[llvm-bugs] [Bug 44638] New: interleaved memory access blocking loop vectorization

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Jan 23 08:14:11 PST 2020


            Bug ID: 44638
           Summary: interleaved memory access blocking loop vectorization
           Product: libraries
           Version: 7.0
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: wxz at marvell.com
                CC: arnaud.degrandmaison at arm.com,
                    llvm-bugs at lists.llvm.org, peter.smith at linaro.org,
                    Ties.Stuij at arm.com

Created attachment 23047
  --> https://bugs.llvm.org/attachment.cgi?id=23047&action=edit
The attached file shows the loop cannot be vectorized because of the high cost
in interleaved memory access

In loop vectorizer, LV, the decision to vectorize a loop is based on the cost
of the vectorized loop. The cost is calculated on the each instruction in the
loop. For interleaved access instruction group, there is one cost for the whole
group. The cost is based on extract/insert model to implement the interleaved
memory access. Usually, this group cost will be very high, especially on
AArch64 which has has a VectorInsertExtractBaseCost set to 3 in
AArch64Subtarget.h. This high cost usually will fail the loop to be vectorized.

On AArch64, there are multiple ways to implement interleaved memory access
without using the vector extract/insert model. For example, TBL instruction can
be used to rearrange and dispatch data from a vector to form a new vector and
it is a single one instruction cost. Also one use zip1/2, umull1/2, smull1/2 to
do this same work instead of extract/insert model.

ti reproduce this problem, compile the attched file with this command

clang -mcpu=thunderx2t99 -march=armv8.1-a+lse -mllvm tbl3.c -ffast-math
-ffp-contract=fast -funroll-loops -finline-functions -fslp-vectorize
-fvectorize -O3 -o tbl3.out

The problem exists in all llvm versions.

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