| Issue |
134026
|
| Summary |
[Flang] `read` statement is slower than Gfortran in SPEC CPU 2017
|
| Labels |
flang
|
| Assignees |
|
| Reporter |
s-watanabe314
|
When measuring 649.fotonik3d_s from SPEC CPU 2017 on the Grace (AArch64) machine, the speed of LLVM was slower than GCC as the number of threads increased. The options specified were `-Ofast -mcpu=neoverse-v2`.
||version|
|-|-|
|GCC|14.2.0|
|LLVM (Release build)|19.1.0|
||1 thread|2 threads|4 threads|8 threads|16 threads|
|-|-|-|-|-|-|
|GCC [s]|525.33|283.96|157.78|**99.93**|**76.15**|
|LLVM [s]|477.86|264.73|153.25|**101.72**|**81.36**|
According to our investigation, one of the reasons seems to be that the `read` statement, called in the non-parallelized initialization process, is slower than Gfortran.
The `read` statement is called approximately 54 million times within the nested loop, so its impact is significant.
The difference in processing time of the `read` statement was verified using the test program `test.f90`. This test was measured using LLVM 20.1.0 built with the build option `-DCMAKE_BUILD_TYPE=Release`.
To minimize the impact of file systems like NFS, the input file `test.dat` was placed in the machine's local directory.
`test.dat` can be generated with the following command:
`python3 -c 'for i in range(54000000): print(i,i,i)' > test.dat`
- test.f90
```fortran
program main
integer, parameter :: num = 54000000
integer, dimension(num) :: a,b,c
integer :: t1,t2, count, countmax
integer :: ii
open(unit=9, file='test.dat', status='old', IOSTAT=ios)
call system_clock(t1, count, countmax)
do ii=1,num
read(9,*) a(ii),b(ii),c(ii)
end do
call system_clock(t2)
print*, "init time : ",real(t2 - t1)/count,"sec"
close(9)
end program main
```
- Compilation commands
```
$ flang-new -O3 -ffast-math -mcpu=neoverse-v2 test.f90
$ gfortran -O3 -ffast-math -mcpu=neoverse-v2 test.f90
```
- Measurement results
||version|time|
|-|-|-|
|GCC|14.2.0|22.496 [s]|
|LLVM (Release build)|20.1.0|42.341 [s]|
The `read` statement appears to be nearly twice as slow in Flang compared to Gfortran.
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