The cprop_hardreg pass is built around the assumption that accessing a
register in a narrower mode is the same as accessing the lowpart of
the register.  This unfortunately is not true for vector registers on
IBM Z. This caused a miscompile of LLVM with GCC 8.5. The problem
could not be reproduced with upstream GCC unfortunately but we have to
assume that it is latent there. The right fix would require
substantial changes to the cprop pass and is certainly something we
would want for our platform. But since this would not be acceptable
for older GCCs I'll go with what Vladimir proposed in the RedHat BZ
and introduce a hopefully temporary and undocumented target hook to
disable that specific transformation in regcprop.c.

--- a/gcc/config/s390/s390.c
+++ b/gcc/config/s390/s390.c
@@ -10488,6 +10488,18 @@ s390_hard_regno_mode_ok (unsigned int regno, machine_mode mode)
   return false;
 }
 
+/* Implement TARGET_NARROW_MODE_REFERS_LOW_PART_P.  */
+
+static bool
+s390_narrow_mode_refers_low_part_p (unsigned int regno)
+{
+  if (reg_classes_intersect_p (VEC_REGS, REGNO_REG_CLASS (regno)))
+    return false;
+
+  return true;
+}
+
+
 /* Implement TARGET_MODES_TIEABLE_P.  */
 
 static bool
@@ -16956,6 +16968,9 @@ s390_case_values_threshold (void)
 #undef TARGET_CASE_VALUES_THRESHOLD
 #define TARGET_CASE_VALUES_THRESHOLD s390_case_values_threshold
 
+#undef TARGET_NARROW_MODE_REFERS_LOW_PART_P
+#define TARGET_NARROW_MODE_REFERS_LOW_PART_P s390_narrow_mode_refers_low_part_p
+
 struct gcc_target targetm = TARGET_INITIALIZER;
 
 #include "gt-s390.h"
--- a/gcc/regcprop.c
+++ b/gcc/regcprop.c
@@ -426,7 +426,8 @@ maybe_mode_change (machine_mode orig_mode, machine_mode copy_mode,
 
   if (orig_mode == new_mode)
     return gen_raw_REG (new_mode, regno);
-  else if (mode_change_ok (orig_mode, new_mode, regno))
+  else if (mode_change_ok (orig_mode, new_mode, regno)
+	   && targetm.narrow_mode_refers_low_part_p (copy_regno))
     {
       int copy_nregs = hard_regno_nregs (copy_regno, copy_mode);
       int use_nregs = hard_regno_nregs (copy_regno, new_mode);
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -5446,6 +5446,16 @@ value that the middle-end intended.",
  bool, (machine_mode from, machine_mode to, reg_class_t rclass),
  hook_bool_mode_mode_reg_class_t_true)
 
+/* This hook is used to work around a problem in regcprop. Hardcoded
+assumptions currently prevent it from working correctly for targets
+where the low part of a multi-word register doesn't align to accessing
+the register with a narrower mode.  */
+DEFHOOK_UNDOC
+(narrow_mode_refers_low_part_p,
+"",
+bool, (unsigned int regno),
+hook_bool_uint_true)
+
 /* Change pseudo allocno class calculated by IRA.  */
 DEFHOOK
 (ira_change_pseudo_allocno_class,
--- a/gcc/hooks.h
+++ b/gcc/hooks.h
@@ -86,6 +86,7 @@ extern void hook_void_tree (tree);
 extern void hook_void_tree_treeptr (tree, tree *);
 extern void hook_void_int_int (int, int);
 extern void hook_void_gcc_optionsp (struct gcc_options *);
+extern bool hook_bool_uint_true (unsigned int);
 extern bool hook_bool_uint_uintp_false (unsigned int, unsigned int *);
 
 extern int hook_int_uint_mode_1 (unsigned int, machine_mode);
--- a/gcc/hooks.c
+++ b/gcc/hooks.c
@@ -498,6 +498,14 @@ hook_void_gcc_optionsp (struct gcc_optio
 {
 }
 
+/* Generic hook that takes an unsigned int and returns true.  */
+
+bool
+hook_bool_uint_true (unsigned int)
+{
+  return true;
+}
+
 /* Generic hook that takes an unsigned int, an unsigned int pointer and
    returns false.  */