[Cfp-interest 1479] Re: complex multiplication and division

[Ian McIntosh]

  BODY { font-family:Arial, Helvetica, sans-serif;font-size:12px; }As
a user, I would not expect the result of a single arithmetic
operation to differ so dramatically between -O0 and -O1, or to differ
so dramatically between GCC and ICC.  I would report this as a GCC
bug, on the grounds that using inline code and using a library
shouldn't differ significantly.  If I could, I would also try LLVM.
 As a former compiler developer, if company lawyers didn't object and
I had access to a GCC and an ICC compiler and maybe LLVM I might look
at the assembly code generated and see why the results are so
different.
 If I still worked for IBM, I might see what result IBM's XLC, GCC
and LLVM compilers (where possible with both double-double and
binary128) produced - likely different again.  ��   I might also
try IBM's PPC MASS library, and their z/OS compiler.
 All that of course would make it a fairly big job . . .  fortunately
for me, I don't have access to any of those.  If somebody wants to
send me assembler code to compare I'll have a look at it though.
  - Ian McIntosh
 On Mon 20/02/10  4:41 AM , paul zimmermann Paul.Zimmermann at inria.fr
sent:
        Dear Damian, 
 on the gcc10 machine from the GCC compile farm, with gcc 6.3.0, I
get: 
 x=(-1.0689346237980500e+252,7.8846096489686452e-145) 
 y=(5.1152592576527620e-318,6.7327111521288819e+78) 
 x/y=(1.1710898434244477e-223,1.5876733750267203e+173) 
 Same on the machine "devel03" from the Plafrim platform with gcc
4.8.5: 
 x=(-1.0689346237980500e+252,7.8846096489686452e-145) 
 y=(5.1152592576527620e-318,6.7327111521288819e+78) 
 x/y=(1.1710898434244477e-223,1.5876733750267203e+173) 
 This is with -O0 (or no -O option), with -O1 or higher I get the MPC
result 
 (since then GCC uses constant folding, calling MPC). 
 Paul 
 > Date: Sun, 9 Feb 2020 22:22:35 +1100 (AEDT) 
 > From: Damian McGuckin  
 >  
 > I tried your program on a GCC 9.2.1 on Power9, and GCC 4.8.1 and
8.? on  
 > Xeon. I was specifically looking at division because something is
very  
 > obviously wrong. You mentioned: 
 >  
 > >  /* MPC gives      
(-3.5164330596706266e-225,1.5876733750267203e+173) */ 
 > >  /* GCC 9.2.1 gives
(1.1710898434244477e-223,1.5876733750267203e+173) */ 
 > >  /* ICC 19.0.4.243:
(-3.5164330596706266e-225,1.5876733750267203e+173) */ 
 >  
 > I get the same answer as MPC. Not your answer. On an ancient Xeon
with GCC  
 > 4.4.7, I get your 1.171.....e-223 answer. A quick guess is that
something  
 > is probably not quite right with your installation of GCC. But I
could be 
 > wrong. 
 >  
 > Regards - Damian 
 >  
 > Pacific Engineering Systems International, 277-279 Broadway, Glebe
NSW 2037 
 > Ph:+61-2-8571-0847 .. Fx:+61-2-9692-9623 | unsolicited email not
wanted here 
 > Views & opinions here are mine and not those of any past or
present employer 
 >  
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