Floating point exception in w3fld1md.F90

[MatthewMasarik-NOAA]

Describe the bug
Running a Debug build with additional flags captures a divide-by-zero floating point exception in w3fld1md.F90.

To Reproduce

• Run grid option b from regtest ww3_ufs1.1/unstr
• Ensure that WW3 builds in Debug cmake type, and additionally, appends these compile flags:

-fno-omit-frame-pointer -fsanitize=address

Expected behavior
forrtl: error (75): floating point exception due to a divide-by-zero occurring in a test condition parameter,

CRIT2=(ABS(TAUDIR-TAUDIRB)*100.0/(TAUDIR+TAUDIRB)*0.5) .GT. 0.1

within subroutine W3FLD1 of w3fld1md.F90. Here TAUDIR = -TAUDIRB, where the values are floating point approximations near Pi. See sample truncated with added write statement output:

  WAVEWATCH III calculating for 2019/12/01 06:36:00 UTC at 22:25:48     
  WAVEWATCH III calculating for 2019/12/01 06:48:00 UTC at 22:25:56     
  WAVEWATCH III calculating for 2019/12/01 07:00:00 UTC at 22:26:04     
  WAVEWATCH III calculating for 2019/12/01 07:12:00 UTC at 22:26:12     
   ******* !!! WARNING - TAUDIR:   3.141582                             
   ******* !!! WARNING - TAUDIRB:  -3.141582                            
  WAVEWATCH III calculating for 2019/12/01 07:24:00 UTC at 22:26:20     
  .                                                                     
  .                                                                     
  WAVEWATCH III calculating for 2019/12/01 10:00:00 UTC at 22:27:54     
   ******* !!! WARNING - TAUDIR:   3.141584                             
   ******* !!! WARNING - TAUDIRB:  -3.141584                            
  WAVEWATCH III calculating for 2019/12/01 10:12:00 UTC at 22:28:01     
  .                                                                     
  .                                                                     
  WAVEWATCH III reached the end of a computation loop at 22:29:12       
                                                                        
  Updating input at 2019/12/01 12:00:00 UTC                             
     Updating currents                                                  
     Updating winds                                                     
                                                                        
  WAVEWATCH III calculating for 2019/12/01 12:12:00 UTC at 22:29:12     
  .                                                                     
  .                                                                     
  WAVEWATCH III calculating for 2019/12/01 15:36:00 UTC at 22:31:09     
  WAVEWATCH III calculating for 2019/12/01 15:48:00 UTC at 22:31:16     
   ******* !!! WARNING - TAUDIR:  -3.141553                             
   ******* !!! WARNING - TAUDIRB:   3.141553                            
                                                                        
   ******* !!! WARNING - TAUDIR:  -3.141553                             
   ******* !!! WARNING - TAUDIRB:   3.141553                            
  WAVEWATCH III calculating for 2019/12/01 16:00:00 UTC at 22:31:23     
  WAVEWATCH III calculating for 2019/12/01 16:12:00 UTC at 22:31:29     
  WAVEWATCH III calculating for 2019/12/01 16:24:00 UTC at 22:31:37     
  WAVEWATCH III calculating for 2019/12/01 16:36:00 UTC at 22:31:43     
  .                                                                     
  .                                                                     
  WAVEWATCH III calculating for 2019/12/01 19:00:00 UTC at 22:33:04     
  WAVEWATCH III calculating for 2019/12/01 19:12:00 UTC at 22:33:11     
  WAVEWATCH III calculating for 2019/12/01 19:24:00 UTC at 22:33:18     
   ******* !!! WARNING - TAUDIR:  -3.141573                             
   ******* !!! WARNING - TAUDIRB:   3.141573                            
                                                                        
   ******* !!! WARNING - TAUDIR:   3.141589                             
   ******* !!! WARNING - TAUDIRB:  -3.141589                            
                                                                        
   ******* !!! WARNING - TAUDIR:   3.141589                             
   ******* !!! WARNING - TAUDIRB:  -3.141589                            
  WAVEWATCH III calculating for 2019/12/01 19:36:00 UTC at 22:33:25     
  WAVEWATCH III calculating for 2019/12/01 19:48:00 UTC at 22:33:32

I hit this error looking for a different issue, so I bypassed it with IF logic. It's just a mathematical bypass though -- I don't know what the science considerations are currently. I rationalized that in these cases, you have a finite number over zero, and taking the liberty to view that as a limit, as the denominator approaching zero, then the term approaches a very large number, and it only needs to be greater than 0.1, so assign it .true..

          IF ((TAUDIR+TAUDIRB).NE.0.) THEN                                               
              CRIT2=(ABS(TAUDIR-TAUDIRB)*100.0/(TAUDIR+TAUDIRB)*0.5) .GT. 0.1            
          ELSE                                                                           
              CRIT2=.TRUE.                                                               
          ENDIF    

Screenshots

 5 0x000000000040fd52 main()  ???:0                                                                            
 6 0x0000000000022555 __libc_start_main()  ???:0                                                               
 7 0x000000000040fc69 _start()  ???:0                                                                          
=================================                                                                              
forrtl: error (75): floating point exception                                                                   
Image              PC                Routine            Line        Source                                     
ww3_shel           0000000001569AFB  Unknown               Unknown  Unknown                                    
libpthread-2.17.s  00002BA86A224630  Unknown               Unknown  Unknown                                    
ww3_shel           00000000014A2996  w3fld1md_mp_w3fld         553  w3fld1md.F90                               
ww3_shel           00000000013FE610  w3srcemd_mp_w3src        2181  w3srcemd.F90                               
ww3_shel           0000000000E0404F  w3wavemd_mp_w3wav        2224  w3wavemd.F90                               
ww3_shel           00000000004737EA  MAIN__                   2573  ww3_shel.F90                               
ww3_shel           000000000040FD52  Unknown               Unknown  Unknown                                    
libc-2.17.so       00002BA86A453555  __libc_start_main     Unknown  Unknown                                    
ww3_shel           000000000040FC69  Unknown               Unknown  Unknown                                    
srun: error: h10c36: task 1355: Aborted (core dumped)                                                          
srun: launch/slurm: _step_signal: Terminating StepId=53400451.0                                                
slurmstepd: error: *** STEP 53400451.0 ON h3c08 CANCELLED AT 2023-12-27T14:40:05 ***

Additional context
Fyi, this new issue #1159 also has floating point divide by zero. It's in a different subroutine, but same module, W3FLD1MD. The related pr #1163.

[mickaelaccensi]

good catch Matthew. The solution seems acceptable

[JessicaMeixner-NOAA]

FYI @breichl