[PATCH v2] Add `--with-install-sysroot=' configuration option

[Maciej W. Rozycki]

On Fri, 22 Nov 2019, Joseph Myers wrote:

> >  As I recall the MIPS sysroot setup (please correct me if I got something 
> > wrong here) was like:
> 
> Yes, that's the sort of layout you get with sysroot suffixes.  See 
> gcc/config/mips/{st.h,t-st} for an example.

 Thanks for the pointer.

> >  Then the right-hand side of /path/to/somewhere (except for usr/) is what 
> > gets printed by `-print-multi-directory' or the left-hand side of output 
> > from `-print-multi-lib', e.g. `sof/el/lib64' for the example above.  
> 
> Rather, it's a suffix (as in SYSROOT_SUFFIX_SPEC, no command-line option 
> to print it),

 Do you mean that there's no option to print SYSROOT_SUFFIX_SPEC on its 
own or that no option prints it as a path component?  If the latter, then 
I think it's an awful shortcoming, because there's no reasonable way for 
a given GCC compilation to determine the layout expected.

> >  Well, I agree we need to have this stuff documented beyond what we 
> > currently have, but I think it applies equally to all the sysroot options 
> > we have, including both the `--sysroot=' GCC driver's option, and the 
> > `--with-sysroot=', `--with-build-sysroot=' and the newly-proposed 
> 
> All three of those refer to the top-level sysroot path, to which a sysroot 
> suffix is appended based on SYSROOT_SUFFIX_SPEC (unless 
> --no-sysroot-suffix is used).
> 
> > `--with-install-sysroot=' `configure' script's options as well.  All we 
> > currently have is this paragraph:
> 
> But this is a path relative to which SYSROOT_SUFFIX_SPEC isn't used at 
> all.

 Can you please show me the two directory layouts, one for `--sysroot=' 
and the other for `--with-install-sysroot=' aka $toolexeclibdir, say for 
the `mips64el-st-linux-gnu' target, and where exactly in GCC installation 
(if anywhere) the `--sysroot=' layout is used?

 Is it that with $toolexeclibdir we have say:

/usr/mips64el-st-linux-gnu/
                          +-> lib/
                          |      +-> 2e/
                          |      \-> 2f/
                          +-> lib32/
                          |        +-> 2e/
                          |        \-> 2f/
                          \-> lib64/
                                   +-> 2e/
                                   \-> 2f/

whereas `--sysroot=/path/to/sysroot' expects:

/path/to/sysroot/
                +-> 2e/
                |     +-> lib/
                |     +-> lib32/
                |     \-> lib64/
                \-> 2f/
                      +-> lib/
                      +-> lib32/
                      \-> lib64/

(and then GCC applies the former scheme to the directories pointed to by 
the `-B' and `-L' options and the latter scheme to the directory pointed 
to by the `--sysroot=' option)?

> >  And last but not least: do we want to hold my proposed change hostage to 
> > a sysroot handling documentation improvement?  It does not appear fair to 
> > me as the situation with said documentation is not a new problem nor one 
> > specific to this newly-added option, and the new option merely played the 
> 
> The proposed new option is, as far as I know, the first one introducing 
> this new kind of sysroot option (one for which the suffix from 
> SYSROOT_SUFFIX_SPEC is never added).

 Thank you for all your input.

 If my understanding as expressed above is correct, then I think the way 
to move forward with this change will be to rename the option to 
`--with-toolexeclibdir=' or suchlike (and adjust documentation 
accordingly) so that it avoids the ambiguity of "sysroot" and is in line 
with the usual `--bindir=', `--libdir=', etc. or less usual 
`--with-slibdir=' options where people can adjust the various installation 
directories according to their requirements or preferences.

 Then on top of this an option like `--enable-sysroot-for-toolexeclibdir' 
can be discussed in the future, that would switch $toolexeclibdir to the 
proper sysroot layout, whether `--with-toolexeclibdir=' has been used or 
not.  Such an option will necessarily have to rely on the presence of a 
GCC option to print SYSROOT_SUFFIX_SPEC/STARTFILE_PREFIX_SPEC for the 
multilib selected.

 If we agree on this plan, I'll post an update patch.

  Maciej