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Re: [PATCH] Temporarily revert Steven's PCH changes for 4.8 (PR pch/54117)


On Thu, Feb 14, 2013 at 11:19:22AM +0100, Steven Bosscher wrote:
> On Thu, Feb 14, 2013 at 12:48 AM, Jakub Jelinek wrote:
> > On Thu, Feb 14, 2013 at 12:41:30AM +0100, Steven Bosscher wrote:
> >> > I agree with David that it might be better to give up pch.
> >>
> >> That'd be a really a good start.
> >
> > You can do it with say following patch instead, PCH is just a compile time
> > optimization, so for -gstabs IMHO it is just fine if we just give up.
> 
> Does that also provent *writing* a PCH with -gstabs?

No, it prevents *reading* of PCH files with -gstabs, so whatever you write
into the PCH file is uninteresting.  The patch can surely be acompanied
by a patch to warn that writing a PCH file is useless in that case, as in
your patch, though the spot you chose for it is too early.  Consider
./xgcc -B ./ -gstabs -o aa.h.gch aa.h -g0
cc1: warning: the "stabs" debug format cannot be used with pre-compiled headers [-Wdeprecated]
which warns even when the header is actually precompiled without debug info.
If you try to use such PCH file with say
./xgcc -B ./ -gstabs aa.c
it will fail to load PCH because of option mismatch (similarly for -g
created PCH file and non-g user), but if you do
./xgcc -B ./ -gstabs aa.c -g0
there should be no reason why it wouldn't work right, so you shouldn't warn
earlier.
So, IMHO you want to warn at the spot which I've changed in my patch,
add
if (pch_file && (write_symbols != NO_DEBUG && write_symbols != DWARF2_DEBUG))
  warning...
there.
And, of course, pch.exp testsuite won't work if -gstabs etc. is in effect,
because it does those hacks which ensure that if PCH isn't loaded, tests
fail (the original header file isn't available when testing PCH load).
That is not a problem for normal PCH uses, where the headers are available,
but pch.exp would need to be tweaked for it somehow.
Supposedly not do any testing if -g (defaulting to non-dwarf) or -gstabs
etc. are present in the default options (try to pre-compile some short
header first, if that fails with the newly added warning, punt),
drop all torture options using -g in it if such test fails with explicit
-g added from the list of torture options for testing, and perhaps add some
effective target if needed (valid-1.c test?).

	Jakub


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