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

[Francois Fayard via llvm-dev]

LLVM is very smart about deleting useless code.

If you want to see vectorized code, I would suggest to write such a function.

int f(int* a, int* b, int n) {
  int ans = 0;
  for (int i = 0; i < n; ++i) {
    ans += a[i] * b[i];
  }
  return ans;
}

That way, the compiler knows nothing about the values of a and b and cannot optimize anything. Also, beware of pointer aliasing which might prevent vectorization or create 2 different paths : one vectorized, one not vectorized.

François Fayard

> On Aug 17, 2017, at 7:01 PM, hameeza ahmed via llvm-dev <llvm-dev at lists.llvm.org> 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,+fxsr,+lzcnt,+mmx,+movbe,+pclmul,+popcnt,+prefetchwt1,+rdrnd,+rdseed,+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 <mailto: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 <mailto: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 <mailto: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 <mailto: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
> 
> 
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