[Bug target/120870] [16/17 regression] CPython miscompiled with preserve_none and CFLAGS="-O2 -march=znver2 -ggdb3"

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120870

--- Comment #51 from Uroš Bizjak <ubizjak at gmail dot com> ---
Prototype patch to confirm Comment #48:

--cut here--
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index e73c2d7f7d0..db571853103 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -496,6 +496,10 @@ ix86_profile_before_prologue (void)
   return flag_fentry != 0;
 }

+/* In no-callee-saved and preserve_none functions,
+   reserve BX_REG for DRAP register.  */
+#define X86_NO_CALLEE_DRAP_REG BX_REG
+
 /* Update register usage after having seen the compiler flags.  */

 static void
@@ -6807,7 +6811,8 @@ ix86_save_reg (unsigned int regno, bool maybe_eh_return,
bool ignore_outlined)

     case TYPE_NO_CALLEE_SAVED_REGISTERS:
     case TYPE_PRESERVE_NONE:
-      if (regno != HARD_FRAME_POINTER_REGNUM)
+      if (regno != HARD_FRAME_POINTER_REGNUM
+         && regno != X86_NO_CALLEE_DRAP_REG)
        return false;
       break;
     }
@@ -7940,6 +7945,11 @@ pro_epilogue_adjust_stack (rtx dest, rtx src, rtx
offset,
 static unsigned int
 find_drap_reg (void)
 {
+  if (cfun->machine->call_saved_registers == TYPE_PRESERVE_NONE
+      || (cfun->machine->call_saved_registers
+         == TYPE_NO_CALLEE_SAVED_REGISTERS))
+    return X86_NO_CALLEE_DRAP_REG;
+
   tree decl = cfun->decl;

   /* Always use callee-saved register if there are no caller-saved
--cut here--