[Bug c/114532] New: gcc -fno-common option causes performance degradation on certain architectures

[h13958451065 at 163 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114532

            Bug ID: 114532
           Summary: gcc -fno-common option causes performance degradation
                    on certain architectures
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: h13958451065 at 163 dot com
  Target Milestone: ---

gcc 10.3 has turnd off -fcommon into fno-common by default, which led to
performance decrease about 7% (CPU intel sapphire rapids) on UnixBench dhry2reg
subtest.

We found that L1-dcache-load-misses increased under -fno-common option than
-fcommon, which we  suspect is the cause of the performance decrease. 
L1 dcache size is 48K per core in CPU intel sapphire rapids (32 KB instruction
+ 48 KB data).

We have compared the binary layout of the executables generated under -fcommon
and -fno-common, we found that the location offset of XXX_Time and XXX_Glob
global variables in BSS section increased about 10K under -fno-common.
Maybe the increase of bss gap influence the probability of L1-dcache-load-miss.

I am not sure about this conclusion, is there any way to verify our conjecture?