In optimization level 3, gdc11 doesn't seem to work as well as gdc10 for base 64 benchmark.
On this site:
https://github.com/kostya/benchmarks#base64

, the poor performance of GDC 11 is well visible, but while version 10 did a good optimization.

I did a base 64 experiment on RISCV64 architecture (other than Intel) with ubuntu-image:

In gdc-11(RISCV64):

 ubuntu@ubuntu:~/dlang/benchmark/benchmarks/base64/gdc-11$ gdc-11 -O3 test.d
 ubuntu@ubuntu:~/dlang/benchmark/benchmarks/base64/gdc-11$ time ./a.out
 encode aaaa... to YWFh...: 1431666688, 20.53
 decode YWFh... to aaaa...: 1073741824, 50.37

 real	1m11.219s
 user	1m10.793s
 sys	0m0.171s
______________________________________________________________________________
In gdc-10(RISCv64):
 ubuntu@ubuntu:~/dlang/benchmark/benchmarks/base64/gdc-10$ gdc-10 -O3 test.d
 ubuntu@ubuntu:~/dlang/benchmark/benchmarks/base64/gdc-10$ time ./a.out
 encode aaaa... to YWFh...: 1431666688, 18.48
 decode YWFh... to aaaa...: 1073741824, 14.55

 real	0m33.447s
 user	0m33.108s
 sys	0m0.220s

source code from :
https://github.com/kostya/benchmarks/blob/master/base64/test.d

On Sunday, 19 September 2021 at 19:04:28 UTC, Mahdi wrote:

It's because of libphobos not being compiled with inlining due to the changes to template linkage emission. If you rebuild Phobos with LTO enabled (if it isn't already) you can get your performance back.

Compile with LTO and try that.

On Sunday, 19 September 2021 at 19:31:27 UTC, max haughton wrote:

Thanks
I experimented with lto but it got worse:

ubuntu@ubuntu:~/dlang/benchmark/benchmarks/base64/gdc-11$ gdc-11 -flto -O3 test.d
ubuntu@ubuntu:~/dlang/benchmark/benchmarks/base64/gdc-11$ time ./a.out
encode aaaa... to YWFh...: 1431666688, 20.83
decode YWFh... to aaaa...: 1073741824, 69.34

real 1m30.447s
user 1m29.369s
sys 0m0.191s

On Sunday, 19 September 2021 at 19:31:27 UTC, max haughton wrote:

Could you please tell me more about these changes? Are they somewhere in the GDC frontend, GCC backend or in the libphobos code itself? Were they intentional or a side effect of something else?

Before finding this forum thread, I happened to submit a similar issue in the gcc bugzilla some days ago: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102765

What is the current status of LTO nowadays? Is it sufficiently reliable and problem-free in GDC11 to be universally recommended for any optimized release builds?