[Bug gcov-profile/94369] 505.mcf_r is 6-7% slower at -Ofast -march=native with PGO+LTO than with just LTO

[jamborm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94369

--- Comment #3 from Martin Jambor <jamborm at gcc dot gnu.org> ---
I did not save the reported number of samples but from the raw sample
numbers and percentage points it seems so:

 (562770/0.4013)/(518450/0.3953) = 1.069

Nevertheless, I did save separately obtained perf stat numbers which
also look similar (and the number of branches might be a clue):

LTO:

         326083.03 msec task-clock:u              #    0.999 CPUs utilized      
                 0      context-switches:u        #    0.000 K/sec              
                 0      cpu-migrations:u          #    0.000 K/sec              
              8821      page-faults:u             #    0.027 K/sec              
     1080945983089      cycles:u                  #                            
      (83.33%)
       21883016095      stalled-cycles-frontend:u #    2.02% frontend cycles
idle     (83.33%)
      435184347885      stalled-cycles-backend:u  #   40.26% backend cycles
idle      (83.33%)
      847570680279      instructions:u            #    0.78  insn per cycle     
                                                  #    0.51  stalled cycles per
insn  (83.34%)
      147428907202      branches:u                #  452.121 M/sec             
      (83.33%)
       13395643229      branch-misses:u           #    9.09% of all branches   
      (83.33%)

     326.436794016 seconds time elapsed

     325.869528000 seconds user
       0.086873000 seconds sys

vs. PGO+LTO:

         347929.80 msec task-clock:u              #    0.999 CPUs utilized      
                 0      context-switches:u        #    0.000 K/sec              
                 0      cpu-migrations:u          #    0.000 K/sec              
              8535      page-faults:u             #    0.025 K/sec              
     1153803509197      cycles:u                  #                            
      (83.33%)
       19911862620      stalled-cycles-frontend:u #    1.73% frontend cycles
idle     (83.33%)
      476343319558      stalled-cycles-backend:u  #   41.28% backend cycles
idle      (83.33%)
      894092414890      instructions:u            #    0.77  insn per cycle     
                                                  #    0.53  stalled cycles per
insn  (83.33%)
      173999066006      branches:u                #  500.098 M/sec             
      (83.33%)
       13698979291      branch-misses:u           #    7.87% of all branches   
      (83.34%)

     348.308607033 seconds time elapsed

     347.711752000 seconds user
       0.090975000 seconds sys