Re: Severe performance problem and proposed solution

[Ralf Corsepius]

Am Mon, 2002-03-11 um 00.12 schrieb Phil Edwards:
> I'm one of the libstdc++-v3 people, and I've been making occasional
> halfhearted attempts to move our configury to 2.5x.  I had another go
> this weekend, which led me to the autoconf mail archives and this thread.
> So here's a status update:
> 
> > > The AC_TRY_RUN/AC_PROG_CC issue is supposed to be solved with
> > > autoconf-2.5x, because AC_PROG_CC doesn't try to run an executable
> > > anymore. I.e. basically, cross-compilation is supposed not to be any
> > > different from native compilation.
> > 
> > Really?  I'll have to poke at this again with 2.52.  It certainly did
> > not work in 2.50.
> 
> And it's still broken in 2.52.  :-( 
Well, ... I am using it for cross-compilation.

I know, there are details, where I do think cross-compilation support is
broken, but this is different topic ...

> AC_PROG_CC tries to do something in
> the "checking for C compiler default output" section which fails (cannot
> create executables) when cross-compiling.  If I edit the configure script
> to ignore the error, I get two steps further:
> 
>     checking for C compiler default output...
>     checking whether the C compiler works... yes
>     checking whether we are cross compiling... yes
>     checking for executable suffix... configure: error: cannot compute 
> EXEEXT: cannot compile and link
This result means that your C compiler is not able to link and/or
compile at the time this check is run.

If building libstdc++ standalone/outside of the gcc-source tree, this
would be a strong indication for your toolchain to be actually broken.

If building inside of the gcc-source-tree, there are various things
which can go wrong. E.g. this check might be run at a time when the
c-toolchain is incomplete and not yet usable. 

It could also be the side-effect of something else, e.g. the toplevel
configure script setting the canonicalization names to something which
is incompatible to autoconf-2.52 (--host etc.), rsp. the toolchain might
also be broken itself (several gnu-cross-toolchains are in pretty bad
shape.)

Could you please check the part of the config.log corresponding to the
failing AC_PROG_CC for the actual reason of the breakdown you are
observing?

> I've seen references in this thread to a macro called AC_NO_EXECUTABLES,
> but that's undocumented in the 2.52 manual, so I don't know what to do
> with that.
Non-documented == Don't use! :)
 
> The AC_PROG_CC documentation says, "See section 11. Manual Configuration,
> for more on support for cross compiling," but that section doesn't say a
> thing about cross-compiling.
This is Akim's baby :)

> I'll wait for 2.53 before trying again.
AFAIK, not much has changed wrt. cross-compilation with 2.53.
(You don't seem to be aware that it already has been released.).

Ralf