[llvm-dev] loop unrolling introduces conditional branch
Mehdi Amini via llvm-dev
llvm-dev at lists.llvm.org
Sat Aug 22 00:27:11 PDT 2015
Can you post the two IR online?
—
Mehdi
> On Aug 21, 2015, at 8:27 PM, Xiangyang Guo via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> I just tried llvm-3.8 (LLVM SVN Repository). With this version, -fno-rtti can help me to compile my code and -irce can help me to do a better job for loop unrolling. However, I still have one question. If I use Clang to compile a piece of c++ code to .bc and then use 'opt -loop-rotate -loop-unroll -irce', I can get what I want. I mean, there is no conditional branch at the end of each unrolled part. However, If I use LLVM API such as IRBuilder (CreateAdd, CreateGEP, CreateLoad and so on) to generate the .bc (I dump the two .bc files and they looks like almost same except the variable name), then 'opt -loop-rotate -loop-unroll -irce'I cannot get what I want. I mean, in this case, there is still loop boundary checking (add, compare, conditional branch) at the end of each unrolled part.
>
> I'm really confused about this. Does Clang do something special? Or do I need to do something else to eliminate the unnecessary loop boundary checking at the end of each unrolled part?
>
> Thanks for your help.
>
> Xiangyang
>
>
> On Fri, Aug 21, 2015 at 11:29 AM, Xiangyang Guo <xguo6 at ncsu.edu <mailto:xguo6 at ncsu.edu>> wrote:
> Hi, Jeremy,
>
> Thanks for your reply. I tried -fno-rtti yesterday and no luck.
>
> Regards,
>
> Xiangyang
>
> On Fri, Aug 21, 2015 at 11:05 AM, Jeremy Lakeman <Jeremy.Lakeman at gmail.com <mailto:Jeremy.Lakeman at gmail.com>> wrote:
> There's been some recent noise on the mailing list about requiring -fno-rtti;
> http://lists.llvm.org/pipermail/llvm-dev/2015-August/089010.html <https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.llvm.org_pipermail_llvm-2Ddev_2015-2DAugust_089010.html&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=iK-4Sl62Seah5JOtnUG-QuscAiOsYnFzalJonc_U6VU&s=ymrIB-O3ZSdNYeAr6O77yr2EXY5oYesx1dTE2lvYifs&e=>
>
> Could that be it?
>
> On Sat, Aug 22, 2015 at 12:21 AM, Xiangyang Guo via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Hi, James and Philip, Thanks for your help.
>
> Based on your advice, I downloaded llvm-3.7. However, with this new version of LLVM, I have the following errors when I compile my previous code:
>
> g++ -o parser main.o `llvm-config --libs all` `llvm-config --ldflags --system-libs` -lpthread -ldl -rdynamic -ltinfo
> main.o:(.data.rel.ro._ZTIN4llvm17GetElementPtrInstE[_ZTIN4llvm17GetElementPtrInstE]+0x10): undefined reference to `typeinfo for llvm::Instruction'
> main.o:(.data.rel.ro._ZTIN4llvm8ICmpInstE[_ZTIN4llvm8ICmpInstE]+0x10): undefined reference to `typeinfo for llvm::CmpInst'
>
> BTW, in my code, I use LLVM API (IRBuilder and so on) to generate one Module and then use PassManager to add several passes. And my Makefile is pretty simple, it looks like this:
> ***********************************************************************************************
> all: parser
>
> OBJS = main.o \
>
> LLVMCONFIG = llvm-config
> CPPFLAGS = `$(LLVMCONFIG) --cxxflags` -std=c++11
> LDFLAGS = `$(LLVMCONFIG) --ldflags --system-libs` -lpthread -ldl -rdynamic -ltinfo
> LIBS = `$(LLVMCONFIG) --libs all`
>
> clean:
> $(RM) -rf parser $(OBJS)
>
> %.o: %.cpp
> g++ -g -c $(CPPFLAGS) -o $@ $<
>
>
> parser: $(OBJS)
> g++ -o $@ $(OBJS) $(LIBS) $(LDFLAGS)
> **********************************************************************************************
> Do you have any idea? Thanks a lot.
>
> Regards,
>
> Xiangyang
>
> On Thu, Aug 20, 2015 at 2:23 PM, James Molloy <james at jamesmolloy.co.uk <mailto:james at jamesmolloy.co.uk>> wrote:
> Hi Xiangyang,
>
> The algorithm for loop unrolling was changed post-3.5 to do more what you'd expect. If you use 3.6 or 3.7 you'll likely get better results.
>
> Cheers,
>
> James
>
> On Thu, 20 Aug 2015 at 18:09 Philip Reames via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> On 08/20/2015 07:38 AM, Xiangyang Guo via llvm-dev wrote:
>> Hi,
>>
>> I want to use loop unrolling pass, however, I find that loop unrolling will introduces conditional branch at end of every "unrolled" part. For example, consider the following code
>>
>> void foo( int n, int array_x[])
>> {
>> for (int i=0; i < n; i++)
>> array_x[i] = i;
>> }
>>
>> Then I use this command "opt-3.5 try.bc -mem2reg -loops -loop-simplify -loop-rotate -lcssa -indvars -loop-unroll -unroll-count=3 -simplifycfg -S", it gives me this IR:
>>
>> define void @_Z3fooiPi(i32 %n, i32* %array_x) #0 {
>> %1 = icmp slt i32 0, %n
>> br i1 %1, label %.lr.ph <https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph_&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=iK-4Sl62Seah5JOtnUG-QuscAiOsYnFzalJonc_U6VU&s=EfR7wZAtVvK4UskGT4q9WtTD1gQPKtWnoc6nhM3hQoQ&e=>, label %._crit_edge
>>
>> .lr.ph <https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph_&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=iK-4Sl62Seah5JOtnUG-QuscAiOsYnFzalJonc_U6VU&s=EfR7wZAtVvK4UskGT4q9WtTD1gQPKtWnoc6nhM3hQoQ&e=>: ; preds = %0, %7
>> %indvars.iv = phi i64 [ %indvars.iv.next.2, %7 ], [ 0, %0 ]
>> %2 = getelementptr inbounds i32* %array_x, i64 %indvars.iv
>> %3 = trunc i64 %indvars.iv to i32
>> store i32 %3, i32* %2
>> %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>> %lftr.wideiv = trunc i64 %indvars.iv.next to i32
>> %exitcond = icmp ne i32 %lftr.wideiv, %n
>> br i1 %exitcond, label %4, label %._crit_edge
>>
>> ._crit_edge: ; preds = %.lr.ph <https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph_&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=iK-4Sl62Seah5JOtnUG-QuscAiOsYnFzalJonc_U6VU&s=EfR7wZAtVvK4UskGT4q9WtTD1gQPKtWnoc6nhM3hQoQ&e=>, %4, %7, %0
>> ret void
>>
>> ; <label>:4 ; preds = %.lr.ph <https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph_&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=iK-4Sl62Seah5JOtnUG-QuscAiOsYnFzalJonc_U6VU&s=EfR7wZAtVvK4UskGT4q9WtTD1gQPKtWnoc6nhM3hQoQ&e=>
>> %5 = getelementptr inbounds i32* %array_x, i64 %indvars.iv.next
>> %6 = trunc i64 %indvars.iv.next to i32
>> store i32 %6, i32* %5
>> %indvars.iv.next.1 = add nuw nsw i64 %indvars.iv.next, 1
>> %lftr.wideiv.1 = trunc i64 %indvars.iv.next.1 to i32
>> %exitcond.1 = icmp ne i32 %lftr.wideiv.1, %n
>> br i1 %exitcond.1, label %7, label %._crit_edge
>>
>> ; <label>:7 ; preds = %4
>> %8 = getelementptr inbounds i32* %array_x, i64 %indvars.iv.next.1
>> %9 = trunc i64 %indvars.iv.next.1 to i32
>> store i32 %9, i32* %8
>> %indvars.iv.next.2 = add nuw nsw i64 %indvars.iv.next.1, 1
>> %lftr.wideiv.2 = trunc i64 %indvars.iv.next.2 to i32
>> %exitcond.2 = icmp ne i32 %lftr.wideiv.2, %n
>> br i1 %exitcond.2, label %.lr.ph <https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph_&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=iK-4Sl62Seah5JOtnUG-QuscAiOsYnFzalJonc_U6VU&s=EfR7wZAtVvK4UskGT4q9WtTD1gQPKtWnoc6nhM3hQoQ&e=>, label %._crit_edge
>> }
>>
>> As you can see, at the end of BB <label>4 and BB<label>7 there are "add", "icmp" and "br" instrcutions to check the boundary. I understand this is for the correctness. However, I would expect the loop unrolling can change my code to something like this:
>>
>> void foo( int n, int array_x[])
>> {
>> int j = n%3;
>> int m = n - j;
>> for (int i=0; i < m; i+=3){
>> array_x[i] = i;
>> array_x[i+1] = i+1;
>> array_x[i+2] = i+2;
>> }
>> for(i=m; i<n; i++)
>> array_x[i] = i;
>> }
>>
>> In this case, the BB<label>4 and BB<label>7 will do not have the "add", "icmp" and "br" instructions because these BBs can be merged together.
>>
>> How can I achieve this? Thanks.
>
> One - rather heavy weight - way to do this would be to add the -irce pass after the loop unroll step. InductiveRangeCheckElimination will introduce a post loop so as to eliminate the range checks in the inner loop. This might not be the ideal transformation for this code, but it might get you closer to what you want.
>
> A couple of caveats:
> - 3.5 isn't recent enough to have a stable IRCE. Download ToT.
> - IRCE requires profiling information on the branches. I'd start by manually annotating your IR to see if it works, then exploring a profile build if it does.
>
> For the record, teaching the unroller to do this transformation (or a creating a new pass) would seem interesting. You might check with Chandler and/or Michael (see recent review threads) for what their plans in this area are.
>>
>> Regards,
>>
>> Xiangyang
>>
>>
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