[llvm-dev] Question about versioning of LVVM IR

[Stanislav Pankevich via llvm-dev]

Hello,

It is likely that I missing something however I have to ask.

We are working on Mull project https://github.com/mull-project which deals
with
LLVM IR in the first place so it is often that we encounter errors like
the following one when we do things like parseAssemblyString(IR, Err,
GlobalCtx);`:

```
test: <string>:7237:187: error: invalid field 'variable'
!1526 = distinct !DIGlobalVariable(name: "test_info_", linkageName:
"_ZN14Hello_sup_Test10test_info_E", scope: !0, file: !1527, line: 4, type:
!1528, isLocal: false, isDefinition: true, variable:
%"class.testing::TestInfo"** @_ZN14Hello_sup_Test10test_info_E,
declaration: !2817)
```

Quick research of recent LLVM commits reveals that stuff is being changed
significantly, example:

commit 7b500b4bdf40cb40cb33bdcf5faf900db4930824
Author: Adrian Prantl <aprantl at apple.com>
Date:   Tue Dec 20 02:09:43 2016 +0000

    [IR] Remove the DIExpression field from DIGlobalVariable.

    ...

The questions are:

1) why do Asm / LL parsers not produce warnings like:

"LLVM IR you are using has version 3.9 which is incompatible with current
supported version 4.0."

2) I didn't find any special version markers in LLVM IR. My guess is that
having them there would allow a developer who is changing LLVM IR format to
put deprecation / backward incompatibility checks so that higher-level
developers, like we are, see the friendly messages/warnings (see 1). It
would
help us a lot to see that the reason is incompatibility, not any other
reasons
like mistakes that we sometimes make.

Is there a reason why this kind of versioning is not being done in
AsmParser/LLParser?

Is it hard to have such functionality in place?

Background: we develop `Mull` as in-source project inside LLVM. We often
use stable LLVM distribution from `brew` and Rust stable/nightly compilers
that are
often one version behind the latest stable version of LLVM. Currently our
solution
is to stick to stable version of LLVM source tree however having a more
friendly
output from AsmParser/LLParser would help us a lot to understand and debug
things faster and easier.

Thanks.

Stan
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