[libcamera-devel] [PATCH] cam: convert to libfmt

[Tomi Valkeinen]

On 10/05/2022 16:22, Laurent Pinchart wrote:
> Hi Tomi,
> 
> Thank you for the patch.
> 
> On Tue, May 10, 2022 at 10:16:17AM +0300, Tomi Valkeinen wrote:
>> This is just a conversation starter, not for merging. I really like
>> libfmt, and I really hate the C++ stream-based string formatting. libfmt
>> is the first library I add to any new C++ project I make.
> 
> You've got it wrong. The first library you should add to any new C++
> project you make is libcamera :-)
> 
>> You can find more information about libfmt from:
>>
>> https://github.com/fmtlib/fmt
>> https://fmt.dev/latest/index.html
>>
>> This patch is just a crude conversion with ugly macros to showcase what
>> the formatting code might look like.
>>
>> libfmt pages suggest that libfmt would be faster and smaller (exe size)
>> than iostreams, but for the size it didn't seem to be true in cam's case
>> as the tests below show. However, simple prints did reduce the exe size,
>> but the few more complex ones increased the size.
>>
>> Size tests with gcc 11.2.0-19ubuntu1
>>
>> - Without libfmt
>>
>> debug		3523400
>> debug lto	3269368
>> release		223056
>> release lto	172280
>>
>> - With libfmt
>>
>> debug		4424256
>> debug lto	4143840
>> release		303952
>> release lto	252640
>>
>> Above shows that cam's size clearly increases with libfmt. However, the
>> increase really comes only from one case, the use of fmt::memory_buffer
>> and std::back_inserter.
> 
> Is this because libfmt is a header-only library ? What if it is built as
> a shared object, what's the impact on code size in cam ?

libfmt supports header-only, but it's not header-only here.

>> Converting that code to use fmt::format() and
>> naive string append gives:
>>
>> release with string append	233680
>> release lto with string append	186936
> 
> What's the impact of switching to string concatenation on runtime ?

What do you mean?

> Would you consider the option of keeping std::stringstream ?

You mean formatting mostly with libfmt, but sometimes with streams? Or 
using libfmt but appending the formatted string to stringstream?

  Tomi