merge from gcc

config/acx.m4

@@ -193,3 +193,34 @@ else
   have_gnat=no
 fi
 ])
+
+dnl 'make compare' can be significantly faster, if cmp itself can
+dnl skip bytes instead of using tail.  The test being performed is
+dnl "if cmp --ignore-initial=2 t1 t2 && ! cmp --ignore-initial=1 t1 t2"
+dnl but we need to sink errors and handle broken shells.  We also test
+dnl for the parameter format "cmp file1 file2 skip1 skip2" which is
+dnl accepted by cmp on some systems.
+AC_DEFUN([ACX_PROG_CMP_IGNORE_INITIAL],
+[AC_CACHE_CHECK([how to compare bootstrapped objects], gcc_cv_prog_cmp_skip,
+[ echo abfoo >t1
+  echo cdfoo >t2
+  gcc_cv_prog_cmp_skip='tail +16c $$f1 > tmp-foo1; tail +16c $$f2 > tmp-foo2; cmp tmp-foo1 tmp-foo2'
+  if cmp t1 t2 2 2 > /dev/null 2>&1; then
+    if cmp t1 t2 1 1 > /dev/null 2>&1; then
+      :
+    else
+      gcc_cv_prog_cmp_skip='cmp $$f1 $$f2 16 16'
+    fi
+  fi
+  if cmp --ignore-initial=2 t1 t2 > /dev/null 2>&1; then
+    if cmp --ignore-initial=1 t1 t2 > /dev/null 2>&1; then
+      :
+    else
+      gcc_cv_prog_cmp_skip='cmp --ignore-initial=16 $$f1 $$f2'
+    fi
+  fi
+  rm t1 t2
+])
+do_compare="$gcc_cv_prog_cmp_skip"
+AC_SUBST(do_compare)
+])