Re: [9fans] assembly syntax in plan 9

[Bakul Shah <bakul@bitblocks.com>]

On Mon, 16 Jan 2012 15:11:24 EST "Joel C. Salomon" <joelcsalomon@gmail.com>  wrote:
> On 01/16/2012 02:03 PM, Bakul Shah wrote:
> > On Jan 16, 2012, at 10:51 AM, Greg Comeau <comeauat9fans@gmail.com> wrote:
> >> What we do in problematic cases with Comeau is to generate code to
> >> arrange for the allocation of the VLA on the heap.  I'm not saying
> >> this is perfect, but at least it gets the feature implementable and up
> >> and running as "a portable implementation" versus perhaps not being
> >> able to implement it at all on some platforms.
> >
> > How do you deal with longjmp?
>
> I recall reading the source for a (mostly-) portable alloca() that
> checked where on the call-stack is was invoked from and released memory
> for any alloca() invocation from lower on the stack.  (The allocations
> themselves were on the heap.)

You are probably referring to Doug Gwyn's portable alloca().
It relies on a static var to keep track of malloced alloca
blocks. At first I thought it won't work in presence of
threads but I guess if you used thread local storage for this,
alloca() might be made to work but it is pretty bletcherous!

Here's what Gwyn had to say about alloca() later....  [From
https://www.securecoding.cert.org/confluence/display/seccode/08.+Memory+Management+(MEM)

    Douglas A. Gwyn

    Never use alloca.  (See my comments in the Gnu source for
    alloca.)  Under C99, you can use VLAs.  Cince they require
    a FIFO lifetime model, neither is as general as dynamic
    allocation.

Funny that he recommends using VLAs instead of alloca()!