[Bug rtl-optimization/78634] New: [7 Regression] 30% performance drop after r242832.

[ysrumyan at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78634

            Bug ID: 78634
           Summary: [7 Regression] 30% performance drop after r242832.
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ysrumyan at gmail dot com
  Target Milestone: ---

Created attachment 40215
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40215&action=edit
test-case to reproduce

We noticed a huge performance regression on x86 for one important benchmark
(the reproduced for which is attached). It is caused by additional
if-conversion which can be seen in ce2 dump:
IF-THEN-ELSE-JOIN block found, pass 1, test 12, then 13, else 14, join 15
scanning new insn with uid = 163.
scanning new insn with uid = 164.
scanning new insn with uid = 165.
scanning new insn with uid = 166.
scanning new insn with uid = 167.
scanning new insn with uid = 168.
scanning new insn with uid = 169.
if-conversion succeeded through noce_try_cmove_arith
deleting insn with uid = 85.
deleting block 14
Removing jump 78.
deleting insn with uid = 78.
deleting insn with uid = 80.
deleting block 13
Merging block 15 into block 12...
changing bb of uid 87
changing bb of uid 88
  from 15 to 12
changing bb of uid 89
  from 15 to 12
Merged blocks 12 and 15.
Conversion succeeded on pass 1.

On AVX2 machine we see:

time ./test1.1124.exe   // build by compiler before r242832.                    

real    0m0.577s
user    0m0.575s
sys     0m0.002s
time ./test1.1125.exe  // build by compiler after r242832.                      

real    0m0.888s
user    0m0.886s
sys     0m0.001s

It is sufficient to compile it with -Ofast option to reproduce on x86.