[llvm-dev] unable to emit vectorized code in LLVM IR

hameeza ahmed via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 17 12:12:54 PDT 2017


Ok. I have managed to vectorize the second loop in the following code. But
the first loop is still not vectorized? Why?

int main(int argc, char** argv) {
int a[1000], b[1000], c[1000]; int g=0;
int aa=atoi(argv[1]), bb=atoi(argv[2]);

for (int i=0; i<1000; i++) {
a[i]=aa+i, b[i]=bb+i;}

for (int i=0; i<1000; i++) {
 c[i]=a[i] + b[i];
g+=c[i];
}

printf("sum: %d\n", g);

return 0;
}

When i executed the optimized IR through jit (lli sum-vec03.ll  5 2) i am
getting following error:

#0 0x00000000013f965c llvm::sys::PrintStackTrace(llvm::raw_ostream&)
/lib/Support/Unix/Signals.inc:402:11
#1 0x00000000013f9b49 PrintStackTraceSignalHandler(void*)
/lib/Support/Unix/Signals.inc:466:1
#2 0x00000000013f7ec3 llvm::sys::RunSignalHandlers()
/lib/Support/Signals.cpp:0:5
#3 0x00000000013f9ea4 SignalHandler(int) /lib/Support/Unix/Signals.inc:256:1
#4 0x00007fcdece96d10 __restore_rt
(/lib/x86_64-linux-gnu/libpthread.so.0+0x10d10)
#5 0x00007fcded2c3038
#6 0x0000000000f4a8fb llvm::MCJIT::runFunction(llvm::Function*,
llvm::ArrayRef<llvm::GenericValue>)
/lib/ExecutionEngine/MCJIT/MCJIT.cpp:538:31
#7 0x0000000000eaff23
llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*,
std::vector<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > > > const&, char const*
const*) /lib/ExecutionEngine/ExecutionEngine.cpp:471:10
#8 0x00000000007be4e9 main /tools/lli/lli.cpp:627:18
#9 0x00007fcdebe2fa40 __libc_start_main
(/lib/x86_64-linux-gnu/libc.so.6+0x20a40)
#10 0x00000000007bc169 _start (/bin/lli+0x7bc169)
Stack dump:
0. Program arguments:lli sum-vec03.ll 5 2
Illegal instruction (core dumped)


What is wrong here? please help.









On Thu, Aug 17, 2017 at 11:51 PM, hameeza ahmed <hahmed2305 at gmail.com>
wrote:

> when i change it to following: then get error: remark: <unknown>:0:0: loop
> not vectorized: call instruction cannot be vectorized
> int main(int argc, char** argv) {
> int a[1000], b[1000], c[1000]; int g=0;
> for (int i=0; i<1000; i++) {
> a[i]=atoi(argv[1]), b[i]=atoi(argv[2]);
>  c[i]=a[i] + b[i];
> g+=c[i];
> }
>
> Here my main goal is that i need to use JIT to perform operations on user
> input file supplied at run time using vector instructions. is it possible
> and achievable through JIT?
> Please help.
>
>
>
> On Thu, Aug 17, 2017 at 11:44 PM, Craig Topper <craig.topper at gmail.com>
> wrote:
>
>> I assume compiler knows that your only have 2 input values that you just
>> added together 1000 times.
>>
>> Despite the fact that you stored to a[i] and b[i] here, nothing reads
>> them other than the addition in the same loop iteration. So the compiler
>> easily removed the a and b arrays. Same with 'c', it's not read outside the
>> loop so it doesn't need to exist. So the compiler turned your loop body
>> back into g+= aa + bb; And since the loop is 1000 iterations and aa and bb
>> never change this got further simplified to (aa+bb)*1000.
>>
>> int main(int argc, char** argv) {
>> int a[1000], b[1000], c[1000]; int g=0;
>> int aa=atoi(argv[1]), bb=atoi(argv[2]);
>> for (int i=0; i<1000; i++) {
>> a[i]=aa, b[i]=bb;
>>  c[i]=a[i] + b[i];
>> g+=c[i];
>> }
>>
>> ~Craig
>>
>> On Thu, Aug 17, 2017 at 11:37 AM, hameeza ahmed <hahmed2305 at gmail.com>
>> wrote:
>>
>>> why is it happening? is there any way to solve this?
>>>
>>> On Thu, Aug 17, 2017 at 10:09 PM, hameeza ahmed <hahmed2305 at gmail.com>
>>> wrote:
>>>
>>>> even if i make my code as follows: vectorized instructions not get
>>>> emitted. What to do?
>>>>
>>>> int main(int argc, char** argv) {
>>>> int a[1000], b[1000], c[1000]; int g=0;
>>>> int aa=atoi(argv[1]), bb=atoi(argv[2]);
>>>> for (int i=0; i<1000; i++) {
>>>> a[i]=aa, b[i]=bb;
>>>>  c[i]=a[i] + b[i];
>>>> g+=c[i];
>>>> }
>>>>
>>>> printf("sum: %d\n", g);
>>>>
>>>> return 0;
>>>> }
>>>>
>>>> On Thu, Aug 17, 2017 at 10:03 PM, Craig Topper <craig.topper at gmail.com>
>>>> wrote:
>>>>
>>>>> Did you remove the printf completely? Meaning that nothing accesses
>>>>> 'c' after the loop? If so it got removed as dead code because it had no
>>>>> visible effect.
>>>>>
>>>>> ~Craig
>>>>>
>>>>> On Thu, Aug 17, 2017 at 10:01 AM, hameeza ahmed <hahmed2305 at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> i removed printf from loop. Now getting no error. but the IR doesnot
>>>>>> contain vectorized code. IR Output is as follows:
>>>>>> ; ModuleID = 'sum-vec.ll'
>>>>>> source_filename = "sum-vec.c"
>>>>>> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
>>>>>> target triple = "x86_64-unknown-linux-gnu"
>>>>>>
>>>>>> ; Function Attrs: norecurse nounwind readnone uwtable
>>>>>> define i32 @main(i32, i8** nocapture readnone) local_unnamed_addr #0 {
>>>>>>   ret i32 0
>>>>>> }
>>>>>>
>>>>>> attributes #0 = { norecurse nounwind readnone uwtable
>>>>>> "correctly-rounded-divide-sqrt-fp-math"="false"
>>>>>> "disable-tail-calls"="false" "less-precise-fpmad"="false"
>>>>>> "no-frame-pointer-elim"="false" "no-infs-fp-math"="false"
>>>>>> "no-jump-tables"="false" "no-nans-fp-math"="false"
>>>>>> "no-signed-zeros-fp-math"="false" "no-trapping-math"="false"
>>>>>> "stack-protector-buffer-size"="8" "target-cpu"="knl"
>>>>>> "target-features"="+adx,+aes,+avx,+avx2,+avx512cd,+avx512er,
>>>>>> +avx512f,+avx512pf,+bmi,+bmi2,+cx16,+f16c,+fma,+fsgsbase,+fx
>>>>>> sr,+lzcnt,+mmx,+movbe,+pclmul,+popcnt,+prefetchwt1,+rdrnd,+r
>>>>>> dseed,+rtm,+sse,+sse2,+sse3,+sse4.1,+sse4.2,+ssse3,+x87,+xsave,+xsaveopt"
>>>>>> "unsafe-fp-math"="false" "use-soft-float"="false" }
>>>>>>
>>>>>> !llvm.ident = !{!0}
>>>>>>
>>>>>> !0 = !{!"clang version 4.0.0 (tags/RELEASE_400/final)"}
>>>>>>
>>>>>> what to do? please help.
>>>>>>
>>>>>>
>>>>>> On Thu, Aug 17, 2017 at 9:57 PM, Nemanja Ivanovic <
>>>>>> nemanja.i.ibm at gmail.com> wrote:
>>>>>>
>>>>>>> Move the printf out of the loop and it should vectorize just fine.
>>>>>>>
>>>>>>> On Thu, Aug 17, 2017 at 6:52 PM, hameeza ahmed <hahmed2305 at gmail.com
>>>>>>> > wrote:
>>>>>>>
>>>>>>>> I want to vectorize the user given inputs. when opt does
>>>>>>>> vectorization user supplied inputs (from a text file) will be added using
>>>>>>>> AVX vector instructions.
>>>>>>>>
>>>>>>>> as you pointed; When i changed my code to following:
>>>>>>>>
>>>>>>>> int main(int argc, char** argv) {
>>>>>>>> int a[1000], b[1000], c[1000];
>>>>>>>> int aa=atoi(argv[1]), bb=atoi(argv[2]);
>>>>>>>> for (int i=0; i<1000; i++) {
>>>>>>>> a[i]=aa, b[i]=bb;
>>>>>>>>  c[i]=a[i] + b[i];
>>>>>>>> printf("sum: %d\n", c[i]);
>>>>>>>>
>>>>>>>> }
>>>>>>>>
>>>>>>>> I am getting error remark: <unknown>:0:0: loop not vectorized: call
>>>>>>>> instruction cannot be vectorized.
>>>>>>>>
>>>>>>>> I am running following commands:
>>>>>>>> clang  -S -emit-llvm sum-vec.c -march=knl -O3 -mllvm
>>>>>>>> -disable-llvm-optzns -o sum-vec.ll
>>>>>>>> opt  -S -O3 -force-vector-width=64 sum-vec.ll -o sum-vec03.ll
>>>>>>>>
>>>>>>>> How to achieve this? Please help.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Thu, Aug 17, 2017 at 10:44 AM, Nemanja Ivanovic <
>>>>>>>> nemanja.i.ibm at gmail.com> wrote:
>>>>>>>>
>>>>>>>>> I'm not sure what you expect to have vectorized here. If you look
>>>>>>>>> at the emitted code, there's no loop. It's just an add and a multiply as
>>>>>>>>> you might expect when adding a loop-invariant sum 1000 times in a loop.
>>>>>>>>>
>>>>>>>>> On Wed, Aug 16, 2017 at 11:38 PM, hameeza ahmed via llvm-dev <
>>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>
>>>>>>>>>> Hello,
>>>>>>>>>> I have written the following code. when i try to vectorize it
>>>>>>>>>> through opt. i am not getting vectorized instructions.
>>>>>>>>>>
>>>>>>>>>> #include <stdio.h>
>>>>>>>>>> #include<stdlib.h>
>>>>>>>>>> int main(int argc, char** argv) {
>>>>>>>>>> int sum=0; int a=atoi(argv[1]); int b=atoi(argv[2]);
>>>>>>>>>> for (int i=0;i<1000;i++)
>>>>>>>>>> {
>>>>>>>>>> sum+=a+b;
>>>>>>>>>> }
>>>>>>>>>>
>>>>>>>>>> printf("sum: %d\n", sum);
>>>>>>>>>> return 0;
>>>>>>>>>> }
>>>>>>>>>> i use following commands:
>>>>>>>>>> clang  -S -emit-llvm sum-main.c -march=knl -O3 -mllvm
>>>>>>>>>> -disable-llvm-optzns -o sum-main.ll
>>>>>>>>>> opt  -S -O3 -force-vector-width=64 sum-main.ll -o sum-main03.ll
>>>>>>>>>>
>>>>>>>>>> why is that so? where am i doing mistake? i am not getting
>>>>>>>>>> vectorized operations rather getting scalar operations.
>>>>>>>>>>
>>>>>>>>>> Please help.
>>>>>>>>>>
>>>>>>>>>> Thank You
>>>>>>>>>>
>>>>>>>>>> Regards
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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