Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!rice!sun-spots-request
From: djones@awesome.Berkeley.EDU
Newsgroups: comp.sys.sun
Subject: SPARC divide - really really slow!
Keywords: Miscellaneous
Message-ID: <4075@brazos.Rice.edu>
Date: 27 Dec 89 04:45:36 GMT
Sender: root@rice.edu
Organization: Sun-Spots
Lines: 45
Approved: Sun-Spots@rice.edu
X-Sun-Spots-Digest: Volume 9, Issue 1, message 5 of 6

I was faced with a program which ran as fast on a SUN 3/60 as it did on a
SUN 4/280, when there should a factor of 2-3 difference if you believe the
MIPS rating.

Using profiling "cc -pg", it became evident that the source is the SPARC
divide instruction -- I gather there is none.  This is, of course, part of
the RISC strategy.  I'm still just a bit surprised that SUN/SPARC hasn't
figured out a way to get integer divisions done a little faster on a SUN
4/280 than on a SUN 3/60!

I was amused to see some of the "functions" that gprof found using up all
my CPU time.  I gather the code checks to see if the numbers are
"not_really_big", or "not_too_big" to do the division (ahem) faster.

So are we stuck with this poor multiply/divide performance in SPARC, or is
this shortcoming being addressed?  Heck, would it be faster to hand off
these operations to the Floating Point chip?

   %  cumulative    self              self    total          
 time   seconds   seconds    calls  ms/call  ms/call name    
 13.9     106.87    36.19                            divloop [4]
 13.8     142.71    35.84                            divloop [5]
  3.3     162.28     8.69                            divide [10]
  3.3     170.84     8.56                            not_really_big [11]
  3.2     179.13     8.29                            divide [12]
  3.1     187.27     8.14                            not_really_big [13]
  3.0     203.11     7.71                            end_regular_divide [15]
  2.9     210.67     7.56                            end_regular_divide [16]
  2.5     223.95     6.50  9326374     0.00     0.00  .rem [18]
  2.3     229.85     5.91  9326374     0.00     0.00  .div [20]
  1.6     239.22     4.27                            got_result [23]
  1.4     242.88     3.66                            got_result [24]
  0.6     248.88     1.69                            do_regular_divide [25]
  0.6     250.43     1.55                            do_regular_divide [26]
  0.5     251.65     1.22                            end_single_divloop [27]
  0.5     254.04     1.19                            end_single_divloop [29]
  0.2     256.09     0.62        4   155.02   155.02  .urem [33]
  0.1     257.94     0.38                            do_single_div [36]
  0.1     258.32     0.38                            do_single_div [37]
  0.1     259.03     0.36        5    71.01    71.01  .udiv [39]
  0.1     259.38     0.35                            not_too_big [40]
  0.1     259.64     0.27                            not_too_big [41]
  0.1     260.28     0.17                            single_divloop [45]
  0.0     260.35     0.07                            single_divloop [48]
  0.0     260.51     0.01                            zero_divide [55]