[NFC][DebugInfo][RemoveDIs] Use iterators to insert in callsite-splitting

[jmorse]

This patch gets call site splitting to use iterators for insertion rather than instruction pointers. When we switch on non-instr debug-info this becomes significant, as the iterators are going to signal whether or not a position is before or after debug-info.

NFC as this isn't going to affect the output of any existing test.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-debuginfo

Author: Jeremy Morse (jmorse)

Changes

This patch gets call site splitting to use iterators for insertion rather than instruction pointers. When we switch on non-instr debug-info this becomes significant, as the iterators are going to signal whether or not a position is before or after debug-info.

NFC as this isn't going to affect the output of any existing test.

---

Full diff: https://github.com/llvm/llvm-project/pull/74455.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/CallSiteSplitting.cpp (+4-4)

diff --git a/llvm/lib/Transforms/Scalar/CallSiteSplitting.cpp b/llvm/lib/Transforms/Scalar/CallSiteSplitting.cpp
index 47af299dbd473..015a0ab35987c 100644
--- a/llvm/lib/Transforms/Scalar/CallSiteSplitting.cpp
+++ b/llvm/lib/Transforms/Scalar/CallSiteSplitting.cpp
@@ -372,10 +372,10 @@ static void splitCallSite(CallBase &CB,
     return;
   }
 
-  auto *OriginalBegin = &*TailBB->begin();
+  BasicBlock::iterator OriginalBegin = TailBB->begin();
   // Replace users of the original call with a PHI mering call-sites split.
   if (CallPN) {
-    CallPN->insertBefore(OriginalBegin);
+    CallPN->insertBefore(*TailBB, OriginalBegin);
     CB.replaceAllUsesWith(CallPN);
   }
 
@@ -399,13 +399,13 @@ static void splitCallSite(CallBase &CB,
       for (auto &Mapping : ValueToValueMaps)
         NewPN->addIncoming(Mapping[CurrentI],
                            cast<Instruction>(Mapping[CurrentI])->getParent());
-      NewPN->insertBefore(&*TailBB->begin());
+      NewPN->insertBefore(*TailBB, TailBB->begin());
       CurrentI->replaceAllUsesWith(NewPN);
     }
     CurrentI->dropDbgValues();
     CurrentI->eraseFromParent();
     // We are done once we handled the first original instruction in TailBB.
-    if (CurrentI == OriginalBegin)
+    if (CurrentI == &*OriginalBegin)
       break;
   }
 }

[OCHyams]

LGTM

[jmorse]

Reverted as some buildbots complained; closer inspection of the final part of the diff reveals the problem:

``

[jmorse]

After being bitten by the buildbots: it turns out in the old version this was a comparison between a freed pointer and another pointer. Turning one of them into an iterator involves dereferencing it, it checking whether it's a sentinel, and subsequently asan firing because of a use-after-free.

[jmorse]

(added inline comment rather than deal with github further); an obvious fix is to perform this check before the call to eraseFromParent. I'll hold off on this as I've got too much in-flight right now.