Hey,

> Congrats on this, Mathieu!

Thank you :)

> Perhaps add “Fixes <https://bugs.gnu.org/39941>.”

Well I think it doesn't. In the bug report 39941, I reported a recent
increase of the cross-compiled disk-image on core-updates (1.5 -> 2.0
GiB), over the last two months. It is not resolved, and this patch fixes
something that has always been around I guess.

>> +     ;; TOOLDIR_BASE_PREFIX is erroneous when using a separate "lib"
>> +     ;; output. Specify it correctly, otherwise GCC won't find its shared
>> +     ;; libraries installed in the "lib" output.
>
> How erroneous is it?  Is there a bug report we could link to?

I didn't find any bug report related. The issue is that in
gcc/Makefile.in, when computing libsubdir_to_prefix:

--8<---------------cut here---------------start------------->8---
libsubdir_to_prefix := \
  $(unlibsubdir)/$(shell echo "$(libdir)" | \
    sed -e 's|^$(prefix)||' -e 's|/$$||' -e 's|^[^/]|/|' \
        -e 's|/[^/]*|../|g')
--8<---------------cut here---------------end--------------->8---

unlibsubdir is ../../../
libdir /gnu/store/xxx-gcc-cross-aarch64-linux-gnu-7.5.0-lib/lib
prefix is /gnu/store/yyy-gcc-cross-aarch64-linux-gnu-7.5.0

then,

libsubdir_to_prefix = ../../../../../..

Which then gives a search path that looks like:

/gnu/store/5qh73asm77554wj43z2wjdrkl3fjxxbp-gcc-cross-aarch64-linux-gnu-7.5.0-lib/lib/gcc/aarch64-linux-gnu/7.5.0/../../../../../../../aarch64-linux-gnu/lib/aarch64-linux-gnu/7.5.0/

So its just that this hack is completely broken for non FHS compliant
path I guess.

Note that on our native toolchain, we have the same issue. If you run:

--8<---------------cut here---------------start------------->8---
 `guix build -e '(@@ (gnu packages gcc) gcc-9)'|tail -1`/bin/gcc -print-search-dirs
--8<---------------cut here---------------end--------------->8---

You can see the same kind of wrong path:

--8<---------------cut here---------------start------------->8---
/gnu/store/347y0zr1a9s2f5pkcncgi3gd0r33qq81-gcc-9.2.0-lib/lib/gcc/x86_64-unknown-linux-gnu/9.2.0/../../../../../../../x86_64-unknown-linux-gnu/lib/x86_64-unknown-linux-gnu/9.2.0/
--8<---------------cut here---------------end--------------->8---

and it still works, because of this snippet in GCC, disabled when
cross-compiling:

--8<---------------cut here---------------start------------->8---
      else if (*cross_compile == '0')
	{
	  add_prefix (&startfile_prefixes,
		      concat (gcc_exec_prefix
			      ? gcc_exec_prefix : standard_exec_prefix,
			      machine_suffix,
			      standard_startfile_prefix, NULL),
		      NULL, PREFIX_PRIORITY_LAST, 0, 1);
	}
--8<---------------cut here---------------end--------------->8---

that produces an extra search-path that looks like:

--8<---------------cut here---------------start------------->8---
/gnu/store/347y0zr1a9s2f5pkcncgi3gd0r33qq81-gcc-9.2.0-lib/lib/gcc/x86_64-unknown-linux-gnu/9.2.0/../../../
--8<---------------cut here---------------end--------------->8---

In short, quite a big mess...

> Apologies for suggesting some more tedious work, but I think it’ll be
> helpful!  :-)

Hehe no worries, here's an updated patch :)

Mathieu