[LLVMdev] Loop vectorizer dosen't find loop bounds

[Hal Finkel]

----- Original Message -----
> Bingo! That works (when coming from C source)
> 
> Now, I have a serious problem. I am not coming from C but I build the
> function with the builder. I am also forced to change the signature
> and
> load the pointers a,b,c afterwards:
> 
> define void @bar([8 x i8]* nocapture readonly %arg_ptr) #0 {
> entrypoint:
>    %0 = bitcast [8 x i8]* %arg_ptr to i32*
>    %1 = load i32* %0, align 4
>    %2 = getelementptr [8 x i8]* %arg_ptr, i64 1
>    %3 = bitcast [8 x i8]* %2 to i32*
>    %4 = load i32* %3, align 4
>    %5 = getelementptr [8 x i8]* %arg_ptr, i64 2
>    %6 = bitcast [8 x i8]* %5 to float**
>    %7 = load float** %6, align 8
>    %8 = getelementptr [8 x i8]* %arg_ptr, i64 3
>    %9 = bitcast [8 x i8]* %8 to float**
>    %10 = load float** %9, align 8
>    %11 = getelementptr [8 x i8]* %arg_ptr, i64 4
>    %12 = bitcast [8 x i8]* %11 to float**
>    %13 = load float** %12, align 8
>    %14 = sext i32 %1 to i64
>    br label %L0
> 
> Now, these pointer (%7,%10,%13) are not qualified with 'restrict' and
> the loop vectorizer gives me the same message:
> 
> LV: We can't vectorize because we can't find the array bounds.
> LV: Can't vectorize due to memory conflicts
> LV: Not vectorizing.
> 
> I asked this a few days ago; now it comes up again: Is there a way to
> qualify a pointer/Value to be 'restrict'?

Currently, no. There will be work in that direction soon. You'll need to extract a sub-function so that you can put 'noalias' on the function arguments.

> 
> Another possible solution would be telling the loop vectorizer that
> it's
> safe to treat all arrays as disjunct. Is this possible?

Yes. Look for llvm.mem.parallel_loop_access in the language reference.

 -Hal

> 
> Frank
> 
> 
> 
> 
> On 28/10/13 15:11, Hal Finkel wrote:
> > ----- 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
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>
> 
> 
> 

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