Re: else ladders practice

On Fri, 22 Nov 2024 12:33:29 -0000 (UTC)
antispam@fricas.org (Waldek Hebisch) wrote:

Bart <bc@freeuk.com> wrote:

 
Sure. That's when you run a production build. I can even do that
myself on some programs (the ones where my C transpiler still
works) and pass it through gcc-O3. Then it might run 30% faster. 

 
On fast machine running Dhrystone 2.2a I  get:
 
tcc-0.9.28rc    20000000
gcc-12.2 -O     64184852
gcc-12.2 -O2    83194672
clang-14 -O     83194672
clang-14 -O2    85763288
 
so with 02 this is more than 4 times faster.  Dhrystone correlated
resonably with runtime of tight compute-intensive programs.
Compiler started to cheat on original Dhrystone, so there are
bigger benchmarks like SPEC INT.  But Dhrystone 2 has modifications
to make cheating harder, so I think it is still reasonable
benchmark.  Actually, difference may be much bigger, for example
in image processing both clang and gcc can use vector intructions,
with may give additional speedup of order 8-16.
 
30% above means that you are much better than tcc or your program
is badly behaving (I have programs that make intensive use of
memory, here effect of optimization would be smaller, but still
of order 2).
 

gcc -O is not what Bart was talking about. It is quite similar to -O1.
Try gcc -O0.
With regard to speedup, I had run only one or two benchmarks with tcc
and my results were close to those of Bart. gcc -O0 very similar to tcc
in speed of the exe, but compiles several times slower. gcc -O2 exe
about 2.5 times faster.
I'd guess, I can construct a case, where gcc successfully vectorized
some floating-point loop calculation and showed 10x speed up vs tcc on
modern Zen5 hardware. But that's would not be typical.