Diagnostic 15328 vectorization support:irregularly indexed load was emulated for the variable <a(index(i))>, part of index is read from memory

Product Version: Intel® Fortran Compiler 15.0 and a later version 

Cause:

A vectorizable loop contains loads from memory locations that are not contiguous in memory (sometimes known as a “gather”). These may be indexed loads, as in the example below, or loads with non-unit stride. The compiler has emulated a hardware gather instruction by issuing individual loads for the different memory locations in software.

The vectorization report generated using Intel® Fortran Compiler's optimization and vectorization report options:

Windows* OS:  /O2  /Qopt-report:4  /Qopt-report-phase:vec    

Linux OS or OS X:  -O2 -qopt-report=4  -qopt-report-phase=vec

Example:

An example below will generate the following  remark in optimization report:

subroutine gathr(n, a, b, index)
   implicit none
   integer, parameter :: RT=8
   integer,                intent(in)  :: n
   integer,  dimension(n), intent(in)  :: index
   real(RT), dimension(n), intent(in)  :: a
   real(RT), dimension(n), intent(out) :: b
   integer                             :: i
   
   do i=1,n
       b(i) = 1.0_RT + 0.1_RT*a(index(i))
   enddo
end subroutine gathr

$ ifort -c -qopt-report=4 -qopt-report-file=stdout gathr.f90  

When using Intel Fortran compiler version 16.0 the following remark is generated:

   [...]

remark #15328: vectorization support: gather was emulated for the variable a:  indirect access  [ gathr.f90(11,31) ]

    [...]

When using Intel Fortran compiler version 17.0 the following remark is generated:

   [...]

remark #15328: vectorization support: irregularly indexed load was emulated for the variable <a(index(i))>, part of index is read from memory   [ gathr.f90(11,31) ]

  [...]

 

The compiler has vectorized the loop by emulating a “gather” instruction in software.

The assembly code contains no gather instructions.

 

See also:

Requirements for Vectorizable Loops

Vectorization Essentials

Vectorization and Optimization Reports

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For more complete information about compiler optimizations, see our Optimization Notice.