[llvm-dev] Skipping construction/destruction of stack allocated objects

Eli Friedman via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 15 19:16:46 PST 2019

On 1/15/2019 6:09 PM, Alexandre Isoard via llvm-dev wrote:
> Hello,
> For performance reasons, I would like to provide a way to skip 
> construction/destruction of objects that are stack allocated. 
> Typically, C-style arrays of std::complex create an initialization 
> loops that is almost always unnecessary.
> I am thinking of providing an __attribute__((uninitialized)) that can 
> be applied to an object declaration:
> {
>     std::complex<float> foo[64][64] __attribute__((uninitialized));
>     // does not need to generate a zeroinitializer loop here
>     somefunction(foo);
>     // does not need to generate a destructor loop here 
> (std::complex<float> don't have one anyway)
> }
> Formally speaking, we won't call constructors/destructors on those 
> objects anymore. We would provide "uninitialized" memory in the same 
> meaning as malloc/free.
> Do you see this as a good idea? Do we already have a better way of 
> achieving this?

The standard way to allocate uninitialized storage in C++ is 
std::aligned_storage.  The proposed extension doesn't really seem like 
an improvement over that.


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