[llvm-dev] Debugging a potential bug when generating wasm32

[Thomas Lively via llvm-dev]

+Sam Clegg <sbc at google.com> Is the expert on this dynamic linking stuff.

A lot of us WebAssembly toolchain folks have been hanging out in the
WebAssembly
Discord <https://discord.gg/6UCUaMP>, especially on the #emscripten
channel, so that would be another good place to ask future
WebAssembly-specific questions.

On Tue, Aug 4, 2020 at 9:06 PM Ömer Sinan Ağacan via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> Sorry if you've seen this message before on llvm.discourse.group or
> elsewhere --
> I've been trying to get to the bottom of this for a while now and asked
> about
> this in a few different platforms before.
>
> I'm currently trying to debug a bug in a LLVM-generated Wasm code. The bug
> could
> be in the code that generates LLVM (rustc) or in the LLVM, I'm not sure
> yet.
> LLVM IR and Wasm can be seen in [1].
>
> The problem is this line:
>
>     (import "GOT.func"
>
> "_ZN4core3fmt3num3imp52_$LT$impl$u20$core..fmt..Display$u20$for$u20$i32$GT$3fmt17h9ba9fea9cadf7bd5E"
> (global (;3;) (mut i32)))
>
> The same symbol is already imported from "env" in the same module:
>
>     (import "env"
>
> "_ZN5core3fmt3num3imp52_$LT$impl$u20$core..fmt..Display$u20$for$u20$i32$GT$3fmt17h9ba9fea9cadf7bd5E"
> (func (;4;) (type 1)))
>
> So there's no need to import it from "GOT.func" and I want to get rid of
> that
> "GOT.func" import.
>
> This LLVM IR is generated when compiling Rust code to a "staticlib", which
> is
> supposed to include *all* dependencies of the code so that it'll be
> linkable
> with code for other languages. Because of the "GOT.func" import this
> module is
> not linkable, it needs to resolve that "GOT.func" import in runtime using
> dynamic linking for Wasm [2].
>
> I'm trying to understand whether this is a rustc bug or an LLVM bug. I'm
> using
> LLVM 10 downloaded from the official web page and rustc nightly. I can
> build
> LLVM from source and use it, but I don't have any experience in LLVM code
> base.
> Questions:
>
> - Given a reference to a symbol, how does LLVM decide how to import it?
>   Currently I see these uses of the problematic symbol in LLVM IR:
>
>   - `store i8* bitcast (i1 (i32*, %"core::fmt::Formatter"*)*
>
> @"_ZN4core3fmt3num3imp52_$LT$impl$u20$core..fmt..Display$u20$for$u20$i32$GT$3fmt17h9ba9fea9cadf7bd5E"
> to i8*), i8** %11, align 4`
>
>   - `store i8* bitcast (i1 (i32*, %"core::fmt::Formatter"*)*
>
> @"_ZN4core3fmt3num3imp52_$LT$impl$u20$core..fmt..Display$u20$for$u20$i32$GT$3fmt17h9ba9fea9cadf7bd5E"
> to i8*), i8** %14, align 4`
>
>   - `store i8* bitcast (i1 (i32*, %"core::fmt::Formatter"*)*
>
> @"_ZN4core3fmt3num3imp52_$LT$impl$u20$core..fmt..Display$u20$for$u20$i32$GT$3fmt17h9ba9fea9cadf7bd5E"
> to i8*), i8** %17, align 4`
>
>   - `declare zeroext i1
>
> @"_ZN4core3fmt3num3imp52_$LT$impl$u20$core..fmt..Display$u20$for$u20$i32$GT$3fmt17h9ba9fea9cadf7bd5E"(i32*
> noalias readonly align 4 dereferenceable(4), %"core::fmt::Formatter"*
> align 4 dereferenceable(36)) unnamed_addr #1`
>
>   First three look very similar so I'm guessing the first three are
> causing one
>   of those imports, and the last one is causing the other import, but I'm
> not
>   sure which one is generating which import. Any ideas?
>
> - Any suggestions on how to debug this? Just knowing which line in the
> LLVM IR
>   listed above causes this "GOT.func" import would be helpful.
>
> Thanks,
>
> Ömer
>
> [1]: https://gist.github.com/osa1/4c672fe8998c8e8768cf9f7c014c61d8
> [2]:
> https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md
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