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?
