doc: we can now build gcc-4.5.1 w/ host gcc-8.2.1

Signed-off-by: Alessandro Rubini <[email protected]>

doc/lm32-toolchain.in

@@ -85,6 +85,7 @@ projects;
 The current status of this work is:
 @itemize @bullet
 @item We have a working 64-bit build of gcc-4.5.3, with newer support libraries;
+@item We can build this with a recent host compiler (@i{gcc-8.2.1});
 @item Newer compilers work, but create bigger binaries and are not suitable
 for our projects, at least the big ones
 @item No work on LLVM is yet completed.
@@ -873,6 +874,42 @@ was the fix for me:
    ln -s libmpfr.so.6 /usr/lib/x86_64-linux-gnu/libmpfr.so.4
 @end example
 
+@c ##########################################################################
+@node Rebuild on Recent Hosts
+@chapter Rebuild on Recent Hosts
+
+With a few additional patches to @i{binutils} and @i{gcc}, I was able
+to rebuild @i{gcc-4.5.3} on Fedora 29, which features @i{gcc-8.2.1}
+as host compiler, which is the most recent at time of writing.
+
+You can compile with the current patch-set part of this package,
+but you must first install a host library:
+
+@example
+   yum install mpfr-devel
+@end example
+
+The problem is a fine issue related to @t{mpfr.h}: auto-configuration
+of the compiler has a wrong patch for including header, and ends up
+picking the host one instead. If the host header is missing,
+compilation would fail. We can ignore the potential version difference
+between the host version and the cross-compiled version; the library
+is stable over time and not really used in our LM32 environments.
+
+The errors were a detail in @i{gcc} documentation (which is built by default,
+so the error was fatal for the overall compilation) and a zillion warnings
+in @i{binutils}, that turn into errors if @t{-Werror} is used.  Thus,
+I removed @i{-Werror}, as a simple solution to an otherwise impossible task.
+
+The resulting binary can successfully run on Debian-8, even if if
+features an older @i{libc} than the one used to build it (2.19
+vs. 2.28), thanks to forward- and backward-compatibility rules in
+system libraries.
+
+I didn't try to build other compiler versions; if needed, there should
+be similar problems as the ones I patched in 4.5.3, but fewer and
+fewer as you build newer versions.
+
 @c ##########################################################################
 @node Things to Do
 @chapter Things to Do