[PATCH] Support g++ 4.8 as a host compiler.

[Roger Sayle]

The recent patch to remove poly_int_pod triggers a bug in g++ 4.8.5's
C++ 11 support which mistakenly believes poly_uint16 has a non-trivial
constructor.  This in turn prohibits it from being used as a member in
a union (rtxunion) that constructed statically, resulting in a (fatal)
error during stage 1.  A workaround is to add an explicit constructor
to the problematic union, which allows mainline to be bootstrapped with
the system compiler on older RedHat 7 systems.

This patch has been tested on x86_64-pc-linux-gnu where it allows a
bootstrap to complete when using g++ 4.8.5 as the host compiler.
Ok for mainline?


2023-10-04  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        * rtl.h (rtx_def::u): Add explicit constructor to workaround
        issue using g++ 4.8 as a host compiler.


diff --git a/gcc/rtl.h b/gcc/rtl.h
index 6850281..a7667f5 100644
--- a/gcc/rtl.h
+++ b/gcc/rtl.h
@@ -451,6 +451,9 @@ struct GTY((desc("0"), tag("0"),
     struct fixed_value fv;
     struct hwivec_def hwiv;
     struct const_poly_int_def cpi;
+#if defined(__GNUC__) && GCC_VERSION < 5000
+    u () {}
+#endif
   } GTY ((special ("rtx_def"), desc ("GET_CODE (&%0)"))) u;
 };