[LLVMdev] Upstreaming PNaCl's IR simplification passes

[Sean Silva]

On Tue, Mar 4, 2014 at 4:04 PM, Mark Seaborn <mseaborn at chromium.org> wrote:

> The PNaCl project has implemented various IR simplification passes that
> simplify LLVM IR by lowering complex features to simpler features.  We'd
> like to upstream some of these IR passes to LLVM.  We'd like to explore if
> this acceptable, and if so, how we should go about doing this.
>
> The immediate reason is that Emscripten is reusing PNaCl's IR passes for
> its new "fastcomp" backend [1].  It would be really useful if PNaCl and
> Emscripten could collaborate via upstream LLVM rather than a branch.
>
> Some background:  There are two related use cases for these IR
> simplification passes:
>
>  1) Simplifying the task of writing a new LLVM backend.  This is
> Emscripten's use case.  The IR simplification passes reduce the number of
> cases a backend has to handle, so they would be useful for anyone else
> creating a new backend.
>

FWIW, this sounds to me like a sufficiently compelling use case to support
getting this in-tree.

-- Sean Silva


>
>  2) Using a subset of LLVM IR as a stable distribution format for portable
> executables.  This is PNaCl's use case.  PNaCl's IR subset omits various
> complex IR features, which we lower using the IR simplification passes [2].
>  Renderscript is an example of another project that uses IR as a stable
> distribution format, though I think currently Renderscript is not
> subsetting IR much.
>
> Some examples of PNaCl's IR simplification passes are:
>
>  * Calling conventions lowering:  ExpandVarArgs and ExpandByVal lower
> varargs and by-value argument passing respectively.  They would be useful
> for any backend that doesn't want to implement varargs or by-value calling
> conventions.
>
>  * Instruction-level lowering:
>     * ExpandStructRegs splits up struct values into scalars, removing the
> "insertvalue" and "extractvalue" instructions.
>     * PromoteIntegers legalizes integer types (e.g. i30 is converted to
> i32).
>
>  * Module-level lowering:  This implements, at the IR level, functionality
> that is traditionally provided by "ld".  e.g. ExpandCtors lowers
> llvm.global_ctors to the __init_array_start and __init_array_end symbols
> that are used by C libraries at startup.
>
> PNaCl's IR simplification passes are modular -- most are independent of
> each other -- so they allow projects to pick and choose which IR features
> to support and which to pre-lower.  The modularity of these passes makes
> them low-maintenance and easy to write targeted tests for.
>
> The code for these passes can be found here:
>
> https://chromium.googlesource.com/native_client/pnacl-llvm/+/master/lib/Transforms/NaCl/
>
> There seems to be plenty of precedent for IR-to-IR lowering passes -- LLVM
> already contains passes such as LowerInvoke, LowerSwitch and LowerAtomic.
>
> The PNaCl team (which I'm a member of) is happy to take on the work of
> maintaining this code, such as updating it as LLVM IR evolves and doing
> code reviews.  We would upstream this gradually, pass by pass, so the
> changes would be manageable.
>
> Cheers,
> Mark
>
> [1] https://github.com/kripken/emscripten/wiki/LLVM-Backend
> [2] https://groups.google.com/forum/#!topic/llvm-dev/lk6dZzwW0ls - PNaCl
> Bitcode reference manual
>
>
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