Re: [PATCH] Add -C when using -Wimplicit-fallthrough and --save-temps (PR preprocessor/78497).

[Jakub Jelinek <jakub at redhat dot com>]

On Thu, Apr 05, 2018 at 02:51:22PM +0200, Martin Liška wrote:
> On 04/04/2018 09:31 PM, Marek Polacek wrote:
> > On Tue, Apr 03, 2018 at 02:29:37PM +0200, Martin Liška wrote:
> >> Hi.
> >>
> >> This helps the warning with --save-temps. Doing that one needs to preserve comments
> >> in preprocessed source file.
> > 
> > Do we really want to only use -C when -Wimplicit-fallthrough is in effect?  I
> > mean, shouldn't we always use -C when -save-temps?
> 
> Why not, Jakub what do you think? Note that it was originally Jakub's idea to do that.

I'd prefer to do that only when we actually care about the comments, it is a
behavior change in any case, and might be undesirable to some people.

Note that we do not care about the comments if -Wimplicit-fallthrough=0
or -Wimplicit-fallthrough=5, but do care for:
-Wimplicit-fallthrough
-Wimplicit-fallthrough=1
-Wimplicit-fallthrough=2
-Wimplicit-fallthrough=3
-Wimplicit-fallthrough=4
-Wextra
-W
(unless -Wno-implicit-fallthrough).  So, it would be desirable to:
1) swap the order, put save-temps to the outer level
2) use
{Wimplicit-fallthrough*:{!Wimplicit-fallthrough=0:{!Wimplicit-fallthrough=5:...}}}
3) verify (including adding testcases) that it doesn't emit comments for the
=0, =5 or -W -Wno-implicit-fallthrough cases, but does for -W etc.

	Jakub