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From 8482662676a4b6ef79a718c8c09943cb15241664 Mon Sep 17 00:00:00 2001
From: Tom Stellard <tstellar@redhat.com>
Date: Tue, 21 Jun 2022 22:22:11 -0700
Subject: [PATCH] [gold] Ignore bitcode from sections inside object files
-fembed-bitcode will put bitcode into special sections within object
files, but this is not meant to be used by LTO, so the gold plugin
should ignore it.
Reviewed By: tejohnson, MaskRay
---
llvm/docs/BitCodeFormat.rst | 3 ++-
llvm/docs/GoldPlugin.rst | 4 ++++
.../tools/gold/X86/Inputs/bcsection-lib.ll | 6 +++++
llvm/test/tools/gold/X86/Inputs/bcsection.s | 5 ++++
llvm/test/tools/gold/X86/bcsection.ll | 23 +++++++++++++++----
llvm/tools/gold/gold-plugin.cpp | 8 +++++++
6 files changed, 43 insertions(+), 6 deletions(-)
create mode 100644 llvm/test/tools/gold/X86/Inputs/bcsection-lib.ll
diff --git a/llvm/docs/BitCodeFormat.rst b/llvm/docs/BitCodeFormat.rst
index 8e81a7daa459..df1f6915d7d5 100644
--- a/llvm/docs/BitCodeFormat.rst
+++ b/llvm/docs/BitCodeFormat.rst
@@ -475,7 +475,8 @@ formats. This wrapper format is useful for accommodating LTO in compilation
pipelines where intermediate objects must be native object files which contain
metadata in other sections.
-Not all tools support this format.
+Not all tools support this format. For example, lld and the gold plugin will
+ignore these sections when linking object files.
.. _encoding of LLVM IR:
diff --git a/llvm/docs/GoldPlugin.rst b/llvm/docs/GoldPlugin.rst
index ce310bc2cf3c..07d2fc203eba 100644
--- a/llvm/docs/GoldPlugin.rst
+++ b/llvm/docs/GoldPlugin.rst
@@ -17,6 +17,10 @@ and above also supports LTO via plugins. However, usage of the LLVM
gold plugin with ld.bfd is not tested and therefore not officially
supported or recommended.
+As of LLVM 15, the gold plugin will ignore bitcode from the ``.llvmbc``
+section inside of ELF object files. However, LTO with bitcode files
+is still supported.
+
diff --git a/llvm/test/tools/gold/X86/Inputs/bcsection-lib.ll b/llvm/test/tools/gold/X86/Inputs/bcsection-lib.ll
new file mode 100644
index 000000000000..ef3557c19cdc
--- /dev/null
+++ b/llvm/test/tools/gold/X86/Inputs/bcsection-lib.ll
@@ -0,0 +1,6 @@
+declare void @elf_func()
+
+define i32 @lib_func() {
+ call void @elf_func()
+ ret i32 0
+}
diff --git a/llvm/test/tools/gold/X86/Inputs/bcsection.s b/llvm/test/tools/gold/X86/Inputs/bcsection.s
index ede1e5c532dd..c523612563b4 100644
--- a/llvm/test/tools/gold/X86/Inputs/bcsection.s
+++ b/llvm/test/tools/gold/X86/Inputs/bcsection.s
@@ -1,2 +1,7 @@
+.global elf_func
+
+elf_func:
+ ret
+
.section .llvmbc
.incbin "bcsection.bc"
diff --git a/llvm/test/tools/gold/X86/bcsection.ll b/llvm/test/tools/gold/X86/bcsection.ll
index 6d3481f8f966..09882d83fe91 100644
--- a/llvm/test/tools/gold/X86/bcsection.ll
+++ b/llvm/test/tools/gold/X86/bcsection.ll
@@ -2,16 +2,29 @@
; RUN: llvm-as -o %t/bcsection.bc %s
; RUN: llvm-mc -I=%t -filetype=obj -triple=x86_64-unknown-unknown -o %t/bcsection.bco %p/Inputs/bcsection.s
-; RUN: llvm-nm --no-llvm-bc %t/bcsection.bco 2>&1 | FileCheck %s -check-prefix=NO-SYMBOLS
-; NO-SYMBOLS: no symbols
+; RUN: llc -filetype=obj -mtriple=x86_64-unknown-unknown -o %t/bcsection-lib.o %p/Inputs/bcsection-lib.ll
-; RUN: %gold -r -o %t/bcsection.o -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext %t/bcsection.bco
-; RUN: llvm-nm --no-llvm-bc %t/bcsection.o | FileCheck %s
+; RUN: %gold -shared --no-undefined -o %t/bcsection.so -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext %t/bcsection.bco %t/bcsection-lib.o
+
+; This test checks that the gold plugin does not attempt to use the bitcode
+; in the .llvmbc section for LTO. bcsection-lib.o calls a function that is
+; present the symbol table of bcsection.bco, but not included in the embedded
+; bitcode. If the linker were to use the bitcode, then the symbols in the
+; symbol table of bcsection.bco will be ignored and the link will fail.
+;
+; bcsection.bco:
+; .text:
+; elf_func
+; .llvmbc:
+; bitcode_func
+;
+; bcsection-lib.o:
+; calls elf_func()
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-unknown"
; CHECK: main
-define i32 @main() {
+define i32 @bitcode_func() {
ret i32 0
}
diff --git a/llvm/tools/gold/gold-plugin.cpp b/llvm/tools/gold/gold-plugin.cpp
index 180c181368e3..294c7a3d6178 100644
--- a/llvm/tools/gold/gold-plugin.cpp
+++ b/llvm/tools/gold/gold-plugin.cpp
@@ -540,6 +540,14 @@ static ld_plugin_status claim_file_hook(const ld_plugin_input_file *file,
BufferRef = Buffer->getMemBufferRef();
}
+ // Only use bitcode files for LTO. InputFile::create() will load bitcode
+ // from the .llvmbc section within a binary object, this bitcode is typically
+ // generated by -fembed-bitcode and is not to be used by LLVMgold.so for LTO.
+ if (identify_magic(BufferRef.getBuffer()) != file_magic::bitcode) {
+ *claimed = 0;
+ return LDPS_OK;
+ }
+
*claimed = 1;
Expected<std::unique_ptr<InputFile>> ObjOrErr = InputFile::create(BufferRef);
--
2.37.1.1.g659da70093