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$ llvm-lto --help
OVERVIEW: llvm LTO linker

USAGE: llvm-lto [options] <input bitcode files>

OPTIONS:

Color Options:

  --color                                 - Use colors in output (default=autodetect)

Generic Options:

  --help                                  - Display available options (--help-hidden for more)
  --help-list                             - Display list of available options (--help-list-hidden for more)
  --version                               - Display the version of this program

LTO Options:

  -O <char>                               - Optimization level. [-O0, -O1, -O2, or -O3] (default = '-O2')
  --check-for-objc                        - Only check if the module has objective-C defined in it
  --disable-verify                        - Do not run the verifier during the optimization pipeline
  --dso-symbol=<string>                   - Symbol to put in the symtab in the resulting dso
  --exported-symbol=<string>              - List of symbols to export from the resulting object file
  -j <uint>                               - Number of backend threads
  --list-dependent-libraries-only         - Instead of running LTO, list the dependent libraries in each IR file
  --list-symbols-only                     - Instead of running LTO, list the symbols in each IR file
  --lto-freestanding                      - Enable Freestanding (disable builtins / TLI) during LTO
  -o <filename>                           - Override output filename
  --print-macho-cpu-only                  - Instead of running LTO, print the mach-o cpu in each IR file
  --restore-linkage                       - Restore original linkage of globals prior to CodeGen
  --save-linked-module                    - Write linked LTO module to file before optimize
  --save-merged-module                    - Write merged LTO module to file before CodeGen
  --set-merged-module                     - Use the first input module as the merged module
  --thinlto                               - Only write combined global index for ThinLTO backends
  --thinlto-action=<value>                - Perform a single ThinLTO stage:
    =thinlink                             -   ThinLink: produces the index by linking only the summaries.
    =distributedindexes                   -   Produces individual indexes for distributed backends.
    =emitimports                          -   Emit imports files for distributed backends.
    =promote                              -   Perform pre-import promotion (requires -thinlto-index).
    =import                               -   Perform both promotion and cross-module importing (requires -thinlto-index).
    =internalize                          -   Perform internalization driven by -exported-symbol (requires -thinlto-index).
    =optimize                             -   Perform ThinLTO optimizations.
    =codegen                              -   CodeGen (expected to match llc)
    =run                                  -   Perform ThinLTO end-to-end
  --thinlto-cache-dir=<string>            - Enable ThinLTO caching.
  --thinlto-cache-entry-expiration=<uint> - Set ThinLTO cache entry expiration time.
  --thinlto-cache-max-size-bytes=<ulong>  - Set ThinLTO cache pruning directory maximum size in bytes.
  --thinlto-cache-max-size-files=<int>    - Set ThinLTO cache pruning directory maximum number of files.
  --thinlto-cache-pruning-interval=<int>  - Set ThinLTO cache pruning interval.
  --thinlto-index=<string>                - Provide the index produced by a ThinLink, required to perform the promotion and/or importing.
  --thinlto-index-stats                   - Print statistic for the index in every input files
  --thinlto-module-id=<string>            - For the module ID for the file to process, useful to match what is in the index.
  --thinlto-prefix-replace=<string>       - Control where files for distributed backends are created. Expects 'oldprefix;newprefix' and if path prefix of output file is oldprefix it will be replaced with newprefix.
  --thinlto-save-objects=<string>         - Save ThinLTO generated object files using filenames created in the given directory.
  --thinlto-save-temps=<string>           - Save ThinLTO temp files using filenames created by adding suffixes to the given file path prefix.
  --use-diagnostic-handler                - Use a diagnostic handler to test the handler interface

# Install LLVM bitcode module (for JITing).
#
# The arguments are:
# $(1) name of the module (e.g. an extension's name or postgres for core code)
# $(2) source objects, with .o suffix
#
# The many INSTALL_DATA invocations aren't particularly fast, it'd be
# good if we could coalesce them, but I didn't find a good way.
#
# Note: blank line at end of macro is necessary to let it be used in foreach
define install_llvm_module
$(MKDIR_P) '$(DESTDIR)${bitcodedir}/$(1)'
$(MKDIR_P) $(sort $(dir $(addprefix '$(DESTDIR)${bitcodedir}'/$(1)/, $(2))))
$(foreach obj, ${2}, $(INSTALL_DATA) $(patsubst %.o,%.bc, $(obj)) '$(DESTDIR)${bitcodedir}'/$(1)/$(dir $(obj))
)
cd '$(DESTDIR)${bitcodedir}' && $(LLVM_BINPATH)/llvm-lto -thinlto -thinlto-action=thinlink -o $(1).index.bc $(addprefix $(1)/,$(patsubst %.o,%.bc, $(2)))

endef

static std::unique_ptr<llvm::ModuleSummaryIndex>
llvm_load_summary(llvm::StringRef path)
{
	llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer> > MBOrErr =
		llvm::MemoryBuffer::getFile(path);

	if (std::error_code EC = MBOrErr.getError())
	{
		ilog(DEBUG1, "failed to open %s: %s", path.data(),
			 EC.message().c_str());
	}
	else
	{
		llvm::MemoryBufferRef ref(*MBOrErr.get().get());

		llvm::Expected<std::unique_ptr<llvm::ModuleSummaryIndex> > IndexOrErr =
			llvm::getModuleSummaryIndex(ref);
		if (IndexOrErr)
			return std::move(IndexOrErr.get());
		elog(FATAL, "failed to load summary \"%s\": %s",
			 path.data(),
			 toString(IndexOrErr.takeError()).c_str());
	}
	return nullptr;
}