[cfe-dev] Adding lifetime begin/end to unnamed temporaries

Arnaud A. de Grandmaison arnaud.degrandmaison at arm.com
Fri Aug 1 08:43:49 PDT 2014

Yes, that’s the only case I could think of. I do not know if this would be used often though.





From: David Blaikie [mailto:dblaikie at gmail.com] 
Sent: 01 August 2014 16:53
To: Arnaud De Grandmaison
Cc: Renato Golin; cfe-dev Developers
Subject: Re: [cfe-dev] Adding lifetime begin/end to unnamed temporaries


On Aug 1, 2014 5:55 AM, "Arnaud A. de Grandmaison" <arnaud.degrandmaison at arm.com> wrote:
> I do not think this can apply to "C-only", as C has no way to express unnamed temporaries (language lawyers may contradict me here ;)

Write a c function that returns a large struct by value and another with that type as a parameter and call the later with the result of the former?

struct big source(void);
void sink(struct big);

sink(source ());

Should get you some anonymous stack usage in c that you can optimize the use of.

> On the other hand for C++, you can have lots of those unnamed temporaries.
> A possible path, along the line of what you suggest, would be to activate lifetime markers only in the non-throwing cases (or when compiled with -fno-exceptions). The exceptional part could come later.
> We could at least get some of the benefits now.
> Cheers,
> Arnaud
> -----Original Message-----
> From: Renato Golin [mailto:renato.golin at linaro.org]
> Sent: 01 August 2014 13:26
> To: Arnaud De Grandmaison
> Cc: Clang Dev
> Subject: Re: [cfe-dev] Adding lifetime begin/end to unnamed temporaries
> On 31 July 2014 17:59, Arnaud A. de Grandmaison <arnaud.degrandmaison at arm.com> wrote:
> > By the way, my patch indeed also affected some debuginfo (some
> > breakpoint location if I remember correctly). I have switched
> > temporarily to something else, but this  patch proved to be extremely
> > complex --- complexity linked to what you can find in the thread about
> > temporary destructors. There is something going wrong in how the
> > scopes are handled and my patch triggers it. I am probably breaking
> > some undocumented or implicit assumptions.  But there is definitely a
> > lot of potential to reduce stack usage, and we saw that on real code.
> This might be a bad idea, but is it possible to start with a C-only implementation, and move on with C++ later? At least we can get the general implementation right, and then only fiddle with exception handling when we need to.
> cheers,
> --renato
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