From: Bonita.Montero@gmail.com
Am 11.12.2025 um 12:46 schrieb bart:
> On 11/12/2025 03:35, Paul wrote:
>> On Wed, 12/10/2025 5:37 PM, bart wrote:
>
>>> A getc loop took 4.3 seconds to read a 192MB file from SSD, on my
>>> Windows PC.
>>>
>>> Under WSL it took 8.4 seconds (8.4/0.5 real/user).
>>>
>>> However reading it all in one go took 0.14 seconds.
>>>
>>> I guess not all 'getc' implementations are the same.
>>
>> #include
>> #include
>> #include
>>
>> /* gcc -Wl,--stack,1200000000Â -o getcbench.exe getcbench.c */
>>
>> int main(int argc, char **argv)
>> {Â FILE* source;
>>
>> Â Â Â int c;Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
            /* getc holder */
>> Â Â Â const int size = 1000*1000*1000;
>> Â Â Â char keep[size];
>
> I didn't see the point of either keeping the array on the stack, or
> using a VLA. I made it static. That also allowed me a choice of
> compilers with no special options needed.
Yes. Under Linux/x64 the default stack size is 8MiB, unter Windows/x64
one MiB.
That's a stack overflow - or should I call it underflow since it grows
downards
- for sure.
>
>> Add some code after the fopen.
>>
>> Â Â Â if (setvbuf(source, NULL, _IOFBF, 65536) != 0)
>> Â Â Â {
>> Â Â Â Â Â Â Â Â fprintf(stderr, "setvbuf() failed\n\n" );
>> Â Â Â Â Â Â Â Â return -1;
>> Â Â Â }
>
> When I added that, it slowed it down! Maybe it was already using a
> bigger buffer.
>
>> Extra code was added so keep[] was not optimized away.
>
> My loop didn't store the characters anywhere; it just bumped a count.
>
> I think it was enough that it was calling an external function,
> 'getc'; a commpiler can't optimise that away.
>
>> Read 1000000000 bytes in 001.075651 seconds
>>
>> Busy sum = FFFFFFFFE216FE9C
>>
>> That's getting close to a gigabyte per second.
>>
>> Summary: The -O2 makes a BIG difference.
>> Â Â Â Â Â Â Â Â Â No idea how it is cheating.
>
> How a look at the generated assembly: is it still making an actual
> call to 'getc', or has it been inlined?
>
> In my case -O2 made little difference, and it was still calling
> getc(). -O2 can't effect such a precompiled function, unless getc() is
> not really an external function: either a macro, or a wrapper.
>
> Also, the generated EXE file actually imports getc from msvcrt.dll,
> which is a library not known to be performant.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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