[LLVMdev] Modifying LoopUnrollingPass
Michael Zolotukhin
mzolotukhin at apple.com
Mon May 4 10:36:21 PDT 2015
Hi Yaduveer,
Vectorizer probably fails because it expects a loop in a certain form, and to convert a loop to this form one need to run some other passes first. For example, when you run “opt -O3”, the following passes are invoked:
-targetlibinfo -tti -no-aa -tbaa -scoped-noalias -assumption-cache-tracker -basicaa -ipsccp -globalopt -deadargelim -domtree -instcombine -simplifycfg -basiccg -prune-eh -inline-cost -inline -functionattrs -argpromotion -sroa -domtree -early-cse -lazy-value-info -jump-threading -correlated-propagation -simplifycfg -domtree -instcombine -tailcallelim -simplifycfg -reassociate -domtree -loops -loop-simplify -lcssa -loop-rotate -licm -loop-unswitch -instcombine -scalar-evolution -loop-simplify -lcssa -indvars -loop-idiom -loop-deletion -loop-unroll -memdep -mldst-motion -domtree -memdep -gvn -memdep -memcpyopt -sccp -domtree -bdce -instcombine -lazy-value-info -jump-threading -correlated-propagation -domtree -memdep -dse -loops -loop-simplify -lcssa -licm -adce -simplifycfg -domtree -instcombine -barrier -float2int -domtree -loops -loop-simplify -lcssa -loop-rotate -branch-prob -block-freq -scalar-evolution -loop-accesses -loop-vectorize -instcombine -scalar-evolution -slp-vectorizer -simplifycfg -domtree -instcombine -loops -loop-simplify -lcssa -scalar-evolution -loop-unroll -instsimplify -loop-simplify -lcssa -licm -scalar-evolution -alignment-from-assumptions -strip-dead-prototypes -globaldce -constmerge -verify
To get this list, you can use the following command:
llvm-as < /dev/null | opt -O3 -disable-output -debug-pass=Arguments
Now, when you get a list of passes to run before the vectorizer, you need to get ‘unoptimized’ IR and run the passes on it - that should give you IR just before the vectorizer.
To get the unoptimized IR, you could use
clang -O3 -mllvm -disable-llvm-optzns -emit-llvm your_source.c -S -o unoptimized_ir.ll
(Please note that we use “-O3 -mllvm -disable-llvm-optzns”, not just “-O0” - that allows us to run analysis passes, but not transformations)
Now you run ‘opt’ with passes preceding the vectorizer to get IR before vectorization:
opt -targetlibinfo -tti -no-aa -tbaa …… -scalar-evolution -loop-accesses unoptimized_ir.ll -S -o ir_before_loop_vectorize.ll
(you might want to remove verifier passes from the list)
And after this you are ready to run the vectorizer:
opt -loop-vectorize ir_before_loop_vectorize.ll -S -o ir_after_loop_vectorize.ll
Hopefully, that’ll resolve the issues you are facing.
Thanks,
Michael
> On May 3, 2015, at 9:18 AM, yaduveer singh <yaduveer99 at gmail.com> wrote:
>
> Hi Michael,
>
> I tried running my sample C program using "LoopVectorizePass" but I was not able to get the output as I was expecting. Every time I got the message
> "LV: Not vectorizing: Cannot prove legality."
>
> Following is the scenario.
>
> c-code
>
> #include <stdio.h>
> int main()
> {
> int i;
> int a=2;
> int sum=0;
> int arr[400];
>
> for(i=0;i<400;i=i+1)
> {
> arr[i]=a+i;
> sum+=arr[i];
> }
> printf("Everything is Done for 1d\n");
> return 0;
> }
>
> following are my command:
>
> yaduveer at yaduveer-Inspiron-3542:~/RP$ clang -S -emit-llvm loop1d.c
> yaduveer at yaduveer-Inspiron-3542:~/RP$ clang -c -emit-llvm loop1d.c
> yaduveer at yaduveer-Inspiron-3542:~/RP$ opt -loop-vectorize -force-vector-width=4 -mem2reg -loop-rotate -indvars -debug -stats loop1d.ll | llvm-dis -o loop1dv1.ll
>
> we found the following message on the terminal:(Attached is the details found on terminal and the 2 .ll files)
>
> LV: Checking a loop in "main" from loop1d.ll
> LV: Loop hints: force=? width=4 unroll=0
> LV: Not vectorizing: Cannot prove legality.
>
> so can you please let me know if I am following correct steps.
> If not, please guide me.
>
>
> Thanks in advance.
>
>
> Regards,
> Yaduveer
>
> On Sun, May 3, 2015 at 12:32 AM, yaduveer singh <yaduveer99 at gmail.com <mailto:yaduveer99 at gmail.com>> wrote:
> Hi Michael,
>
> Thank you very much!
> I will try this.
>
> Regards,
> Yaduveer
>
>
> On Sun, May 3, 2015 at 12:11 AM, Michael Zolotukhin <mzolotukhin at apple.com <mailto:mzolotukhin at apple.com>> wrote:
> Hi Yaduveer,
>
> As far as I remember, unroller in LoopVectorizer pass does what you want to achieve (look for a message "LV: Trying to at least unroll the loops.” to locate this in the code).
>
> Michael
>
> > On May 2, 2015, at 9:00 AM, yaduveer singh <yaduveer99 at gmail.com <mailto:yaduveer99 at gmail.com>> wrote:
> >
> > Hi Zhoulai,
> >
> > I am trying to modify "LoopUnrollPass" in llvm which produces multiple
> > copies of loop equal to the loop unroll factor.Currently, using multicore
> > architecture, say 3 for example and the execution goes like:
> >
> > for 3 cores if there are 9 iterations of loop
> > core instruction
> > 1 0,3,6
> > 2 1,4,7
> > 3 2,5,8
> >
> > But I want to to modify such that it can execute in following way:
> >
> > core instruction
> > 1 0,1,2
> > 2 3,4,5
> > 3 6,7,8
> >
> > I am not able to get where to modify for this. I tried creating a sample
> > pass using original LoopUnrollPass code and run "make", I received
> > following error:
> >
> > loopunrollp.cpp:210:1: error: ‘void
> > llvm::initializeLoopUnrollpPass(llvm::PassRegistry&)’ should have been
> > declared inside ‘llvm’
> > /bin/rm: cannot remove
> > `/home/yaduveer/RP/LLVM/llvm/lib/Transforms/loopunrollp/Debug+Asserts/loopunrollp.d.tmp':
> > No such file or directory
> >
> >
> > Please help
> >
> > Thanks,
> > Yaduveer
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu> http://llvm.cs.uiuc.edu <http://llvm.cs.uiuc.edu/>
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>
>
>
> <messageOnCommandLine.txt><loop1d.c><loop1d.ll><loop1dv1.ll>
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