[llvm-dev] Does anything depend on reg2mem?

Johannes Doerfert via llvm-dev
2022-01-26T21:16:52Z

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.

~ Johannes

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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