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

Thu Aug 17 11:44:10 PDT 2017

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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