[llvm-dev] Functions have two types, one can be mutated but not the other

[Andy Ayers via llvm-dev]

Thanks. Our use case is pretty simple:

Create a function with some placeholder type.
Figure out the actual function type and create some IR that calls the function with that type. Update function’s type via mutateType.
Dump the module. The function’s “declaration” still shows it has the placeholder type.
This doesn’t cause us any problems because the IR is what matters and it’s got the right description.

FWIW we do this because the transformation from an MSIL signature to an ABI-level signature is somewhat involved, and we’ve intertwined the code to generate the right call sequence with the code that figures out what type you end up with once you’ve done all the various transformations, so to start things off, we need a way to refer to the function without knowing precisely what type it will end up with.

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Tuesday, January 19, 2016 3:36 PM
To: Andy Ayers <andya at microsoft.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Functions have two types, one can be mutated but not the other



On Tue, Nov 24, 2015 at 9:25 AM, Andy Ayers via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Function has its own FunctionType* member as well as a Type* member that it inherits from GlobalValue. The latter can be mutated but not the former, leading to potential strange inconsistencies.

While I realize using mutateType is probably going to trigger a bunch of “you’re doing it wrong” replies, it seems like mutateType, as a necessary evil, should be virtual and do the right wrong thing for Functions too.

Thoughts?

I've been perpetrating some of this for the typeless/opaque pointer IR stuff (some details here: https://www.youtube.com/watch?v=OWgWDx_gB1I<https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fwww.youtube.com%2fwatch%3fv%3dOWgWDx_gB1I&data=01%7c01%7candya%40microsoft.com%7c692fcbd2bdf147a7a2fb08d321293eef%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=VWuobdaqbGrRzX0N7VrZw6UubBarZThFRpTl9aoe%2fFc%3d> - or on a few threads on llvm-dev).

I got rid of that extra one in Function - thanks for noticing.

If I recall correctly, even before my work, it wasn't possible to rely solely on mutateType of the operands  so various special cases have been added (ValueMapper.cpp is a good example of some of these special cases)

I'm not sure why those issues might not have come up when dealing with Globals. Perhaps you have some examples (eg: linking bitcode with globals - where the value type gets out of sync with the pointer type? Maybe if we add some more assertions in to check the two values are in sync we'd find cases where they've gotten out of sync?)
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