[llvm] [BOLT][AArch64]`lookupStubFromGroup` Should Check the Last Element Instead of Returning `nullptr` When `lower_bound` Reaches End Iterator (PR #114015)

[via llvm-commits]

https://github.com/liusy58 created https://github.com/llvm/llvm-project/pull/114015

The current implementation of `lookupStubFromGroup` is incorrect. The function is intended to find and return the closest stub using `lower_bound`, which identifies the first element in a sorted range that is not less than a specified value. However, if such an element is not found within `Candidates` and the list is not empty, the function returns `nullptr`. Instead, it should check whether the last element satisfies the condition.

>From c0919e3b48ef08dee5740f4a944eb79d95277808 Mon Sep 17 00:00:00 2001
From: liusy58 <liusy58 at linux.alibaba.com>
Date: Tue, 29 Oct 2024 16:09:31 +0800
Subject: [PATCH] The current implementation of `lookupStubFromGroup` is
 incorrect. The function is intended to find and return the closest stub using
 `lower_bound`, which identifies the first element in a sorted range that is
 not less than a specified value. However, if such an element is not found
 within `Candidates` and the list is not empty, the function returns
 `nullptr`. Instead, it should check whether the last element satisfies the
 condition.

---
 bolt/lib/Passes/LongJmp.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/bolt/lib/Passes/LongJmp.cpp b/bolt/lib/Passes/LongJmp.cpp
index 0b2d00300f46b9..77acd2decc9bd9 100644
--- a/bolt/lib/Passes/LongJmp.cpp
+++ b/bolt/lib/Passes/LongJmp.cpp
@@ -130,9 +130,9 @@ BinaryBasicBlock *LongJmpPass::lookupStubFromGroup(
           const std::pair<uint64_t, BinaryBasicBlock *> &RHS) {
         return LHS.first < RHS.first;
       });
-  if (Cand == Candidates.end())
-    return nullptr;
-  if (Cand != Candidates.begin()) {
+  if (Cand == Candidates.end()) {
+    Cand = std::prev(Cand);
+  } else if (Cand != Candidates.begin()) {
     const StubTy *LeftCand = std::prev(Cand);
     if (Cand->first - DotAddress > DotAddress - LeftCand->first)
       Cand = LeftCand;