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

[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?

[Bug middle-end/114532] gcc -fno-common option causes performance degradation on certain architectures

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-03-30
             Status|UNCONFIRMED                 |WAITING
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
```
Rec_Pointer     Ptr_Glob,
                Next_Ptr_Glob;
int             Int_Glob;
Boolean         Bool_Glob;
char            Ch_1_Glob,
                Ch_2_Glob;
int             Arr_1_Glob [50];
int             Arr_2_Glob [50] [50];
```

Maybe the order of these changed in the layout of the final executable.
In the case of -fcommon, the layout of these are handled by the linker while
with -fno-common, they are handled by compiler into the assembly into the
specific section (and then the sections are combined/laid out by the linker).

So maybe look at the linker map and compare it to what GCC does with
-fno-common in the .s file.