[lld-macho] Avoid running LTO pipeline for no files

[keith]

Summary

If no bitcode files are given as inputs but -object_path_lto is passed
we should avoid running the LTO pipeline.

This mirrors this patch from the ELF port https://github.com/llvm/llvm-project/commit/f3994e4dfa0214b2a09a0e327ba37e6b38bbcdb3

Fixes https://github.com/llvm/llvm-project/issues/61031

[int3]

Thanks!! I think we should follow LLD-ELF whenever reasonable/possible, so I like this better

lld/MachO/Driver.cpp

556 ↗

(On Diff #502246)

personally I think it's nicer to have hasFiles be a member of BitcodeCompiler and have it set by BitcodeCompiler::add() -- in principle the BitcodeCompiler should already know if it has any files, we shouldn't need to tell it twice

lld/test/MachO/lto-object-path.ll

42–44

ultra nit: "NOOBJPATH" refers to the fact that we are not passing -object_path_lto, but "NOLTOPATH" here means we are not passing in any paths to LTO files... the similarity in names seems kind of misleading. Maybe "NOLTOFILES" would be better?

[keith]

Move bool into BitcodeCompiler

[keith]

spacing

lld/MachO/LTO.h

public:
   std::vector<ObjFile *> compile();
 
 private:
   std::unique_ptr<llvm::lto::LTO> ltoObj;
   std::vector<llvm::SmallString<0>> buf;
   std::vector<std::unique_ptr<llvm::MemoryBuffer>> files;
   std::unique_ptr<llvm::raw_fd_ostream> indexFile;
   llvm::DenseSet<StringRef> thinIndices;
+  bool hasFiles = false;
 };
 
 } // namespace lld::macho
 
 #endif

lld/MachO/LTO.cpp

for (const lto::InputFile::Symbol &objSym : objSyms) {
     // load the ObjFile emitted by LTO compilation.
     if (r.Prevailing)
       replaceSymbol<Undefined>(sym, sym->getName(), sym->getFile(),
                                RefState::Strong, /*wasBitcodeSymbol=*/true);
 
     // TODO: set the other resolution configs properly
   }
   checkError(ltoObj->add(std::move(f.obj), resols));
+  hasFiles = true;
 }
 
 // If LazyObjFile has not been added to link, emit empty index files.
 // This is needed because this is what GNU gold plugin does and we have a
 // distributed build system that depends on that behavior.
 static void thinLTOCreateEmptyIndexFiles() {
   DenseSet<StringRef> linkedBitCodeFiles;
   for (InputFile *file : inputFiles)
std::vector<ObjFile *> BitcodeCompiler::compile() {
   FileCache cache;
   if (!config->thinLTOCacheDir.empty())
     cache = check(localCache("ThinLTO", "Thin", config->thinLTOCacheDir,
                              [&](size_t task, const Twine &moduleName,
                                  std::unique_ptr<MemoryBuffer> mb) {
                                files[task] = std::move(mb);
                              }));
 
+  if (hasFiles)
     checkError(ltoObj->run(
         [&](size_t task, const Twine &moduleName) {
           return std::make_unique<CachedFileStream>(
               std::make_unique<raw_svector_ostream>(buf[task]));
         },
         cache));
 
   // Emit empty index files for non-indexed files
   for (StringRef s : thinIndices) {
     std::string path = getThinLTOOutputFile(s);
     openFile(path + ".thinlto.bc");
     if (config->thinLTOEmitImportsFiles)
       openFile(path + ".imports");
   }

lld/test/MachO/lto-object-path.ll

 ; REQUIRES: x86
 ; UNSUPPORTED: system-windows
 
 ; RUN: rm -rf %t; mkdir %t
+; RUN: llc -filetype=obj %s -o %t/test-obj.o
 ; RUN: opt -thinlto-bc %s -o %t/test.o
 ; RUN: opt %s -o %t/test-nonthin.o
 
 ; RUN: %lld %t/test.o -o %t/test
 ; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes CHECK,NOOBJPATH
 
+; RUN: %lld %t/test-obj.o -o %t/test-obj -object_path_lto %t/lto-temps-obj
+; RUN: llvm-nm -pa %t/test-obj | FileCheck %s --check-prefixes CHECK,NOLTOFILES -DDIR=%t
+
 ; RUN: ZERO_AR_DATE=0 %lld %t/test.o -o %t/test -object_path_lto %t/lto-temps
 ; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes CHECK,OBJPATH-DIR -DDIR=%t/lto-temps
 
 ;; Ensure object path is still used if the cache is used
 ; RUN: ZERO_AR_DATE=0 %lld %t/test.o -o %t/test -object_path_lto %t/lto-temps -prune_interval_lto 20 -cache_path_lto %t/cache --thinlto-cache-policy=cache_size_files=1:cache_size_bytes=10
 ; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes CHECK,OBJPATH-DIR -DDIR=%t/lto-temps
 ;; And that dsymutil can read the result
 ; RUN: dsymutil -f -o - %t/test | llvm-dwarfdump - | FileCheck %s --check-prefix=DSYM
 
 ;; Test that the object path hardlinks to the cache when possible
 ;; NB:
 ;; 1) Need to delete current object path contents or hardlink will fail.
 ;; 2) Note the cache policy is different from the test above (which is crafted
 ;;    to ensure that the cache is pruned immediately; we want the opposite here).
 ; RUN: rm -rf %t/lto-temps
 ; RUN: %lld %t/test.o -o %t/test -object_path_lto %t/lto-temps -prune_interval_lto 20 -cache_path_lto %t/cache --thinlto-cache-policy=cache_size_files=10:cache_size_bytes=10000
 ;; Get the link count.
 ; RUN: ls -l %t/lto-temps/1.x86_64.lto.o | tr -s ' ' | cut -f 2 -d ' ' | FileCheck %s --check-prefix=HARDLINK
 
 ;; check that the object path can be an existing file
 ; RUN: touch %t/lto-tmp.o
 ; RUN: ZERO_AR_DATE=0 %lld %t/test-nonthin.o -o %t/test -object_path_lto %t/lto-tmp.o
 ; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes CHECK,OBJPATH-FILE -DFILE=%t/lto-tmp.o
+; RUN: llvm-otool -l %t/lto-tmp.o | FileCheck %s --check-prefixes=MINOS
 
 
 ; CHECK:             0000000000000000                - 00 0000    SO /tmp/test.cpp
 ; NOOBJPATH-NEXT:    0000000000000000                - 03 0001   OSO /tmp/lto.tmp
+; NOLTOFILES-NEXT:   0000000000000000                - 03 0001   OSO [[DIR]]/test-obj.o
 ;; check that modTime is nonzero when `-object_path_lto` is provided
 ; OBJPATH-DIR-NEXT:  {{[0-9a-f]*[1-9a-f]+[0-9a-f]*}} - 03 0001   OSO [[DIR]]/1.x86_64.lto.o
 ; OBJPATH-FILE-NEXT: {{[0-9a-f]*[1-9a-f]+[0-9a-f]*}} - 03 0001   OSO [[FILE]]
 ; CHECK-NEXT:        {{[0-9a-f]+}}                   - 01 0000   FUN _main
 ; CHECK-NEXT:        0000000000000001                - 00 0000   FUN
 ; CHECK-NEXT:        0000000000000000                - 01 0000    SO
 ; CHECK-NEXT:        {{[0-9a-f]+}}                   T _main
 ; DSYM: DW_AT_name ("test.cpp")
 ; HARDLINK: 2
 
+; MINOS: 10.15
+
 target triple = "x86_64-apple-macosx10.15.0"
 target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 
 define void @main() #0 !dbg !4 {
   ret void
 }
 
 !llvm.module.flags = !{ !0, !1 }