Re: [PATCH v2 1/4] [gdb/cli] Skip string copy in source_cache::ensure

Lancelot SIX
2023-10-13T16:34:51Z

Hi Tom,

On Fri, Oct 13, 2023 at 02:19:50PM +0200, Tom de Vries wrote:
> In function source_cache::ensure we have:
> ...
> std::ostringstream output;
> ...
> contents = output.str();
> ...
> The last line causes an unnecessary string copy.
>
> C++20 allows us to skip it, like this:
> ...
> contents = std::move (output).str();
> ...

Looking at cppreference[1], it suggests that this new C++ overload for
str should be equivalent to

content = std::move (*output.rbuf ()).str ();

Should we use this construct instead regardless of C++'s version and
avoid the #ifdef? That being said, I find the C++20 version easier to
read, so it's up to you.

Best,
Lancelot.

[1] https://en.cppreference.com/w/cpp/io/basic_ostringstream/str

>
> Use the more efficient version if compiling with -std=c++20.
>
> Tested on x86_64-linux.
> ---
> gdb/source-cache.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/gdb/source-cache.c b/gdb/source-cache.c
> index 77b357cb42b..7ef54411c69 100644
> --- a/gdb/source-cache.c
> +++ b/gdb/source-cache.c
> @@ -252,7 +252,11 @@ source_cache::ensure (struct symtab *s)
> std::istringstream input (contents);
> std::ostringstream output;
> highlighter->highlight (input, output, lang_name, fullname);
> +#if __cplusplus >= 202002L
> + contents = std::move (output).str();
> +#else
> contents = output.str ();
> +#endif
> already_styled = true;
> }
> catch (...)
> --
> 2.35.3
>