[v3] --enable-pch

[Roger Sayle]

On 1 Jul 2003, Alexandre Oliva wrote:
> On Jun 30, 2003, Benjamin Kosnik <bkoz@redhat.com> wrote:
> > This allows --disable-pch to turn off attempts to use pre compiled
> > headers in C++ runtime land
>
> Could I perhaps talk you into renaming this option such that it's
> clear that it only affects C++ runtime land?  --disable-glibcpp-pch or
> so?  --disable-pch could be expected to disable pch support in GCC,
> and the configure flag is given to the top level, that people will
> often use while thinking of configuring GCC, not just libstdc++.

Hi Alex,

I asked Ben to add --disable-pch support to libstdc++ precisely because
it wasn't disabled by --disable-pch at the top-level.  My problem is
that mainline CVS has a bootstrap failure on i686-pc-cygwin because of
internal PCH problems that give a "not implemented" warning followed by
a segmentation fault.  I'd assumed that just configuring GCC with the
--disable-pch option at the top-level would workaround the bootstrap
failure, but unfortunately we currently always always try to build and
use stdc++.gch independently of the top-level configure flags.

You can see that Ben's fix does the correct thing by not using PCH in
a tree that is configured without it.  One possibility is to additionally
add --disable-glibcpp-pch to disable it selectively, but clearly it should
be possible to disable PCH globally with a single flag.

Does this help explain the motivation for the patch a bit better?
Many thanks to Ben for his work on putting this patch together.

Roger
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