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

Thu Jul 31 09:59:26 PDT 2014

Hi Renato,

I am clearly not a specialist for debug info. It will certainly put some pressure on the debug info generator at opt level >= 01, as the stack slots will be reused, so it needs to place the info at the right place / right time.

You have less live variables, at any given point in the function, but this assumes you have handled the dead variables correctly…

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.

Cheers,

--

Arnaud

From: Renato Golin [mailto:renato.golin at linaro.org] 
Sent: 31 July 2014 10:21
To: Arnaud De Grandmaison
Cc: Clang Dev
Subject: Re: [cfe-dev] Adding lifetime begin/end to unnamed temporaries

Hi Arnaud, 

Can't this affect debugging as well?

I mean, if you run this on O1 or less, you'll have a lot more variables optimised away, especially at the end of the function, and at higher optimisation levels you'll have a lot less variables to care about. 

Cheers, 
Renato 

On 23 Jul 2014 01:23, "Arnaud A. de Grandmaison" <arnaud.degrandmaison at arm.com> wrote:

I am trying to implement the lifetime marker insertion for unnamed temporaries in order to reduce stack consumption.

Inserting the lifetime.begin marker is the easy part; on the other hand,  inserting the lifetime.end gets a bit more complicated, as it involves scoping , exception handling and lifetime extension.

Ideally, the lifetime.end marker should be inserted when the scope is exited, after the cleanups have been run. The problem is that in some cases, an object with a  trivial destructor for example, there is … no cleanup.  The existence of cleanups appears in the AST with the ExpWithCleanups, which wraps the underlying MaterializeTemporaryExpr.

This is used in CodeGenFunction::EmitLValue to start a new RunCleanupsScope when visiting an ExpWithCleanups. My problem is how can I add a RunCleanupScope for MaterializeTemporaryExpr that are not part of an ExpWithCleanups. I can not always add a new scope, because it would mess-up the cleanup ordering --- when there are cleanups, the lifetime.end marker should be emitted after the destructor.

Is there anything obvious I am missing ?

Any hints would be welcome. 

Cheers,

--

Arnaud A. de Grandmaison

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