# Whole Program LLVM in Go [](https://opensource.org/licenses/BSD-3-Clause) [](https://travis-ci.org/SRI-CSL/gllvm) [](https://goreportcard.com/report/github.com/SRI-CSL/gllvm) **TL; DR:** A drop-in replacement for [wllvm](https://github.com/SRI-CSL/whole-program-llvm), that builds the bitcode in parallel, and is faster. A comparison between the two tools can be gleaned from building the [Linux kernel.](https://github.com/SRI-CSL/gllvm/tree/master/examples/linux-kernel) ## Quick Start Comparison Table | wllvm command/env variable | gllvm command/env variable | |-----------------------------|-----------------------------| | wllvm | gclang | | wllvm++ | gclang++ | | wfortran | gflang | | extract-bc | get-bc | | wllvm-sanity-checker | gsanity-check | | LLVM_COMPILER_PATH | LLVM_COMPILER_PATH | | LLVM_CC_NAME ... | LLVM_CC_NAME ... | | | LLVM_F_NAME | | WLLVM_CONFIGURE_ONLY | WLLVM_CONFIGURE_ONLY | | WLLVM_OUTPUT_LEVEL | WLLVM_OUTPUT_LEVEL | | WLLVM_OUTPUT_FILE | WLLVM_OUTPUT_FILE | | LLVM_COMPILER | *not supported* (clang only)| | LLVM_GCC_PREFIX | *not supported* (clang only)| | LLVM_DRAGONEGG_PLUGIN | *not supported* (clang only)| | LLVM_LINK_FLAGS | LLVM_LINK_FLAGS | This project, `gllvm`, provides tools for building whole-program (or whole-library) LLVM bitcode files from an unmodified C or C++ source package. It currently runs on `*nix` platforms such as Linux, FreeBSD, and Mac OS X. It is a Go port of [wllvm](https://github.com/SRI-CSL/whole-program-llvm). `gllvm` provides compiler wrappers that work in two phases. The wrappers first invoke the compiler as normal. Then, for each object file, they call a bitcode compiler to produce LLVM bitcode. The wrappers then store the location of the generated bitcode file in a dedicated section of the object file. When object files are linked together, the contents of the dedicated sections are concatenated (so we don't lose the locations of any of the constituent bitcode files). After the build completes, one can use a `gllvm` utility to read the contents of the dedicated section and link all of the bitcode into a single whole-program bitcode file. This utility works for both executable and native libraries. For more details see [wllvm](https://github.com/SRI-CSL/whole-program-llvm). ## Prerequisites To install `gllvm` you need the go language [tool](https://golang.org/doc/install). To use `gllvm` you need clang/clang++/flang and the llvm tools llvm-link and llvm-ar. `gllvm` is agnostic to the actual llvm version. `gllvm` also relies on standard build tools such as `objcopy` and `ld`. ## Installation To install, simply do (making sure to include those `...`) ``` go get github.com/SRI-CSL/gllvm/cmd/... ``` This should install five binaries: `gclang`, `gclang++`, `gflang`, `get-bc`, and `gsanity-check` in the `$GOPATH/bin` directory. If you are using `go 1.16` you may be forced to install it like this: ``` GO111MODULE=off go get github.com/SRI-CSL/gllvm/cmd/... ``` Hopefully we will have a better fix for this [soon](https://github.com/golang/go/issues/44776)? ## Usage `gclang` and `gclang++` are the wrappers used to compile C and C++. `gflang` is the wrapper used to compile Fortran. `get-bc` is used for extracting the bitcode from a build product (either an object file, executable, library or archive). `gsanity-check` can be used for detecting configuration errors. Here is a simple example. Assuming that clang is in your `PATH`, you can build bitcode for `pkg-config` as follows: ``` tar xf pkg-config-0.26.tar.gz cd pkg-config-0.26 CC=gclang ./configure make ``` This should produce the executable `pkg-config`. To extract the bitcode: ``` get-bc pkg-config ``` which will produce the bitcode module `pkg-config.bc`. For more on this example see [here](https://github.com/SRI-CSL/gllvm/tree/master/examples/pkg-config). ## Advanced Configuration If clang and the llvm tools are not in your `PATH`, you will need to set some environment variables. * `LLVM_COMPILER_PATH` can be set to the absolute path of the directory that contains the compiler and the other LLVM tools to be used. * `LLVM_CC_NAME` can be set if your clang compiler is not called `clang` but something like `clang-3.7`. Similarly `LLVM_CXX_NAME` and `LLVM_F_NAME` can be used to describe what the C++ and Fortran compilers are called, respectively. We also pay attention to the environment variables `LLVM_LINK_NAME` and `LLVM_AR_NAME` in an analogous way. Another useful, and sometimes necessary, environment variable is `WLLVM_CONFIGURE_ONLY`. * `WLLVM_CONFIGURE_ONLY` can be set to anything. If it is set, `gclang` and `gclang++` behave like a normal C or C++ compiler. They do not produce bitcode. Setting `WLLVM_CONFIGURE_ONLY` may prevent configuration errors caused by the unexpected production of hidden bitcode files. It is sometimes required when configuring a build. For example: ``` WLLVM_CONFIGURE_ONLY=1 CC=gclang ./configure make ``` ## Extracting the Bitcode The `get-bc` tool is used to extract the bitcode from a build artifact, such as an executable, object file, thin archive, archive, or library. In the simplest use case, as seen above, one simply does: ``` get-bc -o ``` This will produce the desired bitcode file. The situation is similar for an object file. For an archive or library, there is a choice as to whether you produce a bitcode module or a bitcode archive. This choice is made by using the `-b` switch. Another useful switch is the `-m` switch which will, in addition to producing the bitcode, will also produce a manifest of the bitcode files that made up the final product. As is typical ``` get-bc -h ``` will list all the commandline switches. Since we use the `golang` `flag` module, the switches must precede the artifact path. ## Preserving bitcode files in a store Sometimes, because of pathological build systems, it can be useful to preserve the bitcode files produced in a build, either to prevent deletion or to retrieve it later. If the environment variable `WLLVM_BC_STORE` is set to the absolute path of an existing directory, then WLLVM will copy the produced bitcode file into that directory. The name of the copied bitcode file is the hash of the path to the original bitcode file. For convenience, when using both the manifest feature of `get-bc` and the store, the manifest will contain both the original path, and the store path. ## Debugging The gllvm tools can show various levels of output to aid with debugging. To show this output set the `WLLVM_OUTPUT_LEVEL` environment variable to one of the following levels: * `ERROR` * `WARNING` * `AUDIT` * `INFO` * `DEBUG` For example: ``` export WLLVM_OUTPUT_LEVEL=DEBUG ``` Output will be directed to the standard error stream, unless you specify the path of a logfile via the `WLLVM_OUTPUT_FILE` environment variable. The `AUDIT` level, new in 2022, logs only the calls to the compiler, and indicates whether each call is *compiling* or *linking*, the compiler used, and the arguments provided. For example: ``` export WLLVM_OUTPUT_FILE=/tmp/gllvm.log ``` ## Dragons Begone `gllvm` does not support the dragonegg plugin. ## Sanity Checking Too many environment variables? Try doing a sanity check: ``` gsanity-check ``` it might point out what is wrong. ## Under the hoods Both `wllvm` and `gllvm` toolsets do much the same thing, but the way they do it is slightly different. The `gllvm` toolset's code base is written in `golang`, and is largely derived from the `wllvm`'s python codebase. Both generate object files and bitcode files using the compiler. `wllvm` can use `gcc` and `dragonegg`, `gllvm` can only use `clang`. The `gllvm` toolset does these two tasks in parallel, while `wllvm` does them sequentially. This together with the slowness of python's `fork exec`-ing, and it's interpreted nature accounts for the large efficiency gap between the two toolsets. Both inject the path of the bitcode version of the `.o` file into a dedicated segment of the `.o` file itself. This segment is the same across toolsets, so extracting the bitcode can be done by the appropriate tool in either toolset. On `*nix` both toolsets use `objcopy` to add the segment, while on OS X they use `ld`. When the object files are linked into the resulting library or executable, the bitcode path segments are appended, so the resulting binary contains the paths of all the bitcode files that constitute the binary. To extract the sections the `gllvm` toolset uses the golang packages `"debug/elf"` and `"debug/macho"`, while the `wllvm` toolset uses `objdump` on `*nix`, and `otool` on OS X. Both tools then use `llvm-link` or `llvm-ar` to combine the bitcode files into the desired form. ## Customization under the hood. You can specify the exact version of `objcopy` and `ld` that `gllvm` uses to manipulate the artifacts by setting the `GLLVM_OBJCOPY` and `GLLVM_LD` environment variables. For more details of what's under the `gllvm` hood, try ``` gsanity-check -e ``` ## Customizing the BitCode Generation (e.g. LTO) In some situations it is desirable to pass certain flags to `clang` in the step that produces the bitcode. This can be fulfilled by setting the `LLVM_BITCODE_GENERATION_FLAGS` environment variable to the desired flags, for example `"-flto -fwhole-program-vtables"`. In other situations it is desirable to pass certain flags to `llvm-link` in the step that merges multiple individual bitcode files together (i.e., within `get-bc`). This can be fulfilled by setting the `LLVM_LINK_FLAGS` environment variable to the desired flags, for example `"-internalize -only-needed"`. ## Beware of link time optimization. If the package you are building happens to take advantage of recent `clang` developments such as *link time optimization* (indicated by the presence of compiler flag `-flto`), then your build is unlikely to produce anything that `get-bc` will work on. This is to be expected. When working under these flags, the compiler actually produces object files that are bitcode, your only recourse here is to try and save these object files, and retrieve them yourself. This can be done by setting the `LTO_LINKING_FLAGS` to be something like `"-g -Wl,-plugin-opt=save-temps"` which will be appended to the flags at link time. This will at least preserve the bitcode files, even if `get-bc` will not be able to retrieve them for you. ## Developer tools Debugging usually boils down to looking in the logs, maybe adding a print statement or two. There is an additional executable, not mentioned above, called `gparse` that gets installed along with `gclang`, `gclang++`, `gflang`, `get-bc` and `gsanity-check`. `gparse` takes the command line arguments to the compiler, and outputs how it parsed them. This can sometimes be helpful. ## License `gllvm` is released under a BSD license. See the file `LICENSE` for [details.](LICENSE) --- This material is based upon work supported by the National Science Foundation under Grant [ACI-1440800](http://www.nsf.gov/awardsearch/showAward?AWD_ID=1440800). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.