[llvm-dev] Assigning custom information to IR instruction and passing it to its correspondent in Selection DAG

[Przemyslaw Ossowski via llvm-dev]

Thanks Mehdi for the suggestion.
I want to customize handling of 'load' instruction during processing DAG,
but on IR level I want the 'load' to be treated in different passes as
common load. So I'm afraid that replacing 'load' with Intrinsic would cause
different handling it comparing to LoadInst.
I've found that in the DAGBuilder metadata information connected with
LoadInst instruction is utilized:
bool isInvariant = I.getMetadata(LLVMContext::MD_invariant_load) != nullptr;

Is this metadata somehow protected against dropping - as you mentioned?

Thanks,
Przemek

On Wed, May 1, 2019 at 5:57 AM Mehdi AMINI <joker.eph at gmail.com> wrote:

>
>
> On Tue, Apr 30, 2019 at 3:51 PM Przemyslaw Ossowski via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>> as I wrote in mu previous post I wanted to somehow mark one IR
>> instruction (in this particular case it would be 'load') during dedicated
>> pass, which will set the marking based on neighboring instructions. Next I
>> wanted to somehow to convey this marking from 'load' IR instruction to
>> 'load' SDNode in order to use it during DAG Instruction Selection -
>> information stored in this marking would 'enable' or 'disable' particular
>> pattern... I was thinking about usage metadata for it, but I didn't get any
>> feedback.
>> So now I'm wondering if solution with metadata is the best one or it
>> maybe it doesn't make any sense an something else would be better.
>>
>> I don't have big experience with metadata, but what I've read it would
>> allow for customizing IR instruction without modifying the instruction. Or
>> maybe I'm unnecessarily thinking about metadata, because 'load' instruction
>> has already some fields which could be used for storing 'flags', which
>> might represent any custom information. Obviously I don't want to modify
>> 'load'.
>> Any suggestions?
>>
>
> In general metadata aren't recommended for carrying semantically important
> information as they can be dropped at any time.
>
> What about using a target specific intrinsic? Your pass could convert the
> instructions to your intrinsic that you can then easily match in
> SelectionDAG.
>
> --
> Mehdi
>
>
>
>>
>>
>>
>> On Thu, Apr 25, 2019 at 11:54 AM Przemyslaw Ossowski <
>> przemyslaw.ossowski at googlemail.com> wrote:
>>
>>> Hello,
>>>
>>>
>>>
>>> I’m looking for the best approach which would allow for marking given
>>> LLVM IR instruction during IR custom pass and later utilizing that
>>> information during DAG Instruction Selection in order to match given
>>> pattern based on this marking.
>>>
>>>
>>>
>>> I’m wondering if marking IR instruction utilizing Metadata is good idea.
>>>
>>> But how later pass that information to DAG and appropriately mark in
>>> DAGBuilder SDNode, which represents the earlier marked instruction in IR?
>>>
>>>
>>>
>>> Any suggestion how to implement the most efficiently?
>>>
>>> I’m expecting there are already similar solutions – maybe someone would
>>> be able to indicate the exact example?
>>>
>>>
>>>
>>> Thanks in advance,
>>>
>>> Przemek
>>>
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>
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