[LLVMdev] _Znwm is not a builtin

Chris Lattner clattner at apple.com
Wed May 15 20:46:55 PDT 2013

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?


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