[LLVMdev] _Znwm is not a builtin

[Chris Lattner]

On May 15, 2013, at 8:44 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Wed, May 15, 2013 at 8:28 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Wed, May 15, 2013 at 7:49 PM, Chandler Carruth <chandlerc at google.com> wrote:
> On Wed, May 15, 2013 at 8:31 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> Hi,
> 
> LLVM classifies _Znwm as a builtin by default. After some discussion, the C++ core working group have decreed that that is not correct: calls to "operator new" *can* be optimized, but only if they come from new-expressions, and not if they come from explicit calls to ::operator new. We cannot work around this in the frontend by marking the call as 'nobuiltin' for two reasons:
> 
> 1) The 'nobuiltin' attribute doesn't actually prevent the optimization (see recent patch on llvmcommits)
> 2) We can't block the optimization if the call happens through a function pointer, unless we also annotate all calls through function pointers as 'nobuiltin'
> 
> How feasible would it be to make the 'builtin-ness' of _Znwm etc be opt-in rather than opt-out? Is there some other option we could pursue?
> 
> Wow, this was spectacularly unclear, sorry about that. To avoid confusion, I'm suggesting that we add a 'builtin' attribute, and do not treat a call to _Znwm as a builtin call unless it has the attribute.
>  

It's not clear to me that "builtin" is the right way to model this, but it definitely sounds like this should be an attribute on a call site (as opposed to on the function itself).  What specific kinds of optimizations are we interested in doing to _Znwm calls?

-Chris

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