Re: Compiler bug with aligned members in virtuals + alloca() in VS

From:

Alex Blekhman <xfkt@oohay.moc>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 24 Oct 2006 11:27:51 +0200

Message-ID:

<#XKsv609GHA.3456@TK2MSFTNGP02.phx.gbl>

RichForster wrote:

I'll totally agree in the contrived example, I don't need alloca() but in
real world situations, the overhead of operator new is excessive relative to
the locality of the allocation and the lifetime of the array. I use the
simple alloca() in the example since it was the minimum case that broke the
compiler.

That definitely requires measuring before you draw any
conclusions. Most of the time `operator new' doesn't
allocate memory, it just marks available chunk as taken in
preallocated list of memory chunks. So, roundtrip to CRT and
back is very cheap. At the same time, `_alloca' can cause
page fault while probing stack for required amount of bytes.
Such page fault, being handled by system, will cost much
more than regular call to `new'.

Here's quite interesting research regarding allocations speed:

"Calkins. Memory Allocation Methods in Multithreaded
Programming in C++ Under Windows NT an Applied Intelligence
Whitepaper, May 2000"
http://www.lotn.org/~calkinsc/papers/memorywp.pdf