[llvm-dev] IR with no optimization

[Jingyue Wu via llvm-dev]

On Tue, Feb 9, 2016 at 9:31 AM, Kai Wang via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thanks for the reply.
>
> Yes I'm doing static analysis. I'm trying to do points-to analysis
> actually. I care about whether pointer values point to the same memory
> location. I'm not sure if this is better to be done by Clang or LLVM?
>

That depends on what interface your analysis is going to expose. If the
users of your pointer analysis are other IR passes, LLVM IR sounds a better
place and you don't need to care about variable names or assignments.


>
> How to dump the unoptimized IR? By compiling with -O0?
> Thank you.
>
>
> On Tue, Feb 9, 2016 at 8:20 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> If you're trying to do source level analysis (questions like "is there an
>> assignment of a variable of this name") it may be better to work up in
>> Clang than down in LLVM - LLVM has no guarantees about names (indeed names
>> on instructions are a compiler debugging feature, not a feature that should
>> be used by any optimization, analysis, etc) or preservation of things like
>> loads/stores.
>>
>> You could dump the unoptimized IR, if you're just trying to do some
>> static analysis rather than an optimization - doesn't matter to you,
>> probably, if it doesn't produce a valid kernel in the end.
>>
>> On Tue, Feb 9, 2016 at 8:09 AM, Kai Wang via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi all,
>>>
>>> I'm compiling linux kernel with clang. I want to generate IR with no
>>> optimization. However, kernel can only be compile with -O2 instead of -O0.
>>>
>>> Here is the source code snippet:
>>>
>>> struct zone *next_zone(struct zone *zone)
>>>
>>>  {   pg_data_t **pgdat* = zone->zone_pgdat;
>>>
>>> }
>>>
>>> I want to know there is an assignment from "zone" to "pgdat". I'm trying
>>> to iterate "store" instructions in IR.
>>>
>>> When I compile with -O2, I have the following IR:
>>>
>>> define %struct.zone* @next_zone(%struct.zone* readonly %zone) #0 !dbg !
>>> 214 {
>>>
>>>         call void @llvm.dbg.value(metadata %struct.zone* %zone, i64 0,
>>> metadata !218, metadata !305), !dbg !326
>>>
>>>         %1 = getelementptr inbounds %struct.zone, %struct.zone* %zone,
>>> i64 0, i32 5, !dbg !327
>>>
>>>         %2 = load %struct.pglist_data*, %struct.pglist_data** %1, align
>>> 8, !dbg !327
>>>
>>>         call void @llvm.dbg.value(metadata %struct.pglist_data* %2, i64
>>> 0, metadata !219, metadata !305), !dbg !328 }
>>>
>>> *Store instruction has been optimized, and no variable name in IR.*
>>>
>>> When I comile with -O0, I have the following IR:
>>>
>>> define %struct.zone* @next_zone(%struct.zone* %zone) #0 !dbg !211 {
>>>
>>>          %1 = alloca %struct.zone*, align 8
>>>
>>>          %pgdat = alloca %struct.pglist_data*, align 8
>>>
>>>          store %struct.zone* %zone, %struct.zone** %1, align 8
>>>
>>>          call void @llvm.dbg.declare(metadata %struct.zone** %1,
>>> metadata !297, metadata !265), !dbg !298
>>>
>>>          call void @llvm.dbg.declare(metadata %struct.pglist_data**
>>> %pgdat, metadata !299, metadata !265), !dbg !302
>>>
>>>          %2 = load %struct.zone*, %struct.zone** %1, align 8, !dbg !303
>>>
>>>          %3 = getelementptr inbounds %struct.zone, %struct.zone* %2, i32
>>> 0, i32 5, !dbg !304
>>>
>>>          %4 = load %struct.pglist_data*, %struct.pglist_data** %3, align
>>> 8, !dbg !304
>>>
>>>          *store %struct.pglist_data* %4, %struct.pglist_data** %**pgdat,
>>> align 8, !dbg !302*
>>> There is store instruction. I know there is an assignment. From this
>>> store, I backward traverse until I find variable.
>>> For example, I go through %4->%3->%2->%1->struct.zone. I have variable
>>> name pgdat in IR as well.
>>>
>>> Since kernel can only be compiled with -O2, IR has been optimized a lot.
>>> Is there any way I can know the variable name and there is an assignment
>>> from "zone" to "pgdat"?
>>>
>>> Thank you!
>>> --
>>> Regards,
>>> Kai
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
>
> --
> Regards,
> Kai
>
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