Re: [patch] PR bootstrap/25672: Avoid CFLAGS in CFLAGS_FOR_TARGETS if cross-compiling

[Aldy Hernandez <aldyh at redhat dot com>]

On 03/09/2015 11:35 PM, Jeff Law wrote:
> On 03/09/15 16:22, Aldy Hernandez wrote:
> > Hello gentlemen.
> >
> > The problem here is that we pick up the system's CFLAGS and pass it down
> > to the target libraries.  This causes havoc when, for instance, CFLAGS
> > is -march=x86-64 and the target is powerpc-linux.
> >
> > I don't see in what scenario the toplevel shell's CFLAGS would be of any
> > use in the target libraries, so I've just avoided that code path
> > altogether.
> >
> > Tested by doing a cross-build from x86-64 to ppc64-linux-gnu in a
> > combined tree, and seeing if libgcc starts to build.  Previously we
> > would fail while configuring GCC.  Now, gcc builds fine, and the target
> > libraries start to build.
> >
> > OK for mainline?
> >
> > Aldy
> >
> > curr
> >
> >
> > commit f2d130b93f334c5c680761553624964f87594adf
> > Author: Aldy Hernandez<aldyh@redhat.com>
> > Date:   Mon Mar 9 15:12:33 2015 -0700
> >
> >          PR bootstrap/25672
> >          * configure.ac: Do not initialize CFLAGS_FOR_TARGET from
> > CFLAGS if
> >          cross-compiling.  Similarly for CXX_FOR_TARGET.
> >          * configure: Regenerate.
> I was wondering when someone would come across this oldie-but-goodie...
>
> I'm sure someone, somewhere will complain that
>
> make CFLAGS="whatever" isn't being passed down to the crossses as it has
> been for the last two decades.  And they'll be right.

Well, considering this hasn't worked since 4.0.2, with a brief respite 
in 4.4.0, I wouldn't worry too much.  For that matter, setting 
incompatible CFLAGS won't even survive a toplevel configure on trunk 
right now.  I doubt anyone is dependent on this feature :).

> But in the cross compiler case, I think it's reasonable to expect to
> have to use CFLAGS_FOR_TARGET so that we have separation between the
> host and target flags.  One could easily want to compile the host with
> one set of arguments and the target libraries with something completely
> different.
>
> And note that the various .mt files already use the _FOR_TARGET macros
> to pass down additional flags rather than just CFLAGS or CXXFLAGS.
>
>
>
> Are there any docs that need to change?  I don't recall any offhand, but
> if you could look prior to committing, it'd be appreciated.

Interestingly, there is nothing but a cursory reference to 
CFLAGS_FOR_TARGET in our docs:

@code{BOOT_CFLAGS} does not apply to bootstrapped target libraries.
Since these are always compiled with the compiler currently being
bootstrapped, you can use @code{CFLAGS_FOR_TARGET} to modify their
compilation flags, as for non-bootstrapped target libraries.
Again, if the native compiler miscompiles the stage1 compiler, you may
need to work around this by avoiding non-working parts of the stage1
compiler.  Use @code{STAGE1_TFLAGS} to this end.

Considering our entire cross-compilation section is actually quite 
lacking, I'm going to just commit as is.

Thanks.
Aldy