[LLVMdev] Loop vectorizer dosen't find loop bounds

[Hal Finkel]

----- Original Message -----
> I am trying to vectorize the function
> 
> void bar(float *c, float *a, float *b)
> {
>    const int width = 256;
>    for (int i = 0 ; i < 256 ; ++i ) {
>      c[ i ]         = a[ i ]         + b[ i ];
>      c[ width + i ] = a[ width + i ] + b[ width + i ];
>    }
> }
> 
> using the following commands
> 
> clang -emit-llvm -S loop.c
> opt loop.ll -O3 -debug-only=loop-vectorize -S -o -
> 
> LV: Checking a loop in "bar"
> LV: Found a loop: for.body
> LV: Found an induction variable.
> LV: Found an unidentified write ptr: float* %c
> LV: Found an unidentified write ptr: float* %c
> LV: Found an unidentified read ptr: float* %a
> LV: Found an unidentified read ptr: float* %b
> LV: Found an unidentified read ptr: float* %a
> LV: Found an unidentified read ptr: float* %b
> LV: Found a runtime check ptr:  %arrayidx4 = getelementptr inbounds
> float* %c, i64 %indvars.iv
> LV: Found a runtime check ptr:  %arrayidx14 = getelementptr inbounds
> float* %c, i64 %2
> LV: Found a runtime check ptr:  %arrayidx = getelementptr inbounds
> float* %a, i64 %indvars.iv
> LV: Found a runtime check ptr:  %arrayidx2 = getelementptr inbounds
> float* %b, i64 %indvars.iv
> LV: Found a runtime check ptr:  %arrayidx7 = getelementptr inbounds
> float* %a, i64 %2
> LV: Found a runtime check ptr:  %arrayidx10 = getelementptr inbounds
> float* %b, i64 %2
> LV: We need to do 10 pointer comparisons.
> LV: We can't vectorize because we can't find the array bounds.
> LV: Can't vectorize due to memory conflicts
> LV: Not vectorizing.
> 
> Is there any chance to make this work?

Try adding the restrict keyword to the function parameters:

void bar(float * restrict c, float * restrict a, float * restrict b)

 -Hal

> 
> Frank
> 
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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory