From: anton@mips.complang.tuwien.ac.at
anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:
>OTOH, yesterday I saw what gcc did for the inner loop of the bubble
>benchmark from the Stanford integer benchmarks:
>
> while ( i
> if ( sortlist[i] > sortlist[i+1] ) {
> j = sortlist[i];
> sortlist[i] = sortlist[i+1];
> sortlist[i+1] = j;
> };
> i=i+1;
> };
>
> top=top-1;
> };
>
>gcc-12.2 -O1 produces straighforward scalar code, gcc-12.2 -O3 wants
>to use SIMD instructions:
>
> gcc -01 gcc -O3
>1c: add $0x4,%rax c0: movq (%rax),%xmm0
> cmp %rsi,%rax add $0x1,%edx
> je 35 pshufd $0xe5,%xmm0,%xmm1
>25: mov (%rax),%edx movd %xmm0,%edi
> mov 0x4(%rax),%ecx movd %xmm1,%ecx
> cmp %ecx,%edx cmp %ecx,%edi
> jle 1c jle e1
> mov %ecx,(%rax) pshufd $0xe1,%xmm0,%xmm0
> mov %edx,0x4(%rax) movq %xmm0,(%rax)
> jmp 1c e1: add $0x4,%rax
>35: cmp %r8d,%edx
> jl c0
>
>The version produced by gcc -O3 is almost three times slower on a
>Skylake than the one by gcc -O1 and is actually slower than several
>Forth systems, including gforth-fast. I think that the reason is that
>the movq towards the end stores two items, and the movq at the start
>of the next iteration loads one of these item, i.e., there is partial
>overlap between the store and the load. In this case the hardware
>takes a slow path, which means that the slowdown is much bigger than
>the instruction count suggests.
I was curious if a more recent Intel core had improved on that (and
maybe such a more recent Intel core was targeted by the "optimization"
that caused the slowdown), so I measured it on a P-core of a Core
i3-1315U. The results are as follows:
O1/bubble O3/bubble
424,248,952 2,061,809,866 cpu_core/cycles/
1,536,825,253 1,986,035,580 cpu_core/instructions/
So, more than a factor of 4 on this microarchitecture.
The differences in the topdown analysis are also interesting:
O1
1,177,188,340 cpu_core/topdown-retiring/ # 46.1% Retiring
279,332,826 cpu_core/topdown-bad-spec/ # 10.9% Bad Speculation
778,141,445 cpu_core/topdown-fe-bound/ # 30.5% Frontend Bound
319,237,516 cpu_core/topdown-be-bound/ # 12.5% Backend Bound
0 cpu_core/topdown-heavy-ops/ # 0.0% Heavy Operations
269,356,654 cpu_core/topdown-br-mispredict/ # 10.5% Branch Mispredict
269,356,654 cpu_core/topdown-fetch-lat/ # 10.5% Fetch Latency
59,857,034 cpu_core/topdown-mem-bound/ # 2.3% Memory Bound
O3
1,599,831,263 cpu_core/topdown-retiring/ # 12.9% Retiring
630,236,558 cpu_core/topdown-bad-spec/ # 5.1% Bad Speculation
533,277,087 cpu_core/topdown-fe-bound/ # 4.3% Frontend Bound
9,598,987,583 cpu_core/topdown-be-bound/ # 77.6% Backend Bound
280,169 cpu_core/topdown-heavy-ops/ # 0.0% Heavy Operations
630,236,558 cpu_core/topdown-br-mispredict/ # 5.1% Branch Mispredict
193,918,941 cpu_core/topdown-fetch-lat/ # 1.6% Fetch Latency
5,623,649,291 cpu_core/topdown-mem-bound/ # 45.5% Memory Bound
- anton
--
M. Anton Ertl http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
New standard: https://forth-standard.org/
EuroForth 2023: https://euro.theforth.net/2023
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