[llvm] [MemProf] Fix an incorrect iterator increment (PR #123438)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson created https://github.com/llvm/llvm-project/pull/123438

We pass in a pointer to an Edge iterator to
moveEdgeToExistingCalleeClone, so that it can be correctly updated when
we remove edges during an edge iteration. We were not dereferencing this
pointer in one case, meaning we would increment the pointer and not the
iterator as intended.

This did not cause any issues, as it turns out that we would simply skip
the edge on the next iteration as it was already appropriately handled.
While in theory this incurred some extra compilation time, in practice
for a large application the effect was not significant. I confirmed that
there was no effect to any cloning from the fix.

I plan to send a follow up change to avoid the need to pass in an
iterator at all and simplify / consolidate the handling in the caller,
but want to fix this in case something requires a revert of the follow
on fix.


>From 2f05fbf68dfd84260a4e2c02ec588c6e8b3b8746 Mon Sep 17 00:00:00 2001
From: Teresa Johnson <tejohnson at google.com>
Date: Fri, 17 Jan 2025 18:38:40 -0800
Subject: [PATCH] [MemProf] Fix an incorrect iterator increment

We pass in a pointer to an Edge iterator to
moveEdgeToExistingCalleeClone, so that it can be correctly updated when
we remove edges during an edge iteration. We were not dereferencing this
pointer in one case, meaning we would increment the pointer and not the
iterator as intended.

This did not cause any issues, as it turns out that we would simply skip
the edge on the next iteration as it was already appropriately handled.
While in theory this incurred some extra compilation time, in practice
for a large application the effect was not significant. I confirmed that
there was no effect to any cloning from the fix.

I plan to send a follow up change to avoid the need to pass in an
iterator at all and simplify / consolidate the handling in the caller,
but want to fix this in case something requires a revert of the follow
on fix.
---
 llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp b/llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp
index 61a8f4a448bbd7..6fdf78373ba598 100644
--- a/llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp
+++ b/llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp
@@ -3102,7 +3102,7 @@ void CallsiteContextGraph<DerivedCCG, FuncTy, CallTy>::
   } else {
     // Only moving a subset of Edge's ids.
     if (CallerEdgeI)
-      ++CallerEdgeI;
+      ++(*CallerEdgeI);
     // Compute the alloc type of the subset of ids being moved.
     auto CallerEdgeAllocType = computeAllocType(ContextIdsToMove);
     if (ExistingEdgeToNewCallee) {