[llvm-dev] IR with no optimization

[Kai Wang via llvm-dev]

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?

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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