittests/IR/BasicBlockDbgInfoTest.cpp |  4 ++--
 2 files changed, 12 insertions(+), 16 deletions(-)

diff --git a/llvm/lib/IR/BasicBlock.cpp b/llvm/lib/IR/BasicBlock.cpp
index cf05b11c53963c..e259ec59da0b9e 100644
--- a/llvm/lib/IR/BasicBlock.cpp
+++ b/llvm/lib/IR/BasicBlock.cpp
@@ -961,9 +961,13 @@ void BasicBlock::spliceDebugInfoImpl(BasicBlock::iterator Dest, BasicBlock *Src,
   // Detach the marker at Dest -- this lets us move the "====" DbgRecords
   // around.
   DbgMarker *DestMarker = nullptr;
-  if (Dest != end()) {
-    if ((DestMarker = getMarker(Dest)))
+  if ((DestMarker = getMarker(Dest))) {
+    if (Dest == end()) {
+      assert(DestMarker == getTrailingDbgRecords());
+      deleteTrailingDbgRecords();
+    } else {
       DestMarker->removeFromParent();
+    }
   }
 
   // If we're moving the tail range of DbgRecords (":::"), absorb them into the
@@ -1005,22 +1009,14 @@ void BasicBlock::spliceDebugInfoImpl(BasicBlock::iterator Dest, BasicBlock *Src,
     } else {
       // Insert them right at the start of the range we moved, ahead of First
       // and the "++++" DbgRecords.
+      // This also covers the rare circumstance where we insert at end(), and we
+      // did not generate the iterator with begin() / getFirstInsertionPt(),
+      // meaning any trailing debug-info at the end of the block would
+      // "normally" have been pushed in front of "First". We move it there now.
       DbgMarker *FirstMarker = createMarker(First);
       FirstMarker->absorbDebugValues(*DestMarker, true);
     }
     DestMarker->eraseFromParent();
-  } else if (Dest == end() && !InsertAtHead) {
-    // In the rare circumstance where we insert at end(), and we did not
-    // generate the iterator with begin() / getFirstInsertionPt(), it means
-    // any trailing debug-info at the end of the block would "normally" have
-    // been pushed in front of "First". Move it there now.
-    DbgMarker *TrailingDbgRecords = getTrailingDbgRecords();
-    if (TrailingDbgRecords) {
-      DbgMarker *FirstMarker = createMarker(First);
-      FirstMarker->absorbDebugValues(*TrailingDbgRecords, true);
-      TrailingDbgRecords->eraseFromParent();
-      deleteTrailingDbgRecords();
-    }
   }
 }
 
diff --git a/llvm/unittests/IR/BasicBlockDbgInfoTest.cpp b/llvm/unittests/IR/BasicBlockDbgInfoTest.cpp
index 91a0745a0cc76e..835780e63aaf4f 100644
--- a/llvm/unittests/IR/BasicBlockDbgInfoTest.cpp
+++ b/llvm/unittests/IR/BasicBlockDbgInfoTest.cpp
@@ -141,11 +141,11 @@ TEST(BasicBlockDbgInfoTest, SplitBasicBlockBefore) {
   Function *F = M->getFunction("func");
 
   BasicBlock &BB = F->getEntryBlock();
-  auto I = std::prev(BB.end(), 2);
+  auto I = std::prev(BB.end(), 2); // store i32 2, ptr %1.
   BB.splitBasicBlockBefore(I, "before");
 
   BasicBlock &BBBefore = F->getEntryBlock();
-  auto I2 = std::prev(BBBefore.end(), 2);
+  auto I2 = std::prev(BBBefore.end()); // br label %1 (new).
   ASSERT_TRUE(I2->hasDbgRecords());
 }