Re: [PATCH] Add --enable-host-shared configuration option

[Jeff Law <law at redhat dot com>]

On 10/09/13 18:25, David Malcolm wrote:
> My JIT branch requires embedding GCC's code as a shared library on the
> host.  To do requires building the host code as position-independent,
> which unfortunately incurs a small speed hit.
Obviously this is an opt-in right now, so I'm not terribly concerned.

> My first time attempt at this handled --enable-host-shared by detecting
> it in each child "configure" script (as generated from configure.ac) and
> conditionally appending -fPIC to makefile variables such as CFLAGS,
> CXXFLAGS and LDFLAGS.
>
> This worked when invoking "make" from each subdirectory, but not when
> running "make" in the top-level directory: the top-level make would
> pass in "CFLAGS=-g -O2" to the child makes, overriding their CFLAGS,
> thus erroneously omitting the "-fPIC" flag.
Yea.  This stuff (particularly the overriding of flags) is a mess and 
far from obvious IMHO.  But I don't think fixing that is "in scope" for 
what you're working on.

> This patch instead adds a PICFLAG variable, set up in the child
> configure scripts, to contain -fPIC if configured with
> --enable-host-shared.  The PICFLAG variable is then added to an
> appropriate variable in each child makefile to ensure that it is used
> when building host code.
Seems reasonable -- particularly since you've tested it both with a 
toplevel invocation and invocations in subdirectories.

> There's precedent for both "PICFLAG" and "PIC_FLAG" in the source tree:
> libiberty uses "PICFLAG", libbacktrace usss "PIC_FLAG".  I went with
> "PICFLAG" for the new flags.
But of course...

> The patch uses -fPIC rather than -fpic.
Right.


> Successfully bootstrapped and regtesting on x86_64-unknown-linux.
>
> OK for trunk?
>
> ChangeLog follows inline:
>
> /
> 	* configure.ac: Add --enable-host-shared
> 	* configure: Regenerate.
>
> gcc/
> 	* Makefile.in (PICFLAG): New.
> 	(enable_host_shared): New.
> 	(INTERNAL_CFLAGS): Use PICFLAG.
> 	(LIBIBERTY): Use pic build of libiberty.a if configured with
> 	--enable-host-shared.
> 	* configure.ac: Add --enable-host-shared, setting up new
> 	PICFLAG variable.
> 	* configure: Regenerate.
>
> libbacktrace/
> 	* configure.ac: Add --enable-host-shared, setting up
> 	pre-existing PIC_FLAG variable within Makefile.am et al.
> 	* configure: Regenerate.
>
> libcpp/
> 	* Makefile.in (PICFLAG): New.
> 	(ALL_CFLAGS): Add PICFLAG.
> 	(ALL_CXXFLAGS): Likewise.
> 	* configure.ac: Add --enable-host-shared, setting up new
> 	PICFLAG variable.
> 	* configure: Regenerate.
>
> libdecnumber/
> 	* Makefile.in (PICFLAG): New.
> 	(ALL_CFLAGS): Add PICFLAG.
> 	* configure.ac: Add --enable-host-shared, setting up new
> 	PICFLAG variable.
> 	* configure: Regenerate.
>
> libiberty/
> 	* configure.ac: If --enable-host-shared, use -fPIC.
> 	* configure: Regenerate.
>
> zlib/
> 	* configure.ac: Add --enable-host-shared, setting up new
> 	PICFLAG variable.
> 	* Makefile.am: Add PICFLAG to libz_a_CFLAGS.
> 	* Makefile.in: Regenerate.
> 	* configure: Regenerate.
Presumably other host libraries we depend on such as gmp, mpc, etc are 
available in shared (or at least PIC) form as well?  Obviously these are 
out of our source tree and largely out of our control, but I'm a bit 
curious if there's other host libraries that may cause you headaches in 
the future.

This is fine with the install.texi changes Joseph wanted.

jeff