[LLVMdev] Loop vectorizer dosen't find loop bounds

[Frank Winter]

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'?

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

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
>>