[LLVMdev] Loop vectorizer dosen't find loop bounds
Frank Winter
fwinter at jlab.org
Mon Oct 28 19:30:34 PDT 2013
I am jumping around in memory here. Funny, this appeared after 'jumping
to my subfunction':
--------------------------------------------------------------------------
Sorry! You were supposed to get help about:
mpi-abort
from the file:
help-mpi-api.txt
But I couldn't find that topic in the file. Sorry!
--------------------------------------------------------------------------
Regards the subfunction/main function setup with JIT (original
interface): Is this order correct:?
build subfunction
void* fptr_sub = EE->getPointerToFunction( subfunction );
build main function
void* fptr_main = EE->getPointerToFunction( main function );
call fptr_sub(args); // Here the funny mpi-abort happens.
Not sure if this is correct. First because of the funny business and
second I don't make use of the function pointer to the main function. Is
it correct to JIT it? Do I have to bind it globally to the name
'main_function'?!?
Frank
On 28/10/13 21:23, Hal Finkel wrote:
> ----- 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
>>>>>>
>>>>
>>
>>
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