[llvm-dev] [RFC] LLVM Busybox Proposal

[Tom Stellard via llvm-dev]

On 6/21/21 10:54 AM, Leonard Chan via llvm-dev wrote:
> Hello all,
> 
> When building LLVM tools, including Clang and lld, it's currently possible to use either static or shared linking for LLVM libraries. The latter can significantly reduce the size of the toolchain since we aren't duplicating the same code in every binary, but the dynamic relocations can affect performance. The former doesn't affect performance but significantly increases the size of our toolchain.
> 
> We would like to implement a support for a third approach which we call, for a lack of better term, "busybox" feature, where everything is compiled into a single binary which then dispatches into an appropriate tool depending on the first command. This approach can significantly reduce the size by deduplicating all of the shared code without affecting the performance.
> 
> In terms of implementation, the build would produce a single binary called `llvm` and the first command would identify the tool. For example, instead of invoking `llvm-nm` you'd invoke `llvm nm`. Ideally we would also support creation of `llvm-nm` symlink which redirects to `llvm` for backwards compatibility.
> This functionality would ideally be implemented as an option in the CMake build that toolchain vendors can opt into.
> 
> The implementation would have to replace `main` function of each tool with an entrypoint regular function which is registered into a tool registry. This could be wrapped in a macro for convenience. When the "busybox" feature is disabled, the macro would expand to a `main` function as before and redirect to the entrypoint function. When the "busybox" feature is enabled, it would register the entrypoint function into the registry, which would be responsible for the dispatching based on the tool name. Ideally, toolchain maintainers would also be able to control which tools they could add to the "busybox" binary via CMake build options, so toolchains will only include the tools they use.
> 
> One implementation detail we think will be an issue is merging arguments in individual tools that use `cl::opt`. `cl::opt` works by maintaining a global state of flags, but we aren’t confident of what the resulting behavior will be when merging them together in the dispatching `main`. What we would like to avoid is having flags used by one specific tool available on other tools. To address this issue, we would like to migrate all tools to use `OptTable` which doesn't have this issue and has been the general direction most tools have been already moving into.
> 
> A second issue would be resolving symlinks. For example, llvm-objcopy will check argv[0] and behave as llvm-strip (ie. use the right flags + configuration) if it is called via a symlink that “looks like” a strip tool, but for all other cases it will run under the default objcopy mode. The “looks like” function is usually an `Is` function copied in multiple tools that is essentially a substring check: so symlinks like `llvm-strip`, strip.exe, and `gnu-llvm-strip-10` all result in using the strip “mode” while all other names use the objcopy mode. To replicate the same behavior, we will need to take great care in making sure symlinks to the busybox tool dispatch correctly to the appropriate llvm tool, which might mean exposing and merging these `Is` functions.
> 
> Some open questions:
> - People's initial thoughts/opinions?

I think it's an interesting idea.  My main concern is that adding a new CMake
option for this going to complicate the build system and make future CMake
improvements more difficult.

Do you have any idea of how much performance /
toolchain size gains you will get from this approach?

-Tom

> - Are there existing tools in LLVM that already do this?
> - Other implementation details/global states that we would also need to account for?
> 
> - Leonard
> 
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