On Mon, 2022-08-15 at 12:13 +0100, Richard Purdie via Gcc wrote:
> Hi,
>
> I'm wondering if we'd be able to improve path handling in the -f*-
> prefix-map compiler options to cover relative paths?
>
> Currently it works well for absolute paths but if a file uses a
> relative path or a path with a symlink in, or a non-absolute path, it
> will miss those cases. For relative paths in particular it is
> problematic as you can't easily construct a compiler commandline that
> would cover all relative path options.
>
> At first glance this is relatively straight forward, for example:
>
> Index: gcc-12.1.0/gcc/file-prefix-map.cc
> ===================================================================
> --- gcc-12.1.0.orig/gcc/file-prefix-map.cc
> +++ gcc-12.1.0/gcc/file-prefix-map.cc
> @@ -70,19 +70,25 @@ remap_filename (file_prefix_map *maps, c
> file_prefix_map *map;
> char *s;
> const char *name;
> + char *realname;
> size_t name_len;
>
> + realname = lrealpath (filename);
> +
> for (map = maps; map; map = map->next)
> - if (filename_ncmp (filename, map->old_prefix, map->old_len) == 0)
> + if (filename_ncmp (realname, map->old_prefix, map->old_len) == 0)
> break;
> - if (!map)
> + if (!map) {
> + free (realname);
> return filename;
> - name = filename + map->old_len;
> + }
> + name = realname + map->old_len;
> name_len = strlen (name) + 1;
>
> s = (char *) ggc_alloc_atomic (name_len + map->new_len);
> memcpy (s, map->new_prefix, map->new_len);
> memcpy (s + map->new_len, name, name_len);
> + free (realname);
> return s;
> }
>
> which address a realpath() call into the prefix mapping code. I did
> experiment with this and found it breaks compiling ruby and xen-tools
> which both have code which does:
>
> #include __FILE__
>
> It may be possible to make the remapping conditional of not being
> directly in a #include statement but I didn't find the gcc code
> responsible for that as yet. I also noticed some valgrind tests fails
> after it, I've not looked into why that would be yet.
>
> I wanted to ask if there would be any interest in adding support for
> something like this? I suspect the include/__FILE__ issue is probably a
> latent bug anyway. If anyone has any pointers to the code I could
> improve my patch with I'm also happy to have them!
To answer my own question, something like:
+Index: gcc-12.1.0/libcpp/macro.cc
+===================================================================
+--- gcc-12.1.0.orig/libcpp/macro.cc
++++ gcc-12.1.0/libcpp/macro.cc
+@@ -563,7 +563,7 @@ _cpp_builtin_macro_text (cpp_reader *pfi
+ if (!name)
+ abort ();
+ }
+- if (pfile->cb.remap_filename)
++ if (pfile->cb.remap_filename && !pfile->state.in_directive)
+ name = pfile->cb.remap_filename (name);
+ len = strlen (name);
+ buf = _cpp_unaligned_alloc (pfile, len * 2 + 3);
seems to do roughly what I was wondering about.
I'd be interested to understand whether some patch along the lines I've
mentioned here would stand a chance of being accepted or not.
Cheers,
Richard
