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[DTLTO] Fix handling of multi-module bitcode inputs (#174624)

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This change fixes two issues when processing multi-module bitcode files
in DTLTO:

1. The DTLTO archive handling code incorrectly uses
getSingleBitcodeModule(), which asserts when the bitcode file contains
more than one module.
2. The temporary file containing the contents of an input archive member
was not emitted for multi-module bitcode files. This was due to
incorrect logic for recording whether a bitcode input contains any
ThinLTO modules. In a typical multi-module bitcode file, the first
module is a ThinLTO module while a subsequent auxiliary module is
non-ThinLTO. When modules are processed in order, the auxiliary module
causes the entire bitcode file to be classified as non-ThinLTO, and the
archive-member emission logic then incorrectly skips it.

In addition, this patch adds a test that verifies that multi-module
bitcode files can be successfully linked with DTLTO. The test reproduces
both issues as they existed prior to this change.

SIE Tracker: TOOLCHAIN-21008
This commit is contained in:
Ben Dunbobbin 2026-01-15 00:21:01 +00:00 committed by GitHub
parent d7b6df77d0
commit 663647f1b2
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GPG Key ID: B5690EEEBB952194
4 changed files with 48 additions and 2 deletions
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42
cross-project-tests/dtlto/multimodule.test Normal file
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@ -0,0 +1,42 @@
REQUIRES: x86-registered-target,ld.lld,llvm-ar
# Test that a DTLTO link succeeds with a multi-module (via -fsplit-lto-unit)
# bitcode file. We use an archive, as archive member inputs exercise more of
# the DTLTO specific code than other input file types.
RUN: rm -rf %t && split-file %s %t && cd %t
RUN: %clang -O2 --target=x86_64-linux-gnu -flto=thin -c usebar.cc \
RUN: -fno-rtti -fno-exceptions -fsplit-lto-unit
# Sanity check that multi-module bitcode was produced.
RUN: not llvm-modextract -n 2 usebar.o -o - 2>&1 \
RUN: | FileCheck %s --check-prefix=TWO
TWO: bitcode file contains 2 module(s)
# Create an archive.
RUN: llvm-ar rcs usebar.a usebar.o
# Build with DTLTO.
RUN: %clang -O2 --target=x86_64-linux-gnu -flto=thin -fuse-ld=lld \
RUN: -nostdlib -shared -Wl,--whole-archive,--allow-shlib-undefined usebar.a \
RUN: -fthinlto-distributor=%python \
RUN: -Xthinlto-distributor=%llvm_src_root/utils/dtlto/local.py \
RUN: -Wl,--save-temps
RUN: ls | sort | FileCheck %s
# DTLTO JSON file - confirms DTLTO occurred.
CHECK: .dist-file.json
# .native.o exists - confirms archive member usebar.o participated in DTLTO.
CHECK: {{^}}usebar.a(usebar.o
CHECK-SAME: .native.o
#--- usebar.cc
// Minimal C++ input to exercise multi-module emission with -fsplit-lto-unit.
struct A { virtual int foo(); };
int bar(A *a);
struct B : A { int foo() { return 2; } };
int use() { static B b; return bar(&b); }

2
llvm/include/llvm/LTO/LTO.h
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@ -194,6 +194,8 @@ public:
// Returns the only BitcodeModule from InputFile.
LLVM_ABI BitcodeModule &getSingleBitcodeModule();
// Returns the primary BitcodeModule from InputFile.
LLVM_ABI BitcodeModule &getPrimaryBitcodeModule();
// Returns the memory buffer reference for this input file.
MemoryBufferRef getFileBuffer() const { return MbRef; }
// Returns true if this input file is a member of an archive.

2
llvm/lib/DTLTO/DTLTO.cpp
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@ -146,7 +146,7 @@ lto::DTLTO::addInput(std::unique_ptr<lto::InputFile> InputPtr) {
return Input;
SmallString<64> NewModuleId;
BitcodeModule &BM = Input->getSingleBitcodeModule();
BitcodeModule &BM = Input->getPrimaryBitcodeModule();
// Check if the archive is a thin archive.
Expected<bool> IsThin = isThinArchive(ArchivePath);

4
llvm/lib/LTO/LTO.cpp
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@ -606,6 +606,8 @@ BitcodeModule &InputFile::getSingleBitcodeModule() {
return Mods[0];
}
BitcodeModule &InputFile::getPrimaryBitcodeModule() { return Mods[0]; }
LTO::RegularLTOState::RegularLTOState(unsigned ParallelCodeGenParallelismLevel,
const Config &Conf)
: ParallelCodeGenParallelismLevel(ParallelCodeGenParallelismLevel),
@ -804,7 +806,7 @@ LTO::addModule(InputFile &Input, ArrayRef<SymbolResolution> InputRes,
// If any of the modules inside of a input bitcode file was compiled with
// ThinLTO, we assume that the whole input file also was compiled with
// ThinLTO.
Input.IsThinLTO = IsThinLTO;
Input.IsThinLTO |= IsThinLTO;
auto ModSyms = Input.module_symbols(ModI);
addModuleToGlobalRes(ModSyms, Res,