remapping absolute source paths

[Felix Lee <bdgle@tigerfood.org>]

Say I have a program that has some absolute path names in
the debugging info, like
    /a/x/foo.c

but /a doesn't exist here.  instead I have the source
in /p.

There's no easy way of making that work right in gdb, is
there?  If I say "dir /p", it's not going to look for
/p/x/foo.c.  If I say "dir /p/x" and for every other subdir
in the tree, then gdb will have problems picking /p/x/foo.c
from /p/y/foo.c

I'm thinking either

  1. there should be a way to tell gdb that "/a" in a
     source path should be remapped to "/p"

  2. gdb should try every sub-path of the sourcefile name,
     so it should try
        /p/a/x/foo.c
        /p/x/foo.c
        etc.

1 is less transparent, but it's easier to control
ambiguities, like if for some reason the executable has both
    /a/x/foo.c
    /b/y/foo.c
which seems unlikely, but I could see it happening when
linking several libraries and packages together.

I'm leaning towards implementing 1.  any thoughts?
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