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
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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 > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160209/7573e3f8/attachment-0001.html>
On Tue, Feb 09, 2016 at 08:09:00AM -0800, Kai Wang via llvm-dev wrote:> Since kernel can only be compiled with -O2, IR has been optimized a lot.The former is mostly irrelevant unless they use the optimizer-specific preprocessor flags. Initial IR optimisations can be disabled with -disable-llvm-optzns.> Is there any way I can know the variable name and there is an assignment > from "zone" to "pgdat"?Variable names in IR are syntactical sugar. They serve no real purpose except making it more human readable. What you want should be derived from debug meta data. On the same line, assignment is a C language concept that doesn't really map well to IR -- where subexpressions can be indistinguishable from assignment. Joerg
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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160209/24b90b35/attachment.html>
Thanks for the reply. 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? I guess I can get variable name "zone" and "pgdat "from debug meta data? But how can I know "zone" and "pgdat" point to same memory location? Thank you. On Tue, Feb 9, 2016 at 8:25 AM, Joerg Sonnenberger via llvm-dev < llvm-dev at lists.llvm.org> wrote:> On Tue, Feb 09, 2016 at 08:09:00AM -0800, Kai Wang via llvm-dev wrote: > > Since kernel can only be compiled with -O2, IR has been optimized a lot. > > The former is mostly irrelevant unless they use the optimizer-specific > preprocessor flags. Initial IR optimisations can be disabled with > -disable-llvm-optzns. > > > Is there any way I can know the variable name and there is an assignment > > from "zone" to "pgdat"? > > Variable names in IR are syntactical sugar. They serve no real purpose > except making it more human readable. What you want should be derived > from debug meta data. On the same line, assignment is a C language > concept that doesn't really map well to IR -- where subexpressions can > be indistinguishable from assignment. > > Joerg > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-- Regards, Kai -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160209/86a0cbc7/attachment.html>