[llvm-dev] Wasm, start function, and default globals

[Thomas Lively via llvm-dev]

The start function is not meant for general use, but rather for toolchains
to initialize the state of the runtime where they can guarantee that no
imports will be required.

For getting rid of unused globals and generally optimizing your wasm
modules, I highly recommend using Binaryen's wasm-opt tool.
https://github.com/webassembly/binaryen.

On Tue, Oct 15, 2019 at 6:11 PM Petr Penzin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> There is a good amount of expertise on this in Emscripten community:
> https://github.com/emscripten-core/emscripten
>
> You can also try to ask in one of the WebAssembly repos:
> https://github.com/WebAssembly
>
> I think Luke Imhoff has done some work accessing the start function, I am
> not sure if he is on this list. I personally don't know of a way write to
> start function. There is a bit on it in tool conventions:
>
>
> https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md#start-section
>
> The __heap_base symbol is there because Wasm does not have the memory
> layout C family languages expect. At higher opt level Clang is usually
> pretty good at removing its use.
>
> Best,
>
> Petr
> On 10/15/19 4:50 PM, Bill Ticehurst via llvm-dev wrote:
>
> Apologies if there is a better forum for these questions. Please redirect
> me if so.
>
>
>
> I’ve been using the clang/wasm-ld tools to experiment with some basic
> examples, and there’s a couple things I’m wrestling with.
>
>
>
> 1) How to denote a function as the “start” function (
> https://webassembly.github.io/spec/core/binary/modules.html#start-section)
>
> 2) How to avoid the defaulted __heap_base global.
>
>
>
> I’ve dug around on the samples, mailing lists, and in the wasm-ld code for
> an attribute or flag to specify the start function, but don’t see anything.
> Is this just not implemented (or tracked) yet?
>
>
>
> For 2), I always see the below global in the compiled output even with the
> most trivial code that makes no reference to it (e.g. compiling just a
> simple function that only references values on the implicit operand stack).
>
>
>
> (global (;0;) (mut i32) (i32.const 66560))
>
>
>
> Per https://dassur.ma/things/c-to-webassembly/, looks like this is the
> __heap_base value implicitly provided for modeling the stack/heap as are
> usually present in most C/C++ environments, even though unused in my
> compiled output. Is this something that could/should be optimized away, but
> again just isn’t implemented yet? Any way to suppress this building with
> the current clang/wasm-ld toolchain?
>
>
>
> Thanks,
>
>
>
>    - Bill
>
>
>
>
>
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>
>
>
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