[PATCH] D70500: [WebAssembly] Enable use of wasm-opt and LTO-enabled system libraries

Thu Apr 23 07:32:15 PDT 2020

sunfish added a comment.

In D70500#1998994 <https://reviews.llvm.org/D70500#1998994>, @bernhard wrote:

> Are there plans to offer a way to disable this behavior (or have it optional in the first place)?
>  We'd like to run some custom processing between wasm-ld and wasm-opt which can't happen after the latter due to some of its one-way destructive optimizations (i.e. memory-packing or simplify-globals passes).
>  The only way now is to tell our users to place wasm-opt somewhere where clang can't find it. Or instead of using one clang super-command to manually call -cc1 and wasm-ld separately which is disappointing.
>
> Also, is it even common to place wasm-opt next to the clang executable? Who is this for? Is this documented?

It's for users who want smaller wasm binaries. It's not currently documented, though yes, it would be nice to document it.

That said, details about how LLVM lays out memory or uses globals are intentionally not documented. Just as we don't make any guarantees about exactly which optimizations we do at -O1 vs -O2, we're don't make any guarantees about which optimizations are done in clang vs wasm-ld vs wasm-opt. Clang and wasm-ld are free to do anything wasm-opt does. Could you describe what you want to do in more detail? It may be possible to find alternative approaches, or to design a feature for it with a clear and documentable scope.

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https://reviews.llvm.org/D70500