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Re: [std-proposals] C++ never had a compiler without a stack

From: Rand McRanderson <therandshow_at_[hidden]>
Date: Sat, 19 Aug 2023 09:09:03 -0400
On Tue, Aug 15, 2023 at 11:30 AM David Brown via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On 15/08/2023 16:17, Thiago Macieira via Std-Proposals wrote:
> > On Tuesday, 15 August 2023 06:56:20 PDT Julien Villemure-Fr├ęchette via
> Std-
> > Proposals wrote:
> >> I think the intent of alloca in Posix and VLA in C was specifically to
> have
> >> automatic cleanup semantics for objects with block scoped lifetime. C++
> >> already has the language feature that makes it possible to ensure proper
> >> cleanup at end of scope, so the need for VLA evaporates. If you look
> >> closely, the C language doesn't seem to make any mention of the stack in
> >> the specification of VLAs, I think a pair of malloc/free may be a
> possible
> >> implementation.
> >
> > The difference is that alloca() has determinstic runtime (provided it
> doesn't
> > overflow), wheras malloc() cannot be guaranteed to.
> >
>
> That is one difference, yes. The other key difference is that that run
> time is absolutely minimal - typical "alloca()" calls simply add (or
> subtract) a value to a stack pointer register, which is vastly faster
> than a call to malloc() could ever be. (For real-time systems,
> "deterministic" is more important than "fast". But it's nice to be fast
> too!)
>
> VLA's are often an alternative to alloca() in C, but there are
> differences in the semantics - VLA's are deallocated at the end of the
> scope, while alloca() space is deallocated at the end of the function.
>
> And C++ has no VLA's or any equivalent, so if you want fast, small local
> allocations, you need to use alloca() in C++.
>
>
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Received on 2023-08-19 13:09:17