Use string_file::release in some places
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Commit Message
I found a few spots like:
string_file f;
std::string x = f.string ();
However, string_file::string returns a 'const std::string &'... so it
seems to me that this must be copying the string (? I find it hard to
reason about this in C++).
This patch changes these spots to use release() instead, which moves
the string.
---
gdb/breakpoint.c | 2 +-
gdb/top.c | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)
Comments
On Thu, Sep 21, 2023 at 12:42 PM Tom Tromey via Gdb-patches
<gdb-patches@sourceware.org> wrote:
>
> I found a few spots like:
>
> string_file f;
> std::string x = f.string ();
>
> However, string_file::string returns a 'const std::string &'... so it
> seems to me that this must be copying the string (? I find it hard to
> reason about this in C++).
I admit, you now have me doubting everything I thought I remembered
on this subject! [TODO: reread the standard literature]
As far as I can tell, your assessment is at least consistent with what is
done elsewhere, e.g., location.c, ada-lang,c, and just about everywhere
else except the two locations you've identified in this patch.
Would symtab.c's symbol_to_info_string and valops.c:incomplete_type_hint
also qualify for similar treatment? They use addition assignment, so maybe
that alters the analogy?
In any case:
Reviewed-by: Keith Seitz <keiths@redhat.com>
Keith
On Fri, Sep 22, 2023 at 01:42:22PM -0700, Keith Seitz via Gdb-patches wrote:
> On Thu, Sep 21, 2023 at 12:42 PM Tom Tromey via Gdb-patches
> <gdb-patches@sourceware.org> wrote:
> >
> > I found a few spots like:
> >
> > string_file f;
> > std::string x = f.string ();
> >
> > However, string_file::string returns a 'const std::string &'... so it
> > seems to me that this must be copying the string (? I find it hard to
> > reason about this in C++).
Hi Tom and Keith,
I think your assessment is correct. Such case calls the string copy
operator[1] (i.e. `std::string::string (const std::string &)` - plus the
template expansion).
Same goes for the "return f.string ();" cases from a function returning
a std::string.
FWIW, the changes you made seems good to me.
Reviewed-by: Lancelot Six <lancelot.six@amd.com>
>
> I admit, you now have me doubting everything I thought I remembered
> on this subject! [TODO: reread the standard literature]
>
> As far as I can tell, your assessment is at least consistent with what is
> done elsewhere, e.g., location.c, ada-lang,c, and just about everywhere
> else except the two locations you've identified in this patch.
>
> Would symtab.c's symbol_to_info_string and valops.c:incomplete_type_hint
> also qualify for similar treatment? They use addition assignment, so maybe
> that alters the analogy?
In those cases, because the += operator is used, there is no move
semantics which can be leveraged[2]. This operator can only be used
with:
- const std::string & (what is currently used)
- char
- const char *
- std::initializer_list<char>
C++17 also adds string_view based option, but in all the cases we copy
the data from the argument.
Best,
Lancelot.
>
> In any case:
> Reviewed-by: Keith Seitz <keiths@redhat.com>
>
> Keith
>
[1] https://en.cppreference.com/w/cpp/language/copy_initialization
[2] https://en.cppreference.com/w/cpp/string/basic_string/operator%2B%3D
>>>>> "Keith" == Keith Seitz <keiths@redhat.com> writes:
Keith> Would symtab.c's symbol_to_info_string and valops.c:incomplete_type_hint
Keith> also qualify for similar treatment? They use addition assignment, so maybe
Keith> that alters the analogy?
These two could accumulate all the text into a single string_file.
Maybe that would be slightly better.
Anyway I'm checking this in.
Tom
@@ -7581,7 +7581,7 @@ bp_location::to_string () const
string_file stb;
ui_out_redirect_pop redir (current_uiout, &stb);
print_breakpoint_location (this->owner, this);
- return stb.string ();
+ return stb.release ();
}
/* Decrement reference count. If the reference count reaches 0,
@@ -647,12 +647,12 @@ execute_fn_to_string (std::string &res, std::function<void(void)> fn,
catch (...)
{
/* Finally. */
- res = std::move (str_file.string ());
+ res = str_file.release ();
throw;
}
/* And finally. */
- res = std::move (str_file.string ());
+ res = str_file.release ();
}
/* See gdbcmd.h. */