[BOLT][AArch64] Check Last Element Instead of Returning nullptr in lookupStubFromGroup (#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.

Author: liusy58

bolt/lib/Passes/LongJmp.cpp

@@ -138,9 +138,9 @@ 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;

bolt/test/AArch64/long-jmp-one-stub.s

@@ -0,0 +1,32 @@
+## This test verifies that no unnecessary stubs are inserted when each DotAddress increases during a lookup.
+
+# REQUIRES: system-linux, asserts
+
+# RUN: llvm-mc -filetype=obj -triple aarch64-unknown-unknown %s -o %t.o
+# RUN: %clang %cflags -O0 %t.o -o %t.exe -Wl,-q
+# RUN: link_fdata %s %t.o %t.fdata
+# RUN: llvm-bolt %t.exe -o %t.bolt  \
+# RUN:   --data %t.fdata  | FileCheck %s
+
+# CHECK: BOLT-INFO: Inserted 1 stubs in the hot area and 0 stubs in the cold area. 
+
+  .section .text
+  .global _start
+  .global far_away_func
+
+  .align 4
+  .global _start
+  .type _start, %function
+_start:
+# FDATA: 0 [unknown] 0 1 _start 0 0 100
+    bl far_away_func
+    bl far_away_func
+    ret  
+  .space 0x8000000
+  .global far_away_func
+  .type far_away_func, %function
+far_away_func:
+    add x0, x0, #1
+    ret
+
+.reloc 0, R_AARCH64_NONE