[gcc r14-1873] cprop_hardreg: Enable propagation of the stack pointer if possible

[Philipp Tomsich <ptomsich@gcc.gnu.org>]

https://gcc.gnu.org/g:6a2e8dcbbd4bab374b27abea375bf7a921047800

commit r14-1873-g6a2e8dcbbd4bab374b27abea375bf7a921047800
Author: Manolis Tsamis <manolis.tsamis@vrull.eu>
Date:   Thu May 25 13:44:41 2023 +0200

    cprop_hardreg: Enable propagation of the stack pointer if possible
    
    Propagation of the stack pointer in cprop_hardreg is currenty
    forbidden in all cases, due to maybe_mode_change returning NULL.
    Relax this restriction and allow propagation when no mode change is
    requested.
    
    gcc/ChangeLog:
    
            * regcprop.cc (maybe_mode_change): Enable stack pointer
            propagation.

Diff:
---
 gcc/regcprop.cc | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/gcc/regcprop.cc b/gcc/regcprop.cc
index f426f4fedcd..6cbfadb181f 100644
--- a/gcc/regcprop.cc
+++ b/gcc/regcprop.cc
@@ -422,7 +422,12 @@ maybe_mode_change (machine_mode orig_mode, machine_mode copy_mode,
 
      It's unclear if we need to do the same for other special registers.  */
   if (regno == STACK_POINTER_REGNUM)
-    return NULL_RTX;
+    {
+      if (orig_mode == new_mode)
+	return stack_pointer_rtx;
+      else
+	return NULL_RTX;
+    }
 
   if (orig_mode == new_mode)
     return gen_raw_REG (new_mode, regno);