[LLVMdev] How to use LLVM optimizations with clang

Abdul Wahid Memon engrwahidmemon at gmail.com
Tue Jun 12 03:26:36 PDT 2012 

Hi

Yes, they both are exactly the same.

Regards

Shahzad

On Tue, Jun 12, 2012 at 9:38 AM, Duncan Sands <baldrick at free.fr> wrote:
> Hi, is the comb.ll used here:
>
>
>> $ time lli ./comb.ll
>>
>> then the runtime is
>>
>> real    0m2.671s
>> user    0m2.640s
>> sys     0m0.020s
>>
>> But, if I convert this same file comb,ll in to native binary
>
>
> the same as the comb.ll used here:
>
>> $ clang comb.ll
>
>
> ?
>
> Ciao, Duncan.
>
>
>>
>> and execute it, then the runtime increases alot
>>
>> $ time ./a.out
>>
>> real    0m8.008s
>> user    0m7.964s
>>
>> The binary generated directly by clang have runtime of around 2
>> seconds while generated after LLVM IR have around 8 secs but LLVM
>> bytecode having same optimizations when executed with lli have runtime
>> of around 2 secs.
>>
>> What steps are exactly taken by clang to produce the binary in order
>> to achieve the runtime of around 2 secs. If it is a question of link
>> time optimizations then how can we achieve that?
>>
>> Best Regards
>>
>> Shahzad
>>
>> On Fri, Jun 8, 2012 at 7:29 PM, Abdul Wahid Memon
>> <engrwahidmemon at gmail.com>  wrote:
>>>
>>> Thanks Duncan
>>>
>>> It was really helpful.
>>>
>>> Regards
>>>
>>> Abdul
>>>
>>> On Fri, Jun 8, 2012 at 7:23 PM, Duncan Sands<baldrick at free.fr>  wrote:
>>>>
>>>> Hi,
>>>>
>>>>
>>>>> If I compile the program using the following command line i.e.
>>>>>
>>>>> $ clang -O3 -lm *.c
>>>>
>>>>
>>>>
>>>> this may be doing link time optimization.
>>>>
>>>>
>>>>>
>>>>> then
>>>>>
>>>>> $ time ./a.out
>>>>>
>>>>> real    0m2.606s
>>>>> user    0m2.584s
>>>>> sys     0m0.012s
>>>>>
>>>>> BUT, if I use all the optimizations enabled with -O3 but specify them
>>>>> explicity i.e.
>>>>
>>>>
>>>>
>>>> you can just use "opt -O3" here.
>>>>
>>>>
>>>>>
>>>>> for F in *.c ; do B=`basename $F .c` ; gcc
>>>>> -fplugin=/damm/compilers/dragonegg-3.1.src/dragonegg.so -S -o - $F
>>>>> -fplugin-arg-dragonegg-emit-ir | opt -targetlibinfo -no-aa -tbaa
>>>>> -basicaa -globalopt -ipsccp -deadargelim -instcombine -simplifycfg
>>>>> -basiccg -prune-eh -inline -functionattrs -argpromotion
>>>>> -scalarrepl-ssa -domtree -early-cse -simplify-libcalls
>>>>> -lazy-value-info -jump-threading -correlated-propagation -simplifycfg
>>>>> -instcombine -tailcallelim -simplifycfg -reassociate -domtree -loops
>>>>> -loop-simplify -lcssa -loop-rotate -licm -lcssa -loop-unswitch
>>>>> -instcombine -scalar-evolution -loop-simplify -lcssa -indvars
>>>>> -loop-idiom -loop-deletion -loop-unroll -memdep -gvn -memdep
>>>>> -memcpyopt -sccp -instcombine -lazy-value-info -jump-threading
>>>>> -correlated-propagation -domtree -memdep -dse -adce -simplifycfg
>>>>> -instcombine -strip-dead-prototypes -globaldce -constmerge -preverify
>>>>> -domtree -verify -o $B.ll ; done
>>>>>
>>>>> $ clang *.ll
>>>>
>>>>
>>>>
>>>> Try clang -O3 *.ll
>>>>
>>>> If it makes a difference then that means that clang is linking all these
>>>> files
>>>> together to form one mega file which it is then optimizing.
>>>>
>>>> Ciao, Duncan.
>>>>
>>>>
>>>>>
>>>>> then
>>>>>
>>>>> time ./a.out
>>>>>
>>>>> real    0m7.791s
>>>>> user    0m7.760s
>>>>> sys     0m0.008s
>>>>>
>>>>> Am I missing something here?
>>>>>
>>>>> Though directly compiling files i.e.
>>>>>
>>>>> $ clang *.c
>>>>>
>>>>> results in
>>>>>
>>>>> time ./a.out
>>>>>
>>>>> real    0m10.167s
>>>>> user    0m10.121s
>>>>> sys     0m0.016s
>>>>>
>>>>> Regards
>>>>>
>>>>> Shahzad
>>>>>
>>>>> On Fri, Jun 8, 2012 at 7:00 PM, Duncan Sands<baldrick at free.fr>
>>>>>  wrote:
>>>>>>
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>>
>>>>>>> I tried it with -o - but its producing an error
>>>>>>>
>>>>>>> gcc: fatal error: cannot specify -o with -c, -S or -E with multiple
>>>>>>> files
>>>>>>>
>>>>>>> What you suggest?
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> what I wrote:
>>>>>>
>>>>>>>>  for F in *.c ; do B=`basename $F .c` ; gcc
>>>>>>>> -fplugin=/path/to/dragonegg.so
>>>>>>>> -S -o - $F -fplugin-arg-dragonegg-emit-ir | opt -adce -o $B.ll ;
>>>>>>>> done
>>>>>>>>  clang *.ll
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Thanks to the for loop and passing $F to gcc, you are no longer using
>>>>>> gcc
>>>>>> with
>>>>>> multiple files.  So if you are getting that message then you are don't
>>>>>> what
>>>>>> I
>>>>>> suggested.
>>>>>>
>>>>>> Ciao, Duncan.
>>>>>>
>>>>>>
>>>>>>>>
>>>>>>>> Ciao, Duncan.
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> $ clang *.s
>>>>>>>>>
>>>>>>>>> Regards
>>>>>>>>>
>>>>>>>>> Shahzad
>>>>>>>>>
>>>>>>>>> On Fri, Jun 8, 2012 at 9:10 AM, Duncan Sands<baldrick at free.fr>
>>>>>>>>>  wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Hi Shahzad,
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> Is it possible that we can use LLVM optimization beside O1, O2,
>>>>>>>>>>> O3
>>>>>>>>>>> along with dragonegg plugin?
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> sure, try this:
>>>>>>>>>>
>>>>>>>>>>  gcc -fplugin=path/dragonegg.so ...other_options_here... -S -o -
>>>>>>>>>> -fplugin-arg-dragonegg-emit-ir
>>>>>>>>>> -fplugin-arg-dragonegg-llvm-ir-optimize=0
>>>>>>>>>> |
>>>>>>>>>> opt -pass1 -pass2 ...
>>>>>>>>>>
>>>>>>>>>> Here -fplugin-arg-dragonegg-emit-ir tells it to output LLVM IR
>>>>>>>>>> rather
>>>>>>>>>> than
>>>>>>>>>> target assembler.  You can also use -flto here.
>>>>>>>>>>
>>>>>>>>>> -fplugin-arg-dragonegg-llvm-ir-optimize=0 disables the standard
>>>>>>>>>> set
>>>>>>>>>> of
>>>>>>>>>> LLVM
>>>>>>>>>> optimizations.
>>>>>>>>>>
>>>>>>>>>> In general, if a front-end can produce LLVM IR then you can do
>>>>>>>>>> this,
>>>>>>>>>> by
>>>>>>>>>> outputting the IR and passing it to "opt".
>>>>>>>>>>
>>>>>>>>>> Ciao, Duncan.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Regards
>>>>>>>>>>>
>>>>>>>>>>> Shahzad
>>>>>>>>>>>
>>>>>>>>>>> On Thu, Jun 7, 2012 at 10:59 PM, Abdul Wahid Memon
>>>>>>>>>>> <engrwahidmemon at gmail.com>          wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks alot Chad for these quick and fine responses.
>>>>>>>>>>>>
>>>>>>>>>>>> Regards
>>>>>>>>>>>>
>>>>>>>>>>>> Abdul
>>>>>>>>>>>>
>>>>>>>>>>>> On Thu, Jun 7, 2012 at 10:57 PM, Chad Rosier<mcrosier at apple.com>
>>>>>>>>>>>>  wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> On Jun 7, 2012, at 1:53 PM, Abdul Wahid Memon wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> Thanks again chad. One more question please.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Is it possible that I can use these optimization using
>>>>>>>>>>>>>> dragonegg
>>>>>>>>>>>>>> plugin
>>>>>>>>>>>>>> somehow?
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> I haven't a clue.  Maybe Duncan can answer that?
>>>>>>>>>>>>>
>>>>>>>>>>>>>  Chad
>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Regards
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Shahzad
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Thu, Jun 7, 2012 at 10:51 PM, Chad
>>>>>>>>>>>>>> Rosier<mcrosier at apple.com>
>>>>>>>>>>>>>>  wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Jun 7, 2012, at 1:48 PM, Abdul Wahid Memon wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Thu, Jun 7, 2012 at 10:44 PM, Chad
>>>>>>>>>>>>>>>> Rosier<mcrosier at apple.com>
>>>>>>>>>>>>>>>>  wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On Jun 7, 2012, at 1:39 PM, Abdul Wahid Memon wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Thanks alot Chad for quick response. Does this means that,
>>>>>>>>>>>>>>>>>> we
>>>>>>>>>>>>>>>>>> can
>>>>>>>>>>>>>>>>>> not
>>>>>>>>>>>>>>>>>> use LLVM optimizations except O1, O2, O3, O4 and
>>>>>>>>>>>>>>>>>> unroll-loops
>>>>>>>>>>>>>>>>>> with
>>>>>>>>>>>>>>>>>> clang?
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Try using the -debug-pass=Arguments options to see what
>>>>>>>>>>>>>>>>> passes
>>>>>>>>>>>>>>>>> are
>>>>>>>>>>>>>>>>> being run at each optimization level.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Yes, I have seen those optimization but I want to disable
>>>>>>>>>>>>>>>> all
>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>> default optimization (-O0) and specify them individually to
>>>>>>>>>>>>>>>> test
>>>>>>>>>>>>>>>> their
>>>>>>>>>>>>>>>> effect.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> 1. Generate the bitcode/ir with -O0
>>>>>>>>>>>>>>> 2. Run opt with the optimization in question.  You should be
>>>>>>>>>>>>>>> able
>>>>>>>>>>>>>>> to
>>>>>>>>>>>>>>> specify most any pass with opt.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> E.g.,
>>>>>>>>>>>>>>>>> clang -O[0-3] -mllvm -debug-pass=Arguments foo.c.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> One more thing I would like to know that If I want to
>>>>>>>>>>>>>>>>>> process
>>>>>>>>>>>>>>>>>> multiple
>>>>>>>>>>>>>>>>>> modules with opt at the same time like
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> opt -adce *.bc
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I don't think this will work.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> ok.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> then how is it possible with opt in one go, if I process
>>>>>>>>>>>>>>>>>> all
>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>> bytecode files within Makefile.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> You should be able to define a rule in the Makefile to
>>>>>>>>>>>>>>>>> compile
>>>>>>>>>>>>>>>>> your
>>>>>>>>>>>>>>>>> bitcode/IR files.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> ok. I got the point.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Thanks alot
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Shahzad
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>  Chad
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Thanks.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Shahzad
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On Thu, Jun 7, 2012 at 9:22 PM, Chad
>>>>>>>>>>>>>>>>>> Rosier<mcrosier at apple.com>
>>>>>>>>>>>>>>>>>>  wrote:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> On Jun 7, 2012, at 10:59 AM, Abdul Wahid Memon wrote:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Hello everyone
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> I am trying to use some LLVM optimizations like -die or
>>>>>>>>>>>>>>>>>>>> -adce.
>>>>>>>>>>>>>>>>>>>> Is
>>>>>>>>>>>>>>>>>>>> it
>>>>>>>>>>>>>>>>>>>> possible to use them along clang?
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Or is there a way where these optimization can be passed
>>>>>>>>>>>>>>>>>>>> on
>>>>>>>>>>>>>>>>>>>> to
>>>>>>>>>>>>>>>>>>>> "opt"
>>>>>>>>>>>>>>>>>>>> tool through clang, if opt is being used by clang behind
>>>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>>>> scenes?
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> No, opt only works on llvm IR/bitcode.  You can generate
>>>>>>>>>>>>>>>>>>> it
>>>>>>>>>>>>>>>>>>> like
>>>>>>>>>>>>>>>>>>> this:
>>>>>>>>>>>>>>>>>>> clang -c foo.c -emit-llvm -o foo.bc
>>>>>>>>>>>>>>>>>>> or
>>>>>>>>>>>>>>>>>>> clang -S foo.c -emit-llvm -o foo.ll
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Then you can run the optimization(s):
>>>>>>>>>>>>>>>>>>> opt -adce foo.bc -o foo-adce.bc
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Then you can compile using clang
>>>>>>>>>>>>>>>>>>> clang -c foo-adce.bc -o foo-adce.o
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>  Chad
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Thanks alot
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Regards
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Shahzad
>>>>>>>>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>>>>>>>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>>>>>>>>>>>>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>
>>>>>>
>>>>
>

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