[PATCH] D58312: [WebAssembly] Generalize section ordering constraints

Tue Feb 19 14:21:04 PST 2019

tlively added a comment.

In D58312#1402917 <https://reviews.llvm.org/D58312#1402917>, @binji wrote:

> In D58312#1402909 <https://reviews.llvm.org/D58312#1402909>, @tlively wrote:
>
> > In D58312#1402683 <https://reviews.llvm.org/D58312#1402683>, @binji wrote:
> >
> > > Yeah, I'd still prefer to have a total order if that's not too much effort. The alternative introduces a lot of potential complexity into the consumer, since we'll have a combinatorial explosion of possibilities for where these sections can be located, many of which will likely be untested.
> >
> >
> > I definitely want to reduce overall complexity, not increase it. My assumption is that if consumer code would be simplified by guaranteeing that section A is before section B, then the DAG ordering should be updated to make that ordering required. Sections should only be unordered with respect to each other if consumers are truly agnostic to their order.
> >
> > In this case, producer code was able to be simplified by loosening the ordering constraints with no corresponding complications in consumer code that I am aware of. @binji, does this seem reasonable? Are there potential complications that I am missing?
>
>
> I'm suggesting that we provided an ordering even between sections that otherwise have no constraints, similar to the rest of the wasm format. There's no requirement that the custom sections have an ordering, but I'm worried that we'll end up with consistency issues like we did with the name section (before it was spec'd as being required after the data section).

I'm totally fine with specifying a mostly-linear ordering between sections, including between custom sections that are meaningful to the toolchain. But I think it also makes sense to allow for exceptions to that when it allows for simpler code, as is the case for the unspecified ordering between the producers section and relocation sections. This CL is about changing the mechanism to allow for this exception, not about changing the ordering of existing sections or changing our norm of specifying such orderings. Do you think we should disallow exceptions entirely?

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