[llvm-dev] On Loop Distribution pass

[Dangeti Tharun kumar via llvm-dev]

Dear community,

Our team at IITH have been experimenting with loop-distribution pass in
LLVM. We see the following results on few benchmarks.

clang -O3 -mllvm -enable-loop-distribute -Rpass=loop-distribute file.c

clang -O3 -mllvm -enable-loop-distribute -Rpass-analysis=loop-distribute
file.c


TORCH
<http://crd.lbl.gov/departments/computer-science/PAR/research/previous-projects/torch-testbed/>
:

There are nearly 488 loops in this benchmark. LLVM was not able to
distribute any loop.

TSVC <https://github.com/shantanuatiith/TSVC_>:

There are 151 loops coded in plain ‘C’, none of them got distributed. TSVC
has particularly candidates valid for distribution like the one below. The
inner loop in this example can be distributed.

for (int nl = 0; nl < ntimes/2; nl++) {

for (int i = 1; i < LEN; i++) {

a[i] += c[i] * d[i];

b[i] = b[i - 1] + a[i] + d[i];

}

dummy(a, b, c, d, e, aa, bb, cc, 0.);

}

     MiBench <http://vhosts.eecs.umich.edu/mibench//>:

There are 6539 loops in MiBench. *None* of the loops were distributed by
the loop-distribution pass of LLVM.

     CoMD <https://github.com/exmatex/CoMD>:

CoMD is a reference implementation of typical classical molecular dynamics
algorithms and workloads.

Out of 112 loops none of them are distributed.



For the specific loop I have shown above which is straight forward a good
distribution candidate, the remark is "*loop not distributed: memory
operations are safe for vectorization [-Rpass-analysis=loop-distribute]*".

Can someone reason these results and this remark?


I humbly request the community to correct me, if am missing something in my
analysis.

-- 
Thank you
D Tharun kumar
CS15MTECH11002
9948373970
CSE-IITH
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