[llvm-dev] Skipping construction/destruction of stack allocated objects (original) (raw)

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 15 20:17:05 PST 2019

This seems like a better discussion to have on cfe-dev than on llvm-dev FWIW.

(I also happen to agree with Eli, but that's somewhat irrelevant.)

On Tue, Jan 15, 2019 at 7:16 PM Eli Friedman via llvm-dev < llvm-dev at lists.llvm.org> wrote:

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 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 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::alignedstorage. The proposed extension doesn't really seem like an improvement over that. -Eli -- Employee of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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