[LLVMdev] Loop vectorizer dosen't find loop bounds

[Hal Finkel]

----- Original Message -----
> Thanks for the alternatives!
> 
> I am trying the 'extracting sub-function' approach. However, it seems
> I
> can't get the 'subfunction' to pass the verifier. This is my
> subfunction:
> 
> define void @main_extern([8 x i8]* %arg_ptr) {
> entrypoint:
>    %0 = getelementptr [8 x i8]* %arg_ptr, i32 0
>    %1 = bitcast [8 x i8]* %0 to i64*
>    %2 = load i64* %1
>    %3 = getelementptr [8 x i8]* %arg_ptr, i32 1
>    %4 = bitcast [8 x i8]* %3 to i64*
>    %5 = load i64* %4
>    %6 = getelementptr [8 x i8]* %arg_ptr, i32 2
>    %7 = bitcast [8 x i8]* %6 to float**
>    %8 = load float** %7
>    %9 = getelementptr [8 x i8]* %arg_ptr, i32 3
>    %10 = bitcast [8 x i8]* %9 to float**
>    %11 = load float** %10
>    %12 = getelementptr [8 x i8]* %arg_ptr, i32 4
>    %13 = bitcast [8 x i8]* %12 to float**
>    %14 = load float** %13
>    call void @main(i64 %2, i64 %5, float* %8, float* %11, float* %14)
>    ret void
> }
> 
> Looks good to me. However the verify pass fails:
> 
> /svn/llvm/include/llvm/Support/Casting.h:97: static bool
> llvm::isa_impl_cl<To, const From*>::doit(const From*) [with To =
> llvm::GlobalVariable; From = llvm::GlobalValue]: Assertion `Val &&
> "isa<> used on a null pointer"' failed.
> 
> I have no idea what this tries to tell me. Any idea?

That's a bug (you're hitting an internal assertion failure). You could try removing one instruction at a time to try and narrow it down and/or file a bug report.

 -Hal

> 
> Frank
> 
> 
> 
> On 28/10/13 15:27, Hal Finkel wrote:
> > ----- 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