Hello world,
I found that the mlow-precision-div option have a fix number of newton iterations, which is 2 for float type and 3 for double type.
I noticed that if I alter the numbers of newton iterations as following, it could leads to faster performance in SPEC2017 fpspeed test on AArch64, with less but acceptable precision.
Before change:
frecpe s2, s8
frecps s4, s2, s8
fmul s2, s2, s4
frecps s4, s2, s8
fmul s2, s2, s4
fmul s10, s2
after change:
frecpe s2, s8
frecps s4, s2, s8
fmul s2, s2, s4
fmul s10, s2
The detail of the improvement is shown as following: (change the number of newton iterations for float to 1 and double to 2)
Test case
Improvement
603.bwaves_s
7.92%
607.cactuBSSN_s
Output miscompare
619.lbm_s
32.34%
621.wrf_s
Output miscompare
627.cam4_s
Output miscompare
628.pop2_s
Output miscompare
638.imagick_s
-0.97%
644.nab_s
9.09%
649.fotonik3d_s
Output miscompare
654.roms_s
-3.45%
This may benefit the performance of some test cases which do not have a high demand on precision.
Considering the precision of div is already lower than the IEEE standard when this option is on. Why the precision is fixed by the magic number 2 and 3?
Should we provide a parameter so that users can alter this value according to their needs?
