[LLVMdev] Performance problems with FORTRAN allocatable arrays

[Duncan Sands]

Hi Wonsun, can you please provide a testcase.

Best wishes, Duncan.

> I've noticed that LLVM does a bad job of optimizing array indexing
> code for FORTRAN arrays declared using the ALLOCATABLE keyword.
>
> For example if you have something like the following:
>
> DOUBLE PRECISION,ALLOCATABLE,DIMENSION(:,:,:,:) :: QAV
> ...
> ALLOCATE( QAV( -2:IMAX+2,-2:JMAX+2,-2:KMAX+2,ND) )
> ...
> DO L = 1, 5
>     DO K = K1, K2
>        DO J = J1, J2
>           DO I = I1, I2
>              II  =  I + IADD
>              IBD = II - IBDD
>              ICD = II + IBDD
>
>              QAV(I,J,K,L) = R6I * (2.0D0 * Q(IBD,J,K,L,N) +
>>                                      5.0D0 * Q( II,J,K,L,N) -
>>                                              Q(ICD,J,K,L,N))
>           END DO
>        END DO
>     END DO
> END DO
>
> Most of the code needed to calculate the address of QAV(I,J,K,L)
> should be hoisted out of the loop since J, K, and L are constant
> inside the loop. But I'm not seeing this happening because LLVM's
> alias analysis cannot distinguish between the loads of the array
> dimensions for QAV and the store to QAV(I,J,K,L). I've tried all the
> alias analyses available in the standard distribution, including type
> based analysis and scalar evolution. But if you think about it, the
> array dimensions of QAV is 'metadata' and should not alias with any
> actual accesses in the program. I've compiled the same code with GCC
> and it was able to hoist most of the address calculations out as
> expected. GCC was able to hoist address calculations for Q also.
>
> This is an actual piece of code in SPECCPU2006 437.leslie3d and the
> loop I analyzed is in line 1630 of file tml.f. 437.leslie3d suffers
> horrible performance problems because of this and similar problems.
>
> Is there anyway to enable this optimization? Is there a way to flag in
> the IR that a particular locations is array dimension meta data?
>
> Thanks,
> Wonsun
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