Why is this code slow?

[Lance Bachmeier]

On Tuesday, 26 March 2024 at 14:25:53 UTC, Lance Bachmeier wrote:
> On Sunday, 24 March 2024 at 19:31:19 UTC, Csaba wrote:
>> I know that benchmarks are always controversial and depend on 
>> a lot of factors. So far, I read that D performs very well in 
>> benchmarks, as well, if not better, as C.
>>
>> I wrote a little program that approximates PI using the 
>> Leibniz formula. I implemented the same thing in C, D and 
>> Python, all of them execute 1,000,000 iterations 20 times and 
>> display the average time elapsed.
>>
>> Here are the results:
>>
>> C: 0.04s
>> Python: 0.33s
>> D: 0.73s
>>
>> What the hell? D slower than Python? This cannot be real. I am 
>> sure I am making a mistake here. I'm sharing all 3 programs 
>> here:
>>
>> C: https://pastebin.com/s7e2HFyL
>> D: https://pastebin.com/fuURdupc
>> Python: https://pastebin.com/zcXAkSEf
>>
>> As you can see the function that does the job is exactly the 
>> same in C and D.
>>
>> Here are the compile/run commands used:
>>
>> C: `gcc leibniz.c -lm -oleibc`
>> D: `gdc leibniz.d -frelease -oleibd`
>> Python: `python3 leibniz.py`
>>
>> PS. my CPU is AMD A8-5500B and my OS is Ubuntu Linux, if that 
>> matters.
>
> As others suggested, pow is the problem. I noticed that the C 
> versions are often much faster than their D counterparts. (And 
> I don't view that as a problem, since both are built into the 
> language - my only thought is that the D version should call 
> the C version).
>
> Changing
>
> ```
> import std.math:pow;
> ```
>
> to
>
> ```
> import core.stdc.math: pow;
> ```
>
> and leaving everything unchanged, I get
>
> C: Avg execution time: 0.007918
> D (original): Avg execution time: 0.102612
> D (using core.stdc.math): Avg execution time: 0.008134
>
> So more or less the exact same numbers if you use 
> core.stdc.math.

And then the other thing is changing

```
const int BENCHMARKS = 20;
```

to

```
enum BENCHMARKS = 20;
```

which should allow substitution of the constant directly into the 
rest of the program, which gives

```
Avg execution time: 0.007564
```

On my Ubuntu 22.04 machine, therefore, the LDC binary with no 
flags is slightly faster than the C code compiled with your flags.