[Gc] returning stack address without a warning

Andrew Haley aph at redhat.com
Thu Oct 26 04:31:24 PDT 2006

Eric Deplagne writes:
 > On Thu, 26 Oct 2006 11:33:20 +0100, Andrew Haley wrote:
 > > Eric Deplagne writes:
 > >  > > My goal with -Wall and similar is to at least document any remaining
 > >  > > warnings in the source.  The collector inherently does a few things that
 > >  > > should generate warnings in "normal" code, e.g. it returns the address
 > >  > > of a local to get an upper bound on the stack pointer.  This also really
 > >  > > applies only to gc7.
 > >  > 
 > >  > If there's nothing clever I didn't notice on the way, using an helper
 > >  > function is enough to prevent the compiler from noticing what we're doing
 > >  > to that poor local variable...
 > > 
 > > No, that's a bad idea.  Later versions of the compiler may well see
 > > through this trick and you'll get the warning again.  It is better
 > > either to use a properly supported way to the the stack address or to
 > > live with the warning than to go through such contortions.
 >   Well... The first part of your "or" statement is obviouly false,
 >   there is no supported way to get the stack address...

Well, I'm talking about gcc.  There is no official ISO way of doing
it, true, but what is being done here isn't portable in any case.  (Of
course there cannot be an ISO way of doing it, since ISO C doesn't
even mandate the use of a stack.)  So, if you're using gcc you might
as well do it in a properly supported way, and leave the warning for
other compilers.

 >   As for a compiler noticing this trick, it might happen, but I really
 >   doubt it, as long as the helper function doesn't get inlined...

Bingo.  It's perfectly reasonable for the compiler to inline very
small functions.

 >   IMHO one more function away is not such a "contortion" that it's not worth it...
 >   And for the case a compiler was clever enough to still bark, I guess I would then
 >   cope with the warning, having done what is possible to avoid it...

So, if you are prepared to cope with a warning at some time in the
future, why not simply cope with it now?  Contorting code to avoid
compiler warnings is, as they say, "the tail wagging the dog".


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