From c9022a29b4d3c0c7b82c1ae795405c238a14bde1 Mon Sep 17 00:00:00 2001
From: Maksim Panchenko <maks@fb.com>
Date: Thu, 29 May 2025 19:24:34 -0700
Subject: [PATCH] [BOLT][AArch64] Detect veneers with missing data markers
 (#142069)

The linker may omit data markers for long absolute veneers causing BOLT
to treat data as code. Detect such veneers and introduce data markers
artificially before BOLT's disassembler kicks in.
---
 bolt/lib/Rewrite/RewriteInstance.cpp | 13 +++++++++++++
 bolt/test/AArch64/veneer-lld-abs.s   | 13 ++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index ad062ea3622d..69c39cf5e5ba 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -1341,6 +1341,19 @@ void RewriteInstance::discoverFileObjects() {
         }
       }
     }
+
+    // The linker may omit data markers for absolute long veneers. Introduce
+    // those markers artificially to assist the disassembler.
+    for (BinaryFunction &BF :
+         llvm::make_second_range(BC->getBinaryFunctions())) {
+      if (BF.getOneName().starts_with("__AArch64AbsLongThunk_") &&
+          BF.getSize() == 16 && !BF.getSizeOfDataInCodeAt(8)) {
+        BC->errs() << "BOLT-WARNING: missing data marker detected in veneer "
+                   << BF << '\n';
+        BF.markDataAtOffset(8);
+        BC->AddressToConstantIslandMap[BF.getAddress() + 8] = &BF;
+      }
+    }
   }
 
   if (!BC->IsLinuxKernel) {
diff --git a/bolt/test/AArch64/veneer-lld-abs.s b/bolt/test/AArch64/veneer-lld-abs.s
index 7e6fe2d12706..b22301db66c5 100644
--- a/bolt/test/AArch64/veneer-lld-abs.s
+++ b/bolt/test/AArch64/veneer-lld-abs.s
@@ -6,10 +6,21 @@
 # RUN:    -fuse-ld=lld -Wl,-q
 # RUN: llvm-objdump -d %t.exe | FileCheck --check-prefix=CHECK-INPUT %s
 # RUN: llvm-objcopy --remove-section .rela.mytext %t.exe
-# RUN: llvm-bolt %t.exe -o %t.bolt --elim-link-veneers=true --lite=0
+# RUN: llvm-bolt %t.exe -o %t.bolt
 # RUN: llvm-objdump -d -j .text  %t.bolt | \
 # RUN:   FileCheck --check-prefix=CHECK-OUTPUT %s
 
+## Occasionally, we see the linker not generating $d symbols for long veneers
+## causing BOLT to fail veneer elimination.
+# RUN: llvm-objcopy --remove-symbol-prefix=\$d %t.exe %t.no-marker.exe
+# RUN: llvm-bolt %t.no-marker.exe -o %t.no-marker.bolt \
+# RUN:   2>&1 | FileCheck %s --check-prefix=CHECK-BOLT
+# RUN: llvm-objdump -d -j .text  %t.no-marker.bolt | \
+# RUN:   FileCheck --check-prefix=CHECK-OUTPUT %s
+
+# CHECK-BOLT-NOT: BOLT-WARNING: unable to disassemble instruction
+# CHECK-BOLT: BOLT-WARNING: missing data marker detected in veneer __AArch64AbsLongThunk_far_function
+
 .text
 .balign 4
 .global far_function