[llvm-dev] Does anything depend on reg2mem?

[Johannes Doerfert via llvm-dev]

On 1/26/22 16:00, Roman Lebedev wrote:
> On Thu, Jan 27, 2022 at 12:17 AM Johannes Doerfert via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> The web is full of scripts and descriptions that tell you to run reg2mem.
>>
>> If we delete it we will get regular emails that things don't work anymore.
>> That said, if we introduce an alias reg2mem -> SROA, we should be fine.
> Note that reg2mem is an opposite transform to that of SROA/mem2reg.

Ups, misread. Need more or less coffee.


>
> NACK to the removal, unless we also remove all of the functionality
> that said pass uses, otherwise we are just reducing test coverage,
>
>> ~ Johannes
> Roman
>
>> On 1/26/22 13:55, Reid Kleckner via llvm-dev wrote:
>>> I looked into this back when we were working on Windows EH around four
>>> years ago, and at the time, so far as I could tell, no, nothing depends on
>>> reg2mem. I asked Dan Gohman about it at the time, and he said that
>>> essentially SSAUpdater was introduced and rolled out across the codebase to
>>> address these CFG manipulation use cases properly.
>>>
>>> I believe there may exist passes that call into reg2mem helper functions to
>>> demote specific values into memory across certain operations such as
>>> setjmp, for example. Parts of the code may still be in use, but IMO the
>>> pass can be deleted.
>>>
>>> On Sun, Jan 16, 2022 at 5:10 AM Russell Wallace via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> According to the documentation,
>>>>
>>>>> This file demotes all registers to memory references. It is intended to
>>>> be the inverse of mem2reg. By converting to load instructions, the only
>>>> values live across basic blocks are alloca instructions and load
>>>> instructions before phi nodes. It is intended that this should make CFG
>>>> hacking much easier. To make later hacking easier, the entry block is split
>>>> into two, such that all introduced alloca instructions (and nothing else)
>>>> are in the entry block.
>>>>
>>>> It does make sense that converting everything back to memory references,
>>>> would make CFG hacking easier. For example, simplifycfg would have an
>>>> easier job if it requires reg2mem to have been run first.
>>>>
>>>> Does simplifycfg, or any other pass, actually have a 'reg2mem must have
>>>> been run first' dependency?
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